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1Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N 4N1; 3Department of Physiology, Faculty of Medicine, University of Montreal, Montreal, Quebec, Canada H3C 3J7; and 2Department of Bioengineering, University of California, San Diego, La Jolla, California 92093-0412
Submitted 16 September 2003 ; accepted in final form 28 December 2003
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONREFERENCES |
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C receptor; calcium current; hyperpolarization-activated current
NPR-C was originally referred to as a "clearance receptor"; however, in the heart and in gastrointestinal smooth muscle, NPR-C is known to be functionally linked to adenylyl cyclase via a pertussis toxin-sensitive G protein, Gi (3, 5, 37, 38). NPR-C receptors are disulfide-linked homodimers that have a single transmembrane domain. Therefore, they differ significantly from the traditional heptahelical receptor and heterotrimeric Gi protein complex (4, 39). Importantly, NPR-C includes a specific Gi-activator domain within the intracellular portion of the receptor, a motif that was first described for the insulin-like growth factor receptor (41). This Gi-activator sequence is characterized by the presence of two NH2-terminal basic residues and a COOH-terminal BBXXB motif, where B and X are basic and nonbasic residues, respectively (41, 42). In NPR-C, a 17-amino acid sequence (R469–R485) within the 37-amino acid intracellular domain is responsible for activation of Gi (44, 52). A shorter 12-amino acid fragment of NPR-C containing a Gi-activator peptide sequence has been shown to be equipotent with the 17-amino acid fragment in terms of its ability to inhibit adenylyl cyclase (44). In this way, natriuretic peptides, when bound to NPR-C, are able to activate Gi proteins and decrease cAMP levels via the inhibition of adenylyl cyclase (3, 44).
Recently, Mangoni et al. (31) and Mangoni and Nargeot (32) described a procedure for isolating single myocytes from the sinoatrial (SA) node region of the mouse heart. The myogenic pacemaker function of these specialized myocytes is due to their ability to generate a slow diastolic depolarization, during which the membrane potential slowly approaches the threshold for eliciting an action potential (1). In the mammalian heart, several different ionic currents have been proposed to play a role in the generation of the slow diastolic depolarization. These include a delayed rectifier K+ current and at least four voltage-dependent inward currents: the hyperpolarization-activated current (If), the sustained inward current, and T- and L-type Ca2+ currents [ICa(T) and ICa(L)] (8, 11, 13, 23). Two of these inward currents in particular, ICa(L) and If, are thought to contribute to the generation of the diastolic depolarization phase of the myogenic pacemaker activity (1, 31, 48).
Recently, we described a novel electrophysiological effect mediated by NPR-C in the heart (45). In this study on bullfrog single atrial myocytes, CNP, when bound to NPR-C, was found to markedly shorten the action potential by inhibiting ICa(L). This inhibitory effect on ICa(L) was mimicked by the NPR-C-selective agonist cANF (2) and maintained in the presence of the NPR-B antagonist HS-142-1 (33), thus demonstrating the involvement of NPR-C (34).
The main goal of the present study was to determine the ionic mechanism that is responsible for a natriuretic peptide effect on heart rate in the adult mouse. An initial focus was to determine whether CNP can modulate cAMP-sensitive currents in isolated mouse SA node myocytes. Our findings demonstrate that CNP has a negative chronotropic effect in adult mouse heart and provide strong evidence that this is mediated by NPR-C. In the isolated SA node myocytes, this negative chronotropic effect of CNP is mainly due to the selective inhibition of ICa(L), because another cAMP-sensitive pacemaker current, If, was unaffected by CNP superfusion.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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After the heart was allowed to stabilize for 15–20 min, baseline ECG recordings were made, and the effects of isoproterenol (Iso, 5 x 10–9 M) and cANF (10–7 M) on the ECG were recorded. The R-R interval (used as an indicator of heart rate) was monitored in real time as the experimental compounds were added to the temperature-controlled perfusate. In each experiment, Iso was added to mimic a level of adrenergic tone, which was maintained throughout the experiment. After a steady state was achieved in the presence of Iso, cANF was added to the perfusate.
Single cell preparation. Cells from the SA node region were isolated from the hearts of adult male C57BLK6 mice (25–30 g) according to the methods of Mangoni and Nargeot (32). Briefly, adult mice were anesthetized with methoxyflurane. The heart was excised and placed in Tyrode solution consisting of (in mM) 140 NaCl, 5.4 KCl, 1.2 KH2PO4, 1.0 MgCl2, 1.8 CaCl2, 5.55 glucose, and 5 HEPES, with pH adjusted to 7.4 with NaOH, at 35° C. The SA node region of the heart was isolated by separating the atria from the ventricles, cutting open the superior and inferior venae cavae, and pinning the tissue so that the crista terminalis could be identified. The SA node area is demarcated by boundaries consisting of the crista terminalis, the interatrial septum, and the openings of the two great veins. This SA node region was cut into strips, which were transferred to a "low-Ca2+-Mg2+-free" solution containing 140 mM NaCl, 5.4 mM KCl, 1.2 mM KH2PO4, 0.2 mM CaCl2, 50 mM taurine, 18.5 mM glucose, 5 mM HEPES, and 1 mg/ml BSA, with pH adjusted to 6.9 with NaOH. SA node tissue strips were digested in 5 ml of low-Ca2+-Mg2+-free solution containing 4.2 mg of collagenase (type II, Worthington; 268 U/mg), 80 µl of elastase (Worthington; 88 U/ml), and 65.2 µl of 1 mg/100 µl protease solution (type XIV, Sigma; 4.3 U/mg) for 20–23 min. Then the tissue was transferred to 5 ml of modified KB solution (in mM: 100 potassium glutamate, 10 potassium aspartate, 25 KCl, 10 KH2PO4, 2 MgSO4, 20 taurine, 5 creatine, 0.5 EGTA, 20 glucose, and 5 HEPES, and 1% BSA, with pH adjusted to 7.2 with KOH), and digestion was continued for 5–9 min at 35° C under mechanical agitation with a wide-bore pipette. This procedure yielded a sufficient number of SA node myocytes with cellular automaticity that was recovered after readapting the cells to a physiological concentration of Ca2+ (by the addition of a solution containing 10 mM NaCl and 1.8 mM CaCl2 and then a normal Tyrode solution containing 1 mg/ml BSA). Cells were confirmed to be from the SA node by their spontaneous beating and expression of If, which was demonstrated under voltage-clamp conditions. The University of Calgary Animal Resource Centre approved all experimental protocols and animal procedures used in this study.
Solutions and drugs. The recording chamber was superfused with a control Tyrode solution (22–23° C) containing (in mM) 140 NaCl, 5 KCl, 1 MgCl2, 1 CaCl2, 10 HEPES, and 5 glucose, with pH adjusted to 7.4 with NaOH. This solution was used for recording SA node electrophysiological responses and for studying If. To record ICa(L), the following external solution was used (in mM): 140 TEA-Cl, 2 CaCl2, 1 MgCl2, 10 HEPES, and 5 glucose, with pH adjusted to 7.4 with CsOH. The pipette filling solution used for recording If consisted of (in mM) 135 KCl, 1 CaCl2, 10 EGTA, 4 Mg-ATP, 1 MgCl2, 6.6 Na-phosphocreatine, and 10 HEPES, with pH adjusted to 7.2 with KOH. For recording ICa(L), the pipette contained (in mM) 135 CsCl, 10 EGTA, 4 Mg-ATP, 1 MgCl2, 6.6 Na-phosphocreatine, and 10 HEPES, with pH adjusted to 7.2 with TEA-OH. In experiments examining the effects of the Gi-activator peptide on ICa(L), 0.01 mM GTP
S was added to the recording pipette. GTPS was included on the basis of the findings of Pagano and Anand-Srivastava (44), which demonstrated a more robust effect of this Gi-activator peptide in the presence of GTPS. Gi-activator peptides have been shown to facilitate the binding of GTPS to Gi protein (41).
Iso and cAMP were purchased from Sigma (St. Louis, MO). CNP and cANF, an NPR-C-selective agonist that has no effect on the NPR-B-cGMP pathway (2), were purchased from Peninsula Laboratories (San Carlos, CA). The Gi-activator peptide (provided by M. B. Anand-Srivastava) (44) was stored in solution at –70° C and synthesized by standard solid-phase techniques and purified (95–99%) by high-performance liquid chromatography (Peninsula Laboratories).
Electrophysiological protocols. The whole cell configuration of the patch-clamp technique (17) was used for voltage-clamp studies of single SA node myocytes. Micropipettes were pulled from borosilicate glass (with filament, 1.5 mm OD, 0.86 mm ID; Sutter Instrument, Novato, CA) using a Flaming/Brown pipette puller (model p-87, Sutter Instrument). The resistance of these pipettes was 5–10 M
when they were filled with recording solution.
Microelectrodes were positioned with a hydraulic micromanipulator (SD Instruments) mounted on the stage of an inverted microscope (Nikon Diaphot). Seal resistances were 2–15 G. Rupturing the sarcolemma in the patch resulted in access resistances of 5–15 M. Series resistance compensation was usually not employed because of the small capacitance of SA node myocytes (typically 15–30 pF) and low current densities. The calculated voltage error in the absence of series resistance compensation was 3–5 mV and therefore was ignored. When it was used, series resistance compensation averaged 80–85% when a Multiclamp 700A or Axopatch 200B amplifier (Axon Instruments, Foster City, CA) was used. There was no significant difference between currents recorded with and those recorded without series resistance compensation. These sets of results were therefore combined. Voltage-clamp command waveforms and resulting capacitative and ionic current changes were digitized using a Digidata 1322A interfaced with pCLAMP 8 software (Axon Instruments). Data were stored on computer for offline analysis.
Current-voltage (I-V) relations for If were generated by applying a series of 2-s voltage-clamp steps in 10-mV increments from –35 to –135 mV from a holding potential of –60 mV. Peak ICa(L) was recorded by first applying a depolarizing step from –60 to –40 mV to inactivate the Na+ current (18). Immediately after this prepulse, 250-ms voltage-clamp steps were applied from –50 to +70 mV in 10-mV increments. This voltage-clamp protocol allows the measurement of ICa(L) carried by the CaV1.2 (
1C) family of channels. The peak inward current was measured, and I-V relations were plotted. When the effects of acetylcholine (ACh) on If were tested, Ba2+ (10–5 M) was included in the external solution to block the inwardly rectifying ACh-sensitive K+ current (26).
Statistical analysis. Summary data are presented as means ± SE. The data were analyzed using an ANOVA with Dunnett's multiple comparison procedure (in most cases) or a paired Student's t-test (Fig. 1) to identify significant differences. In all instances, P < 0.05 was considered significant.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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-adrenergic stimulation, Iso was added initially, and then cANF was added to selectively activate NPR-C in the presence of "adrenergic tone." Figure 1A shows representative ECG data for which the R-R interval (used as an indicator of heart rate) is labeled in control conditions, after the addition of Iso (5 x 10–9 M), and in the presence of Iso + cANF (10–7 M). In this experiment, Iso decreased the R-R interval from 178 to 112 ms. Subsequent application of cANF resulted in an R-R interval increase to 160 ms. The averaged effects of Iso and cANF on the R-R, P-R, and Q-T intervals are summarized in Table 1. As expected, the R-R and P-R intervals were significantly decreased by Iso application. Subsequent application of cANF significantly increased both of these ECG parameters.
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Effects of CNP on ICa(L). To explore the ionic mechanism(s) of these NPR-C-mediated effects on heart rate, voltage-clamp measurements of ICa(L) were made on single SA node myocytes (32). Our previous work demonstrated that CNP can inhibit basal ICa(L) and also showed that the effect is more pronounced in the presence of -adrenergic receptor stimulation (45). Therefore, the following experiments are conducted in the presence of Iso (5 x 10–9 M). Representative raw data are illustrated in Fig. 2A. On average, peak ICa(L) was about –9 pA/pF in the presence of Iso (5 x 10–9 M). Application of CNP (10–7 M) in this setting significantly decreased peak current to approximately –4 pA/pF (Fig. 2B).
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We have previously demonstrated that, in bullfrog atrial myocytes, CNP can inhibit ICa(L) by binding to NPR-C (45). Accordingly, the next series of experiments were designed to evaluate this hypothesis in mouse SA node myocytes. Figure 3 illustrates the effects of cANF (10–8 M), which is a selective agonist of NPR-C with no capacity to bind to NPR-B or activate the cGMP second messenger pathway (2). The raw data in Fig. 3A show that Iso approximately doubles peak ICa(L) from –5 to –10 pA/pF. After the addition of cANF, peak ICa(L) is reduced to about –6 pA/pF. On average, cANF decreased peak ICa(L) from –13 to about –6 pA/pF. In summary, these electrophysiological findings (Figs. 2 and 3) suggest that the effects of CNP on ICa(L) are mediated by NPR-C.
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Effects of Gi-activator peptide on ICa(L). Recently, two independent research groups published results that demonstrate that NPR-C contains a specific Gi-activator sequence (44, 52). These studies showed that amino acids 469–485, which are located in the middle region of the intracellular domain of NPR-C, are necessary and sufficient for activation of Gi proteins in the heart.
This important finding was used in our experimental design to provide further evidence for an NPR-C-mediated effect on ICa(L). This Gi-activator peptide (the 17-amino acid fragment of NPR-C containing the Gi-activator sequence) was applied intracellularly to single SA node myocytes by including it in the micropipette filling solution. The time course of the effects of the Gi-activator peptide is illustrated in Fig. 4A. Myocytes were first exposed to Iso (10–7 M) for 
3 min. During the subsequent impalement of the cell, the Gi-activator peptide (10–7 M), which was included in the recording pipette, was able to diffuse into the cell under conventional whole cell recording conditions. In the presence of Iso, peak ICa(L) was approximately –12 pA/pF, and it was decreased significantly as the Gi-activator peptide entered the cell over the course of 200 s to about –3 pA/pF.
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NPR-C-mediated effects have been shown to result from activation of the Gi family of G proteins, which are known to inhibit adenylyl cyclase activity (3, 44). Accordingly, we reasoned that inhibition of ICa(L) by the Gi-activator peptide should be antagonized in the presence of elevated cAMP levels. To test this hypothesis, the Gi-activator peptide (10–7 M) and cAMP (10–5 M) were added to the recording pipette with the expectation that each compound could enter the myocyte under whole cell recording conditions. Figure 4B illustrates that when a high concentration of cAMP is in the micropipette, peak ICa(L) increased from –4 to –15 pA/pF. The time course and magnitude of this effect of cAMP on ICa(L), in the presence of the Gi-activator peptide, are similar to the increase in ICa(L) that has been observed in the presence of cAMP alone (16, 45). These findings demonstrate that this effect of cAMP developed fully in the presence of the inhibitory Gi-activator peptide.
Effects of CNP on If. To evaluate the specificity of these inhibitory effects of CNP on ICa(L) in mouse SA node myocytes, If was measured. If is strongly modulated by intracellular cAMP levels (1, 32). Accordingly, CNP, which activates Gi protein, inhibits adenylyl cyclase, and decreases cAMP levels, may inhibit If.
Raw data and plots of the time course of the effects of Iso (10–7 M) and cANF (10–8 M) on If in SA node myocytes are illustrated in Fig. 5, A and B, respectively. As expected, Iso significantly increased If from –38 to –50 pA/pF (Fig. 5B). Note, however, that cANF failed to significantly inhibit this current. The I-V curves that summarize this data set show that neither cANF (to selectively activate NPR-C; Fig. 5C) nor CNP (Fig. 5D) has any significant effect on If in the presence of Iso.
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To gain further insight into this negative result, ACh (5 x 10–6 M) was applied to SA node myocytes. SA node myocytes were stimulated with Iso (10–7 M) and then with CNP (10–8 M). ACh was added in the presence of Iso and CNP. Figure 6A illustrates representative measurements of If. CNP caused a very small reduction in current, whereas ACh, in combination with CNP, markedly reduced If. As shown in the I-V relation that summarizes these findings (Fig. 6B), the effects of CNP on If were not significant. In contrast, the effects of ACh on this current were more pronounced and consistent. These results confirm that If is sensitive to cAMP levels and that ACh can inhibit this current (1, 32) but that CNP has no effect on If in these same myocytes.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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Our results show that CNP can have a negative chronotropic effect on the Langendorff-perfused mouse heart that is mediated by NPR-C. The data in Fig. 1 were obtained using the selective NPR-C agonist cANF, which has no capacity to activate the NPR-B-cGMP pathway but strongly inhibits adenylyl cyclase after binding to NPR-C (2). CNP elicited a negative chronotropic effect very similar to that shown for cANF in Fig. 1 (data not shown). In addition to this effect on heart rate (R-R interval), cANF also significantly increased the P-R interval (Table 1). The P-R interval is an indicator of conduction across the atria and through the atrioventricular node (35). The increased P-R interval in the presence of cANF indicates that conduction has slowed within the atrioventricular conduction system. Because NPR-C has approximately the same affinity for all natriuretic peptides, we suggest that ANP and BNP could have effects on heart rate and ECG intervals similar to those identified for cANF and CNP in the present study.
After the effects of CNP on heart rate were defined, the ionic mechanism(s) for these CNP-induced changes in heart rate was examined in voltage-clamp experiments using single SA node myocytes from the primary pacemaker region of the adult mouse heart. The SA node expresses at least two cAMP-sensitive currents, ICa(L) and If, which can contribute to generation of diastolic depolarization or the pacemaker phase of the action potential (1, 31). ICa(L) current density increases dramatically when the channel is phosphorylated by PKA (20, 22). The PKA-mediated phosphorylation of ICa(L) increases or decreases when cAMP levels in the cell increase or decrease, respectively (22). If current density is also altered by changing levels of intracellular cAMP (1); however, in contrast to ICa(L), this effect appears not to be mediated by PKA. Instead, cAMP binds directly to the intracellular COOH-terminal portion of the HCN channel subunits and alters its biophysical properties (14, 23). As described previously, NPR-C includes a Gi-activator sequence, which inhibits adenylyl cyclase and, thus, decreases cAMP levels in the presence of natriuretic peptides (3, 44, 52). We anticipated that ICa(L) and If would be inhibited by CNP because we previously showed that CNP can inhibit L-type Ca2+ current in isolated bullfrog atrial cells (45).
Our results demonstrate that CNP potently inhibits ICa(L) in single SA node myocytes (Fig. 2). The involvement of NPR-C was confirmed by using cANF, which is a selective NPR-C agonist (Fig. 3). It is important that the Gi-activator peptide (Fig. 4), which consists of the specific 17-amino acid segment of NPR-C known to inhibit adenylyl cyclase activity (44, 52), also substantially decreased ICa(L). This effect of CNP is selective, inasmuch as If was not significantly changed by CNP (Fig. 5). Although CNP had no effect on If, ACh was able to significantly inhibit this current (Fig. 6), a result that provides a useful positive control for the responsiveness of If to cAMP levels in the cell (1). Altering cAMP level in SA node myocytes may result in a significant shift in the activation curve of If (1). It is theoretically possible that CNP could alter the activation kinetics of this current without significantly altering the magnitude of If. Until the effects of CNP on If activation are measured experimentally, this possibility cannot be completely ruled out. However, in the present study, we show that the magnitude of If was increased in the presence of Iso and decreased in the presence of ACh, whereas CNP had no effect on the magnitude of If. On the basis of these electrophysiological findings, we conclude that the major effect of CNP in the mouse SA node is a selective inhibition of ICa(L) due to binding to NPR-C with no significant change in another cAMP-sensitive current, If.
Selectivity of CNP effects. Our initial working hypothesis was that ICa(L) and If would be inhibited by CNP, because the biophysical properties of both are modulated by cAMP levels in these pacemaker myocytes. However, this was not the case in the mouse SA node. The ability of CNP to selectively inhibit ICa(L) without affecting If suggests that the HCN channels that are responsible for If are functionally isolated from the pool of intracellular cAMP that is modulated by the CNP-NPR-C signaling pathway and mediates the inhibition of ICa(L). Intracellular compartmentation of cAMP has been described previously in the mammalian heart (7, 46, 47). For example, it is known that although 1-adrenergic receptor activation in ventricular myocytes leads to cAMP-mediated phosphorylation of Ca2+ channels, phospholamban, troponin I, and ryanodine receptors, 2-adrenergic receptor activation results in the exclusive phosphorylation of L-type Ca2+ channels (25). Thus activation of 1-adrenergic receptors leads to an increase in cAMP levels throughout the cell, whereas 2-adrenergic signaling results in an increase in cAMP levels that is spatially restricted and is sometimes described as occurring in microdomains. One key factor leading to the compartmentation of 2-adrenergic receptor-mediated cAMP signaling may be the functional coupling of the receptor to Gs and Gi proteins (10, 24). In this way, activation of Gi prevents an increase in cAMP levels throughout the entire myoplasm. Conversely, 1-adrenergic receptors are coupled only to Gs proteins and, therefore, can result in a more global rise in intracellular cAMP levels (7). Other factors that may be involved in this compartmentation phenomenon are spatially localized phosphatase (24) and/or phosphodiesterase activity (21). It is not known whether NPR-C is colocalized with L-type Ca2+ channels or whether compartmentation is responsible for the ability of CNP to inhibit ICa(L) but not If.
Significance of CNP effects. CNP is present and sarcolemmal receptors for it are expressed in the heart of all mammals that have been examined, including mouse, rat, pig, and human (4, 6, 9, 39, 40, 49). NPR-C appears to be the receptor most prominently expressed in cardiac myocytes, whereas the guanylyl cyclase-linked NPR-B may be restricted to the nonmyocyte population of cells in the heart (15). Thus it is anticipated that the effects of CNP on the mouse SA node described in the present study will apply to other mammals.
The levels of all natriuretic peptides in the heart, including CNP, increase substantially during heart failure (50). Plasma BNP levels are often used as an indicator of systolic function and the onset of heart failure (12, 36, 43). In situations of increased contractile force and/or stretch in the heart (such as those that occur in normal physiology or during pathophysiological conditions such as heart failure), the release of natriuretic peptides, including CNP, is augmented. Under these circumstances, the NPR-C-mediated inhibition of ICa(L) may produce a negative chronotropic effect in the SA node.
In summary, our results identify a novel negative chronotropic effect of CNP in the mouse heart. In SA node myocytes this response is mediated by NPR-C and is due to a strong inhibition of ICa(L). In contrast, If is unaltered by CNP.
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ACKNOWLEDGMENTS |
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This work was supported by operating grants from the Canadian Institute of Health Research (69-6100) and the Heart and Stroke Foundation of Canada to W. R. Giles, as well as Canadian Institute of Health Research Grant MOP-13661 to M. B. Anand-Srivastava. A. E. Lomax is the recipient of postdoctoral fellowships from the Heart and Stroke Foundation of Canada and the Alberta Heritage Foundation for Medical Research. R. A. Rose is the recipient of research studentship awards from the Heart and Stroke Foundation of Canada and the Alberta Heritage Foundation for Medical Research.
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FOOTNOTES |
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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.
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2-adrenergic receptor signaling to L-type Ca2+ channels. Biophys J 79: 2547–2556, 2000.
-adrenergic agonists. Proc Natl Acad Sci USA 93: 295–299, 1996.
2-adrenergic signaling. J Biol Chem 274: 22048–22052, 1999.
2-Adrenergic cAMP signaling is uncoupled from phosphorylation of cytoplasmic proteins in canine heart. Circulation 99: 2458–2465, 1999.
2-adrenergic receptors based on characteristics in primary structure. J Biol Chem 267: 8342–8346, 1992.
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) on Iso-stimulated ICa(L) (
). Values are means ± SE of 7 cells. *Significant inhibition of ICa(L) by CNP.
3 min. As shown in raw current traces (inset), peak ICa(L) increased to a steady-state level of approximately –12 pA/pF after stimulation with Iso. After the Gi-activator peptide entered this myocyte, peak ICa(L) decreased to about –3 pA/pF. This inhibitory effect is apparent in the time-course experiment. As indicated by the horizontal bar, Iso was present throughout the experiment. The Gi-activator peptide, which began to diffuse into the myocyte at time 0, started to elicit an inhibitory effect at 
) on Iso-stimulated If (