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Am J Physiol Heart Circ Physiol 280: H2545-H2553, 2001;
0363-6135/01 $5.00
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Vol. 280, Issue 6, H2545-H2553, June 2001

Mechanisms of coronary artery depolarization by uridine triphosphate

Donald G. Welsh and Joseph E. Brayden

Department of Pharmacology, University of Vermont, Burlington, Vermont 05405


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

We sought to define the basic mechanisms by which pyrimidine nucleotides constrict rat coronary resistance arteries. Uridine triphosphate (UTP) caused a dose-dependent constriction in coronary arteries stripped of endothelium. UTP also depolarized and increased cytosolic Ca2+ in coronary smooth muscle cells. Nisoldipine, an antagonist of voltage-operated Ca2+ channels, blocked the rise in cytosolic Ca2+ and reduced UTP-induced vasoconstriction by ~75% which suggests a prominent role for depolarization in this constrictor response. The ionic basis of UTP-induced depolarization was subsequently explored in coronary smooth muscle cells using whole-cell patch-clamp electrophysiology. In the absence of K+ and with CsCl in the pipette, UTP (40 µM) activated a sustained inwardly rectifying current (-0.66 ± 0.10 pA/pF at -60 mV). A 100 mM reduction in bath Na+ shifted the reversal potential of this current (from -2 ± 1 to -28 ± 4 mV) and reduced the magnitude (from -2.26 ± 0.61 to -0.51 ± 0.11 pA/pF). In addition to activating a depolarizing cation current, UTP inhibited hyperpolarizing outward currents. Specifically, UTP inhibited ATP-sensitive and voltage-dependent K+ currents yet had no effect on inwardly rectifying and Ca2+-activated K+ channels. This study indicates that electromechanical coupling is integral to pyrimidine-induced constriction in coronary resistance arteries.

resistance; nonselective cation channels; potassium; smooth muscle; calcium


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

PYRIMIDINE NUCLEOTIDES such as uridine triphosphate (UTP) and uridine diphosphate (UDP) are signaling molecules released from cells in response to mechanical stimulation (19) and cellular injury (4). Within the purine/pyrimidine P2 receptor family, uracil nucleotides bind with highest affinity to the P2Y receptors (33); this elicits both dilation and constriction (22-24) in resistance arteries. Dilation arises from the activation of endothelial P2Y receptors and the augmented production of nitric oxide (22, 25) and other endothelial-derived factors (33). In contrast, arterial constriction induced by pyrimidine nucleotides is caused by activation of P2Y receptors located directly on vascular smooth muscle cells (40).

Interest in pyrimidine nucleotides has been sparked by findings indicating that these agents are important in the genesis of vasospasm (39, 43) which is a leading cause of morbidity and mortality (2, 20, 21, 42). It has been suggested that vasospasm occurs when pyrimidine nucleotides (released from erythrocytes, leukocytes, and platelets) tonically activate smooth muscle P2Y receptors (39, 43). Normally, pyrimidine nucleotides liberated from these sources have limited access to smooth muscle P2Y receptors due to the confluent layer of endothelial cells (4). Accessibility, however, dramatically increases after arterial rupture (i.e., hemorrhage) or during disease states (i.e., atherosclerosis) when endothelial integrity is compromised (1, 10, 21). Upon activation, P2Y receptors stimulate a signal-transduction cascade that elevates cytosolic Ca2+ levels. This response in turn is thought to directly contribute to vascular spasm by enhancing cross-bridge formation.

Pyrimidine nucleotides could elevate cytosolic Ca2+ in vascular smooth muscle by increasing Ca2+ influx or release or by decreasing Ca2+ extrusion. Although recent studies have noted that pyrimidine nucleotides mobilize Ca2+ from both intracellular and extracellular sources (39, 43), it is the extracellular component that ultimately sustains the rise in cytosolic Ca2+ and maintains arterial constriction. This sustained rise in cytosolic Ca2+ is in large part blocked by voltage-operated Ca2+ channel antagonists (39, 43) which indicates an important role for membrane depolarization in pyrimidine-induced responses. The ionic basis of pyrimidine-induced depolarization remains unclear although it could arise from the activation of depolarizing inward current (via Cl- or cation channels) and/or from the inhibition of a hyperpolarizing outward current [via inwardly rectifying (Kir), ATP-sensitive (KATP), voltage-dependent (KV), or Ca2+-activated (KCa) K+ channels].

The purpose of this study was to identify the ion channels that underlie UTP-induced depolarization in coronary resistance arteries. Membrane potential (Em), cytosolic Ca2+, and diameter measurements revealed that electromechanical coupling was integral to UTP-induced responses in intact coronary arteries. With the use of whole cell patch-clamp electrophysiology, further experiments have demonstrated the ability of UTP to activate a sustained depolarizing cation current and to inhibit KATP and KV currents. We conclude that these changes in ion-channel conductance will drive the smooth muscle cell depolarization and consequently the rise in cytosolic Ca2+ required to sustain the constriction induced by pyrimidine nucleotides.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals and tissues. Female Sprague-Dawley rats (12-16 wk old) were used in this study. Rats were euthanized with an injection of pentobarbitone (130 mg/kg ip). The heart was removed and placed in cold bicarbonate-buffered physiological salt solution (PSS) containing (in mM): 119 NaCl, 4.7 KCl, 25 NaHCO3, 1.7 KH2PO4, 1.2 MgSO4, 1.6 CaCl2, and 10 glucose, at pH 7.4. The septal and right-anterior descending arteries were dissected from the surrounding tissue and cut into 2-3-mm segments (~180-µm diameter). These intact segments were either mounted in an arteriograph or used in the isolation of coronary smooth muscle cells.

Intact resistance arteries. Intact coronary arteries were mounted in an arteriograph chamber (Living Systems, Burlington, VT) and pressurized to 15 mmHg for measurements of diameter and cytosolic Ca2+ (28). The low intravascular pressure minimizes the activity of voltage-operated Ca2+ channels under resting conditions by maintaining smooth muscle Em between -50 and -55 mV. Endothelial cells were removed from all arteries by passing an air bubble through the lumen of the cannulated artery for 2 min; successful removal was confirmed by the loss of acetylcholine-induced dilations. Diameter was monitored using an automated video dimension-analyzer system (Living Systems). To assess cytosolic Ca2+, coronary arteries were incubated for 80 min in modified PSS containing pluronic acid (0.5%) and the membrane-permeable acetyoxymethyl ester analog of fura 2 (fura 2-AM; 2 µM). Absolute changes in cytosolic Ca2+ were measured using IonOptix microfluorimetry equipment (IonOptix, Milton, MA) and were calculated ratiometrically (excitation at 340 nm and 380 nm; emission at 510 nm) as previously described (15). Fluorescent ratios were converted to Ca2+ concentrations using an apparent dissociation constant (Kd) of 282 nm (15).

To assess Em, coronary arteries were mounted onto a small-vessel wire myograph (initial tension = 300 mg). A glass microelectrode filled with 3 M KCl (tip resistance = 60-80 MOmega ) was carefully inserted into the vessel wall. The criteria for a successful cell penetration were: 1) a sharp negative Em deflection on entry, 2) a stable potential for at least 1 min after entry, and 3) a sharp return to 0 mV upon removal of the electrode from the cells. Em recordings were typically limited to 5-10 min, thus multiple impalements were required to assess Em during control conditions and in the presence of 40 µM UTP. To facilitate the duration of each electrical recording, these experiments were conducted in the presence of nisoldipine (1 µM), a voltage-operated Ca2+ antagonist.

Single smooth muscle cells. Smooth muscle cells were enzymatically isolated from septal and right-anterior descending arteries as previously described (31). Briefly, arteries were cut into 2-mm segments and placed in isolation solution of the following composition (in mM): 60 NaCl, 85 sodium glutamate, 5.6 KCl, 2 MgCl2, 10 glucose, 0.1 CaCl2, and 10 HEPES, at pH 7.4 and 37°C. At the end of a 10-min equilibration period, artery segments were placed in isolation solution containing 1 mg/ml each of albumin, papain, and dithioerythritol for 20 min. Artery segments were then placed for 15 min in a second isolation solution containing 1 mg/ml each of collagenase type F and hyaluronidase. The tissue was subsequently washed twice in isolation solution and triturated with a polished wide-bore pipette. Cells were stored on ice and used the same day.

Perforated and conventional whole cell patch clamp techniques were used to monitor ionic currents in coronary smooth muscle cells. Recording electrodes (resistance 4-7 MOmega ) were pulled from borosilicate glass, and dental wax was applied to the exterior surface to minimize pipette capacitance. Voltage-clamped cells were maintained at a holding potential of -50 or -60 mV for 10-15 min before experimentation. In general, whole cell currents were monitored under control conditions and in the presence of UTP (with or without ion-channel antagonist) while voltage was slowly ramped (0.167 mV/ms; range varied with experimental protocol) or stepped (20-mV increments; 800-ms duration). The KATP current was, however, assessed at a constant holding potential of -60 mV. A 1-M NaCl-agar salt bridge between the bath and the Ag/AgCl reference electrode was used to minimize offset potentials during bath exchanges. Liquid junction potentials were measured as previously described (27) and were <2 mV with any solution change. Membrane currents were filtered at 1 kHz, digitized at 5 kHz, and recorded using pCLAMP 8.0 software (Axon Instruments, Foster City, CA). Data were subsequently analyzed using Clampfit 8.0 software. Cell capacitance was measured with the cancellation circuitry in the voltage-clamp amplifier (Axopatch 200A amplifier, Axon Instruments). Patch-clamp recordings were performed at room temperature (21°C); the bath and pipette solutions used to isolate specific currents are summarized in Table 1. To explore the ion selectivity of cation currents, bath Cl- and Na+ concentrations were reduced by 100 mM by substituting Na-aspartate or N-methyl-D-glucamine-Cl for NaCl.

                              
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  		Table 1.   Bath and pipette solutions used to isolate specific currents

RT-PCR analysis of UTP-sensitive P2Y receptors. Approximately 100 smooth muscle cells that were enzymatically isolated from coronary arteries were placed in RNase- and DNase-free collection tubes. After total RNA extraction (RNeasy mini kit; Qiagen, Valencia, CA), first-strand cDNA was synthesized using the superscript RT kit (Qiagen). Reverse transcription was also performed in the absence of RT to control for DNA contamination. Subsequently 5-10 µl of each first-strand cDNA reaction was placed in a PCR solution (40-45 µl; Applied Biosystems, Branchburg, NJ) containing 1.4 mM MgCl2, 0.02 mM forward and reverse primers (Great American Gene, Ramona, CA), 0.25 mM deoxynucleotide triphosphates, 1× reaction buffer, and 2.5 U AmpliTaq Gold DNA polymerase. PCRs were hot started (94°C for 10 min) and then exposed to 35-40 cycles of 94°C for 60 s, 60°C for 90 s, and 72°C for 60 s. Forward and reverse primers specific for rat UTP-sensitive P2Y receptors (P2Y2, P2Y4, and P2Y6) were designed using Vector NTI software and were as follows: P2Y2F, 5'-TTCCACGTCACCCGCACCCTCTTATTACT-3'; P2Y2R, 5'-CGATTCCCCAACTCACACATACAAATGATTG-3'; P2Y4F, 5'-GTAACTGCCCACCCTCGTCTACTA-3'; P2Y4R, 5'-GTCCGCCCACCTGCTGAT-3'; P2Y6F, 5'-CGCAACCGCACTGTCTG-3'; and P2Y6R, 5'-TCTCTGCCTCTGCCACTTG-3'. Primers yielded product sizes of 510, 794, and 468 bp for P2Y2, P2Y4, and P2Y6, respectively. Reaction products were run on a 1.5% agarose gel.

Chemicals, drugs, and enzymes. Buffer reagents, collagenease type F, hyaluronidase, dithioerythritol, GdCl3, BaCl2, tetraethylammonium (TEA), and 4-aminopyridine (4-AP) were obtained from Sigma (St. Louis, MO). Papain was purchased from Worthington Biochemical (Lakewood, NJ); pinacidil and glibenclamide were obtained from Research Biochemical (Natick, MA). Molecular Probes (Eugene, OR) supplied pluronic acid and fura 2-AM. Pinacidil, glibenclamide, and amphotericin B were dissolved in DMSO to final solvent concentrations of 0.05%. Nisoldipine (a gift from Miles Pharmaceuticals, West Haven, CT) was dissolved in ethyl alcohol to a final solvent concentration of 0.05%.

Statistical analysis. Data are expressed as means ± SE, and n indicates the number of animals. Comparisons of data were made using paired t-tests. Data were considered to be significantly different at P <=  0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

UTP depolarizes, elevates cytosolic Ca2+, and constricts coronary arteries. Pyrimidine nucleotides elicited a dose-dependent constriction (UTP, EC50 = 3.2 µM; UDP, EC50 = 8.5 µM) in coronary resistance arteries pressurized to 15 mmHg and stripped of endothelium (see Fig. 1A). UTP (40 µM) also depolarized coronary arterial myocytes from -54 ± 2 to -37 ± 1 mV (see Fig. 1B) and elevated cytosolic Ca2+ from 108 ± 15 to 180 ± 23 nM (see Fig. 2A). In the presence of UTP, nisoldipine (1 µM), a voltage-operated Ca2+ channel antagonist, lowered cytosolic Ca2+ from 180 ± 23 to 110 ± 16 nM and dilated the coronary arteries by 74 ± 7% (see Fig. 2). These results suggest that the UTP-induced depolarization elevates cytosolic Ca2+ through the activation of voltage-dependent Ca2+ channels, and this increase contributes to the vasoconstriction.


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  		Fig. 1.   Pyrimidine nucleotides depolarize and constrict isolated coronary arteries. A: uridine triphosphate (UTP; n = 6; EC50 = 3.2 µM) and uridine diphosphate (UDP; n = 3; EC50 = 8.5 µM) induce dose-dependent constriction of intact coronary resistance arteries. B: summary of effects of UTP on smooth muscle membrane potential (n = 4). Data are means ± SE; *significant difference from control.



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  		Fig. 2.   UTP induces a dihydropyridine-sensitive increase in cytosolic Ca2+. A: representative cytosolic Ca2+ and diameter response of intact, pressurized coronary arteries to UTP (40 µM) in the absence or presence of nisoldipine (1 µM) in normal and Ca2+-free physiological salt solution (PSS). B: cytosolic Ca2+ concentration under control conditions and in the presence of UTP ± nisoldipine and Ca2+-free PSS. Data are means ± SE; *significant difference from control.

mRNA for UTP-sensitive P2Y receptors. The sustained vasomotor, Em, and cytosolic Ca2+ responses to UTP were consistent with the activation of UTP-sensitive P2Y receptors. To test whether mRNA for P2Y2, P2Y4, and P2Y6 was present in coronary smooth muscle, the total RNA from ~100 cells was extracted, reverse transcripted to produce cDNA, and subjected to PCR amplification. PCR products with the predicted sizes for P2Y2, P2Y4, and P2Y6 receptors were observed on the agarose gel (see Fig. 3).


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  		Fig. 3.   Coronary artery smooth muscle cells contain mRNA for P2Y2, P2Y4, and P2Y6 receptors. Total RNA extracted from ~100 coronary smooth muscle cells was subjected to RT-PCR analysis. Reactions without reverse transcriptase (-) were run alongside cDNA templates (+) to verify the absence of genomic DNA contamination. M, molecular mass marker; sizes measured in base pairs.

UTP activates a cation current. UTP-induced depolarization could arise from an increase in inward current (Na+ or Cl- conductances) and/or an inhibition of K+ conductance. To examine the effect of UTP on Na+ and Cl- conductances, the perforated patch-clamp technique was used to monitor whole cell currents before and after the application of UTP. In the absence of K+ ions and at a holding potential of -60 mV, UTP activated a sustained inward current (magnitude of 0.66 ±0.10 pA/pF; n = 6; see Fig. 4A). The current activated by UTP demonstrated inward rectification, reversed near 0 mV, and was unaffected by the presence of nisoldipine (see Fig. 4, B and C). The reversal potential (Erev) of the UTP-activated current was unaffected by a 100-mM reduction in bath Cl- (control, -3 ± 1 mV; low Cl-, -2 ± 2 mV; n = 5; see Fig. 5A). In contrast, a 100 mM reduction in Na+ (see Fig. 5B) shifted Erev (from -2 ± 1 mV to -28 ± 4 mV; n = 5) in close correspondence with the hyperpolarizing shift in the Na+ equilibrium potential (ENa; from 0 to -32 mV) and reduced the magnitude of the UTP-activated current at -120 mV (from -2.26 ± 0.61 to -0.51 ± 0.11 pA/pF; n = 5). Gadolinium (30 µM), a trivalent cation that blocks smooth muscle cation conductances (30, 38), effectively antagonized the cation current activated by UTP (see Fig. 5C). These findings are consistent with the hypothesis that UTP activates a vascular smooth muscle cation conductance.


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  		Fig. 4.   UTP activates an inward current (I) in isolated coronary smooth muscle cells. A: effects of UTP (40 µM) on steady-state current (holding potential = -60 mV; perforated patch clamp). B: effects of UTP on whole cell current during a voltage (V)-step protocol. Step pulses were applied (-120 to 60 mV; 600 ms; holding potential = -60 mV) in the absence or presence of UTP (40 µM). Dashed line represents zero current. C: I-V relationship of the UTP-activated current (n = 4) as determined from the step-pulse protocol in B. Data are means ± SE.



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  		Fig. 5.   Reversal potential (Erev) of the UTP-activated inward current shifts with the Na+ equilibrium potential (ENa) but not the Cl- equilibrium potential (ECl). Ramp pulses (-120 to 60 mV; 0.167 mV/ms; holding potential = -60 mV) were applied before and after the application of UTP. A: Erev is not affected when ECl is shifted from +1 to +31 mV by lowering the Cl- concentration from 135 to 35 mM. B: Erev follows the shift in ENa from 0 to -42 mV caused by lowering the NaCl concentration from 135 to 35 mM. C: summary of the effects of Gd3+ (30 µM; n = 5) on the UTP-activated cation current in isolated coronary smooth muscle cells.

UTP inhibits KATP and KV channels. Because a reduction in K+ conductance can lead to depolarization, this study examined the effects of UTP on the four predominant types of K+ channels in coronary smooth muscle. With 140 mM and 20 mM K+ in the pipette and bath, respectively, we observed a Ba2+-sensitive inward current (see Fig. 6A), which reversed (-41 ± 2 mV; n = 7) in close proximity to the K+ equilibrium potential (EK; -43 mV). This Ba2+-sensitive current, previously identified as Kir (34), was unaffected by the application of UTP (40 µM). We next determined whether UTP had effects on KATP. In the presence of pinacidil (10 µM), a KATP channel opener, the application of 40 µM UTP significantly reduced steady-state KATP current (holding potential of -60 mV; see Fig. 6B); the magnitude of this reduction relative to the glibenclamide-sensitive current (10 µM) was 47 ± 3%. The pinacidil and glibenclamide concentrations used in this study have been previously shown not to influence Kir activity (43).


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  		Fig. 6.   UTP inhibits ATP-sensitive (KATP) but not inwardly rectifying (Kir) currents. A: Ba2+-sensitive difference currents in the absence and presence of UTP (30 µM BaCl2). Ba2+-sensitive Kir currents were monitored (voltage ramps, -120 to 60 mV; 0.167 mV/ms; perforated patch) in the presence and absence of UTP. B: UTP inhibits KATP currents. KATP currents were monitored in isolated coronary cells (140 mM KCl + 10 µM pinacidil; holding potential -60 mV; conventional whole cell) under control conditions and in the presence of UTP (40 µM) and glibenclamide (10 µM). C: effects of UTP on Kir (n = 6) and KATP (n = 6) currents. Kir currents were measured at -120 mV and were defined as that component of the inward current sensitive to Ba2+ (30 µM). KATP currents were measured at -60 mV and were defined as that component of the inward current that was sensitive to glibenclamide. Data are means ± SE; *significant difference from control.

Given that P2Y receptors are coupled to a transduction pathway that activates protein kinase C (PKC; 18, 26, 29) and that PKC can in turn influence both KCa (6) and KV (3) currents, we next examined the ability of UTP to modulate these K+ channel conductances. Using a pipette solution that clamped free Ca2+ at 100 nM, voltage-ramp protocols revealed the presence of an outward current sensitive to 1 mM TEA (see Fig. 7A). This TEA-sensitive current, which predominantly reflects whole cell KCa activity (17, 35), was unaffected by the addition of 40 µM UTP (see Fig. 7, A and C). Because there is no effective means to fully antagonize smooth muscle KV channels, whole cell KV activity cannot be assessed as a difference current. Consequently, to examine the effects of UTP on these channels, we adopted an approach similar to that of Robertson and Nelson (35) whereby that proportion of the outward current insensitive to KCa channel antagonists (1 mM TEA and 100 nM iberiotoxin) was primarily assumed to reflect the KV current. Using this criteria, we found that KV currents were significantly reduced by UTP (from 25.2 ± 2.0 to 19.3 ± 2.2 pA/pF; see Fig. 7B) and further reduced by addition of 4-AP (from 19.3 ± 2.2 to 8.8 ± 1.2 pA/pF; see Fig. 7C), which is an antagonist that partially blocks smooth muscle KV currents (35).


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  		Fig. 7.   UTP inhibits voltage-dependent (KV) but not Ca2+-activated (KCa) currents. Ramp pulses (-60 to 100 mV; 600 ms; holding potential = -60 mV; perforated patch) were applied before and after UTP (40 µM) application. A: KCa current, defined as the component of the whole cell current that was sensitive to tetraethylammonium (TEA; 1 mM), was unaffected by UTP. B: KV current, defined as the residual current that remained in the presence of TEA (1 mM) and iberiotoxin (100 nM), was significantly reduced in the presence of UTP. C: summary of the effects of UTP on KCa and KV currents as measured at +100 mV. Data are means ± SE; *significant difference from control; **significant difference from UTP + 4-AP.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Pyrimidine nucleotides are important signaling compounds released from several different cells during mechanical stress (19) and cellular injury (4). The activation of smooth muscle P2Y receptors by these factors leads to a sustained rise in cytosolic Ca2+ and a maintained vasoconstriction. Here we report that in coronary arteries, UTP causes a sustained constriction and a rise in cytosolic Ca2+ that depends in large part on membrane depolarization and the activation of voltage-operated Ca2+ channels. In addition we provide the first direct demonstration that UTP-induced depolarization depends on the activation of a depolarizing inward current and the inhibition of two different K+ currents. In particular our results demonstrate that UTP activates a novel inwardly rectifying cation conductance and inhibits both KATP and KV currents in coronary resistance arteries.

UTP response of rat coronary arteries. The influence of UTP on resistance arteries is mediated by P2Y receptors and the associated signal-transduction pathways that are coupled to the trimeric G proteins, Gq and possibly G12-13 (5, 18). Upon excitation, Gq stimulates phospholipase Cbeta , augments the production of inositol trisphosphate (InsP3) and diacylgycerol (DAG), and activates PKC (18, 26, 29). Gq and G12-13 have also been suggested to modulate RhoA and Rho-dependent kinase (5, 37). These signal-transduction pathways regulate vascular tone via mechanisms that may or may not depend on changes in Em. Consistent with depolarization being a principal mechanism underlying UTP-induced vasoconstriction, we documented the ability of UTP to: 1) depolarize vascular smooth muscle, 2) activate Ca2+ entry through voltage-operated Ca2+ channels, and 3) elicit a vasoconstriction that is in large part nisoldipine sensitive. In keeping with these observations, previous studies have noted the efficacy of Ca2+ channel blockers on UTP-induced contractions in both canine coronary (23) and rat mesenteric (16) arteries. Without selective antagonists, this study could not directly ascertain which P2Y receptor subtypes were responsible for UTP-induced vasoconstriction. However, based on the RT-PCR findings and the known sensitivity of P2Y receptors for UTP (33), such responses would likely depend on P2Y2, P2Y4, and/or P2Y6 activation.

It is important to recognize that in addition to activating voltage-operated Ca2+ channels, pyrimidine nucleotides can mobilize Ca2+ from the sarcoplasmic reticulum (SR; 40, 44). In isolated smooth muscle cells the mobilization of SR Ca2+ is observed as an initial transient that precedes a lower but sustained rise in cytosolic Ca2+ (40, 44). In contrast, in intact coronary arteries, UTP caused a sustained Ca2+ elevation with no apparent initial Ca2+ transient (present study). The absence of the transient response in the intact artery may indicate that SR Ca2+ is not simultaneously mobilized throughout the entire population of vascular smooth muscle cells in the arterial wall. Indeed, recent observations have shown that SR Ca2+ mobilization by UTP is asynchronous among smooth muscle cells (13). Asynchronous Ca2+ release would, because of an averaging effect, limit our ability to resolve the contribution of the SR to the UTP-induced rise in cytosolic Ca2+. In any case, the steady-state elevation in cytosolic Ca2+ noted in this study appears to be mediated primarily by Ca2+ entry through voltage-dependent Ca2+ channels.

Ionic basis of UTP-induced depolarization. Membrane potential in vascular smooth muscle reflects the dynamic interplay between multiple inward and outward ion-channel currents. Observations from this investigation are the first to indicate that UTP evokes the depolarization of vascular smooth muscle by activating a cation conductance. The UTP-activated cation current exhibits inward rectification, does not rapidly desensitize, and is blocked by Gd3+. P2Y receptor recruitment has not been previously associated with cation current activation in vascular cells but rather with the activation of Ca2+-sensitive Cl- currents in coronary (41) and pulmonary (11) artery myocytes. However, other G protein-coupled receptors have been reported to modulate cation currents in vascular smooth muscle. For example, Helliwell and Large (12) have noted that alpha 1-adrenoceptors stimulate a cation current in rabbit portal vein with properties similar to the UTP-activated cation current observed in the present study. Interestingly, the portal vein cation current appears to depend on the production of DAG but not the activation of PKC (12). The molecular identity of the UTP-activated cation current present in rat coronary vascular smooth muscle cells, the specific subtype of P2Y receptor involved in activation of the cation current, and the precise mechanisms and pathways by which this current is modulated are important areas for future study.

The depolarization induced by UTP may not only depend on the activation of a depolarizing cation current but also on the inhibition of K+ channels. In support of this view, the present study revealed the ability of UTP to inhibit both KATP and KV currents. The selective inhibition of these K+ channel conductances likely results from the activation of PKC, a family of serine/threonine kinases that can influence the phosphorylation state of ion-channel proteins. Past studies have noted that direct activators of PKC (3, 7) as well as numerous vasoconstrictor agents that activate phospholipase C and PKC (7, 9) cause substantial inhibition of smooth muscle K+ currents (i.e., KATP and KV).

In the present study, the activation of P2Y receptors by UTP had no discernable effect on whole cell KCa activity in rat coronary smooth muscle cells when internal Ca2+ was held constant at 100 nM. These findings are similar to those of Strobaek and colleagues (41) and indicate that the G protein-coupled pathways activated by UTP do not directly influence KCa channels. However, under physiological conditions UTP can dramatically influence local Ca2+ events (i.e., Ca2+ sparks and waves) and such modulation could alter KCa channel activity in vascular smooth muscle. For example, Jaggar and Nelson (13) report that pyrimidine nucleotides reduce the frequency of Ca2+ sparks in cerebral artery myocytes and that such a reduction is responsible for the decrease in the frequency and amplitude of spontaneous transient outward currents. Thus it could be hypothesized that pyrimidine nucleotides do indeed limit KCa channel activity under physiological conditions and that this in turn contributes to the UTP-induced depolarization of vascular smooth muscle. In an interesting contrast to this study, Sanchez-Fernandez and colleagues (36) reported that UTP induces an oscillatory increase in the open probability of the KCa channel. The oscillatory nature of this latter response raises the possibility that oscillatory or wavelike Ca2+ events, which can be induced by UTP (13), might underlie these responses. Clearly, further investigations are required to determine the relationships between intracellular Ca2+ events and the KCa channel and consequently whether the modulation of this channel by these events either contributes to or otherwise alters the depolarization induced by UTP.

Physiological implications. The voltage dependency of KV and KCa channels implies that inhibition of these channels should have little effect on hyperpolarized smooth muscle. Indeed, functional studies conducted on vessels with Em values ranging from -60 to -50 mV have shown that KV (4-AP) and KCa (iberiotoxin) channel antagonists have minimal effects on smooth muscle Em and arterial diameter (8, 14). In contrast, KATP channels are active in coronary artery smooth muscle throughout a broad range of Em values (32). It is within this context that one can appreciate the importance of cation current activation and KATP channel inhibition to the initiation of UTP-induced depolarization and vasoconstriction (see Fig. 1). In depolarized smooth muscle there will be a substantial steady-state KV current (35). Thus, under these conditions, the targeted inhibition of KV channels by agonists (i.e., UTP) should cause further depolarization with the steady-state Em value ultimately determined by the effects of UTP on cation, KATP, and KV channels. As noted earlier, coronary vasospasm may in part depend on the release of pyrimidine nucleotides (from leukocytes and platelets) and the activation of smooth muscle P2Y receptors (39). If this is indeed the case, then results from this study indicate that agents specifically designed to minimize cation current activation, prevent the inhibition of KV or KATP channels, or specifically activate these or other K+ channels could provide an effective means of preventing or reversing coronary vasospasm.

The present study has shown that UTP-induced vasoconstriction in coronary arteries depends to a large degree on the activation of voltage-operated Ca2+ channels and increased Ca2+ entry which occur in response to depolarization of the vascular smooth muscle cells. Using patch-clamp electrophysiology to examine the ionic basis of pyrimidine nucleotide-induced depolarization, we found that UTP activated a sustained inwardly rectifying cation current and inhibited KATP and KV currents in coronary artery smooth muscle cells. We propose that UTP depolarizes coronary smooth muscle through potent and coordinated actions on multiple types of ion channels.


    ACKNOWLEDGEMENTS

The authors are grateful to Kerry Siebert and Suzanne Brett Welsh for technical assistance.


    FOOTNOTES

This work was supported by National Institutes of Health Grant HL-58231 and the Medical Research Council of Canada (D. G. Welsh).

Address for reprint requests and other correspondence: J. E. Brayden, Rm. B-329, Given Bldg., Dept. of Pharmacology, Univ. of Vermont, Burlington, VT 05405 (E-mail: brayden{at}salus.med.uvm.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 21 August 2000; accepted in final form 10 January 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Am J Physiol Heart Circ Physiol 280(6):H2545-H2553
0363-6135/01 $5.00 Copyright © 2001 the American Physiological Society


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