INTRODUCTION

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CHRONIC HYPOXIA AND HYPOXEMIA are commonly associated with impaired oxygenation but may also result from prolonged residence at high altitude. The systemic circulation exhibits attenuated vasoreactivity after prolonged hypoxic exposure that persists on restoration of normoxia (4, 5, 9, 23). This diminished vasoconstrictor reactivity appears to be a local vascular effect, because it is observed after denervation of the sympathetic innervation system (21) as well as in blood vessels isolated from animals exposed to this stimulus (5, 9, 23). However, the vasoconstrictor response to agonists such as phenylephrine (PE) in vivo is composed of a direct agonist-induced response that results from ligand-receptor binding as well as a secondary myogenic contribution that is elicited by increased intraluminal pressure. Evidence exists that both components of resistance-vessel vasoconstriction are affected by exposure to chronic hypoxia. For example, we have recently demonstrated that the response to PE is reduced in mesenteric resistance arteries from chronically hypoxic (CH) rats compared with controls when these vessels are held at constant intraluminal pressure (9). Similarly, Toporsian and Ward (23) observed reduced myogenic vasoconstriction in isolated diaphragmatic arterioles over a range of pressures. Although the altered direct agonist-induced component of vasoconstriction that follows chronic hypoxia has been examined in several studies, the mechanism of altered myogenic reactivity in these vessels has not been elucidated.

Increased intraluminal pressure and vessel distention stretch vascular smooth muscle (VSM) cells, which results in membrane depolarization (10), Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCC; Ref. 25), and myogenic vasoconstriction (20). We have recently demonstrated (6) that VSM cells in superior mesenteric arteries (SMA) isolated from CH rats are hyperpolarized compared with normoxic controls. Hyperpolarization of VSM cells after prolonged hypoxic exposure could be hypothesized to oppose stretch-induced depolarization thus limiting Ca²⁺ influx and therefore could be responsible for the previously described reduced myogenic vasoconstriction under these conditions. Therefore we examined vasoconstriction as well as changes in VSM cell resting membrane potential (E_m) and vessel-wall Ca²⁺ concentration ([Ca²⁺]) in response to increasing intraluminal pressure in mesenteric resistance arteries isolated from both normoxic control rats and rats exposed to hypoxia for 48 h. Furthermore, because a previous report demonstrated that removal of the endothelium attenuated pressure-induced vasoconstriction of diaphragmatic resistance arteries isolated from both control and CH rats, experiments were also performed using endothelium-denuded vessels and during nitric oxide synthase (NOS) inhibition to evaluate a potential role for endothelium-derived factors as mediators of blunted myogenic responsiveness after chronic hypoxia.

	METHODS

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Animals. Male Sprague-Dawley rats (body wt 300-350 g; Harlan Industries) were provided with fresh bedding, food, and drinking water and a 12:12-h light-dark cycle was maintained. A total of 50 rats were used for this study. Each experiment employed a single vessel from an individual rat. CH rats were exposed to hypobaric hypoxia at a barometric pressure of 380 Torr for 48 h, whereas control rats were housed in identical cages at ambient pressure (barometric pressure, ~630 Torr) in our laboratory in Albuquerque, NM (elevation 1,670 m). We have previously documented that arterial PO₂ for rats under identical control conditions is 73 ± 1 compared with 44 ± 2 Torr under hypoxic conditions (9). The duration of hypoxic exposure used for this study was selected based on previous reports that demonstrated attenuated vascular reactivity following 48-h hypoxic exposure for the renal and mesenteric circulations in vivo (9, 16). In addition, mesenteric and diaphragmatic arteries isolated from rats exposed to hypoxia for 48 h demonstrate attenuated reactivity compared with normoxic controls (5, 9, 23). Before experimentation, rats were deeply anesthetized with pentobarbital sodium (32.5 mg ip) and killed by exsanguination after vessels were harvested according to a protocol approved by the Institutional Animal Care and Use Committee of the University of New Mexico School of Medicine.

Isolated vessel preparation. Mesenteric resistance arteries (interior diameter at 70 Torr, 100-200 µm) from control and CH rats were isolated and pressurized. The chest and abdomen of anesthetized rats were opened, and heparin (100 U in 0.1 ml) was injected into the heart to prevent clotting. The mesenteric arcade was excised and transferred to ice-cold dissecting solution [that contained 3 mM MOPS (pH 7.4), 145 mM NaCl, 5 mM KCl, 1 mM MgSO₄, 2.5 mM CaCl₂, 1 mM KH₂PO₄, 0.02 mM EDTA, 2 mM pyruvate, 5 mM glucose, and 1% bovine serum albumin]. The arcade was secured in a Silastic-coated petri dish that contained dissecting solution. Veins were removed, and resistance-artery branches were cleaned of adipose tissue and transferred to a beaker of dissecting solution. Third-order vessel segments were dissected from the cleaned branches, transferred to a vessel chamber (Living Systems), cannulated with glass micropipettes, and secured with ligatures. Vessels were slowly pressurized to 70 Torr with physiological saline solution [PSS, which contained (in mM) 129.8 NaCl, 5.4 KCl, 0.83 MgSO₄, 19 NaHCO₃, 1.8 CaCl₂, and 5.5 glucose] using a servo-controlled peristaltic pump (Living Systems) and superfused (5 ml/min) with warmed (37°C) PSS aerated with a normoxic gas mixture (21% O₂-6% CO₂-73% N₂). After a 30-min equilibration period, intraluminal pressure was slowly increased to 120 Torr, and vessels were stretched to remove visible bends. Pressure was then reduced to 70 Torr for an additional 30-min equilibration period.

Endothelium removal. For experiments involving endothelium-denuded vessels, endothelial cell integrity was demonstrated before removal by administration of ACh (1 µM) in the presence of PE (10 µM). Removal of the endothelium was accomplished by passage of a 1-ml air bubble through the vessel lumen. Endothelium-denuded arteries were then perfused and superfused with warmed, aerated PSS for 30 min to wash out endothelium-derived factors, and vessels were repressurized to 70 Torr. After a 30-min equilibration period, the vessels were constricted with PE (10 µM), and ACh (1 µM) was administered to assess the efficacy of endothelium-removal procedures. Control vessels were constricted by 51.4 ± 9.2% relative to the Ca²⁺-free vessel diameter in response to 10 µM PE, whereas this treatment constricted vessels from CH rats by 31.9 ± 4.4%. ACh-induced vasodilation was abolished by these procedures (see Fig. 4), which suggests that endothelial function was effectively impaired.

VSM cell E_m. VSM cell E_m was recorded from pressurized mesenteric resistance arteries using glass intracellular microelectrodes. VSM cells were impaled by microelectrodes (tip resistance, 100-150 M) filled with 3 M KCl, which were inserted into pressurized arteries through the adventitial surface. A Neuroprobe 1600 amplifier (A-M Systems) was used for recording membrane potential. Analog output from the amplifier was low-pass filtered at 1 kHz and routed to a Tektronix RM502A oscilloscope and a Gould chart recorder. Criteria for acceptance of membrane potential recordings were 1) an abrupt negative deflection of potential as the microelectrode was advanced into a cell, 2) stable membrane potential for at least 1 min, and 3) an abrupt change in potential to ~0 mV after the electrode was retracted from the cell. Membrane potentials were recorded at intraluminal pressures of 40, 70, and 120 Torr. Movement of the vessel during pressure changes precluded maintained VSM cell impalement and continuous recordings (24); therefore, potential was measured from different VSM cells at each pressure. Occasionally recordings from several VSM cells were made at a particular pressure for each animal. The potentials of all VSM cells recorded for a particular rat at that pressure were averaged and considered as a single replicate for statistical purposes. VSM cell E_m was recorded from both endothelium-intact and endothelium-denuded resistance arteries (n = 5 for both control and CH arteries in each group).

Pressure-induced vasoconstriction and vessel-wall [Ca²+]. Pressurized resistance arteries were loaded with the cell-permeant ratiometric Ca²⁺-sensitive fluorescent dye fura 2-acetoxymethyl ester (fura 2-AM; Molecular Probes). Fura 2-AM was dissolved in anhydrous DMSO at a concentration of 1 mM. Immediately before loading, fura 2-AM was mixed with a 20% solution of Pluronic acid in DMSO and this mixture was diluted with PSS to yield a final concentration of 2 µM fura 2-AM and 0.05% Pluronic acid. Vessels were incubated in this solution for 45 min at room temperature in the dark. Administration of fura 2-AM to the abluminal surfaces of pressurized resistance arteries has been shown to preferentially load VSM cells (17). The diluted fura 2-AM solution was aerated with a normoxic gas mixture during the loading period. Vessels were equilibrated for 20 min with warmed, aerated PSS after the loading period to wash out excess dye and to allow for esterification of AM groups.

Calculations and statistics. All data are expressed as means ± SE. Values of n refer to the number of animals in each group. Myogenic tone was calculated as the percent difference in inner diameter at a particular pressure when vessels were superfused with Ca²⁺-free vs. Ca²⁺-replete PSS. Paired t-tests were used to compare vessel diameters in Ca²⁺-containing vs. Ca²⁺-free PSS at each pressure examined. Unpaired t-tests were used to compare myogenic tone, vessel-wall [Ca²⁺], and E_m between control and CH groups at each pressure step. A probability of 0.05 was accepted as statistically significant for all comparisons.

	RESULTS

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Endothelium-intact vessels. VSM cells in endothelium-intact mesenteric resistance arteries isolated from CH rats were hyperpolarized compared with controls at each pressure examined (Fig. 1B). The VSM cell E_m for both groups became less negative (depolarized) as intraluminal pressure was increased (Fig. 1B). In addition, at intraluminal pressures >20 Torr, the inner diameter of endothelium-intact resistance arteries isolated from normoxic control rats was significantly smaller when superfused with Ca²⁺-containing PSS compared with Ca²⁺-free PSS (passive diameter; Fig. 2A). In contrast, the diameter of vessels isolated from chronic hypoxia and superfused with Ca²⁺-containing PSS differed from the passive diameter only at 120 Torr, which was the highest pressure applied (Fig. 2B). Furthermore, the degree of myogenic tone present in arteries isolated from CH rats was less than that of vessels from control rats at intraluminal pressures between 60 and 100 Torr (Fig. 3). Vessel-wall [Ca²⁺] increased with increasing intraluminal pressure for resistance arteries isolated from both groups of rats (Fig. 4). However, the vessel-wall [Ca²⁺] was diminished in resistance arteries from CH rats compared with normoxic control rats at each pressure examined (Fig. 4).

View larger version (18K): [in this window] [in a new window]   Fig. 1.   A: original recording of vascular smooth muscle (VSM) cell resting membrane potential (Em) in a mesenteric resistance artery pressurized to 70 Torr. B: VSM cell membrane potential as a function of intraluminal pressure for endothelium-intact mesenteric resistance arteries isolated from control and chronically hypoxic (CH) rats (n = 5 for both groups). *P  0.05 vs. control.

View larger version (23K): [in this window] [in a new window]   Fig. 2.   Inner diameter of endothelium-intact mesenteric resistance arteries isolated from control (A) and CH (B) rats as a function of intraluminal pressure (n = 5 for both groups). *P  0.05 vs. vessels superfused with Ca2+-free physiological saline solution (PSS).

View larger version (17K): [in this window] [in a new window]   Fig. 3.   Myogenic tone as a function of intraluminal pressure for mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups). *P  0.05 vs. control.

View larger version (18K): [in this window] [in a new window]   Fig. 4.   Vessel-wall Ca2+ concentration ([Ca2+]) as a function of intraluminal pressure for mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups). *P  0.05 vs. control.

Endothelium-denuded vessels. The endothelium-dependent vasodilator ACh reversed PE-induced vasoconstriction of endothelium-intact mesenteric resistance arteries isolated from both control and CH rats (Fig. 5). After air was passed through the vessel lumen, ACh-induced vasodilation was abolished for resistance arteries from both groups (Fig. 5), which suggests that the endothelium was removed or functionally impaired by this procedure. Similar to the responses observed for endothelium-intact vessels, VSM cells for endothelium-denuded resistance arteries isolated from both groups of rats were depolarized by increasing intraluminal pressure (Fig. 6). However, in contrast to the relative hyperpolarization of VSM cells in endothelium-intact arteries from CH rats compared with control rats (see Fig. 1), E_m values of VSM cells in endothelium-denuded vessels isolated from control and CH rats were not different (Fig. 6). The inner diameter was smaller when endothelium-denuded resistance vessels from control rats were superfused with Ca²⁺-containing PSS compared with Ca²⁺-free PSS (Fig. 7A). Interestingly, the inner diameter of endothelium-denuded vessels from CH rats superfused with Ca²⁺-containing PSS was smaller than the passive diameter, which suggests that endothelium removal restored myogenic behavior in these resistance arteries (Fig. 7B). In addition, the degree of myogenic tone did not differ between endothelium-denuded vessels isolated from control and CH rats (Fig. 8). Furthermore, pressure-induced increases in vessel-wall [Ca²⁺] for endothelium-denuded resistance arteries were similar between CH and control groups (Fig. 9).

View larger version (16K): [in this window] [in a new window]   Fig. 5.   Reversal of phenylephrine (PE)-induced vasoconstriction by ACh for endothelium-intact and -denuded resistance arteries isolated from control (CON) and CH rats. *P  0.05 vs. endothelium-intact arteries.

View larger version (17K): [in this window] [in a new window]   Fig. 6.   VSM cell membrane potential as a function of intraluminal pressure for endothelium-denuded mesenteric resistance arteries isolated from control and CH rats; n = 5 for both groups. There were no significant differences.

View larger version (25K): [in this window] [in a new window]   Fig. 7.   Inner diameter of endothelium-denuded mesenteric resistance arteries isolated from control (A) and CH (B) rats as a function of intraluminal pressure (n = 5 for both groups). *P  0.05 vs. vessels superfused with Ca2+-free PSS.

View larger version (17K): [in this window] [in a new window]   Fig. 8.   Myogenic tone as a function of intraluminal pressure for endothelium-denuded mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups). There were no significant differences.

View larger version (16K): [in this window] [in a new window]   Fig. 9.   Vessel-wall [Ca2+] as a function of intraluminal pressure for endothelium-denuded mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups). There were no significant differences.

NOS-inhibited vessels. In the presence of the NOS inhibitor L-NNA (100 µM), the diameter of endothelium-intact vessels isolated from control rats was smaller when superfused with Ca²⁺-containing PSS than with Ca²⁺-free PSS (Fig. 10A). Unlike endothelium-denuded vessels, no differences were observed in the diameters of L-NNA-treated resistance arteries from CH rats when superfused with Ca²⁺-containing or Ca²⁺-free PSS (Fig. 10B). Furthermore, L-NNA-treated vessels from CH rats developed less myogenic tone than L-NNA-treated arteries from control rats (Fig. 11). In addition, vessel-wall [Ca²⁺] was lower in L-NNA-treated vessels from CH rats compared with NOS-inhibited arteries isolated from normoxic controls (Fig. 12).

View larger version (26K): [in this window] [in a new window]   Fig. 10.   Inner diameter of nitric oxide synthase (NOS)-inhibited endothelium-intact mesenteric resistance arteries isolated from control (A) and CH (B) rats as a function of intraluminal pressure (n = 5 for both groups). *P  0.05 vs. vessels superfused with Ca2+-free PSS.

View larger version (19K): [in this window] [in a new window]   Fig. 11.   Myogenic tone as a function of intraluminal pressure for NOS-inhibited mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups). *P  0.05 vs. control. L-NNA, N-nitro-L-arginine.

View larger version (20K): [in this window] [in a new window]   Fig. 12.   Vessel-wall [Ca2+] as a function of intraluminal pressure for NOS-inhibited mesenteric resistance arteries isolated from control and CH rats (n = 5 for both groups); *P  0.05 vs. control.

	DISCUSSION

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The major findings of this study are 1) VSM cells in mesenteric resistance arteries isolated from CH rats are persistently hyperpolarized compared with controls, 2) VSM cell hyperpolarization is associated with blunted myogenic reactivity and decreased vessel-wall [Ca²⁺], 3) removal of the endothelium from resistance arteries isolated from CH rats enhances myogenic reactivity and restores VSM cell E_m and vessel-wall [Ca²⁺] to control levels, and 4) NOS inhibition does not affect blunted myogenic responsiveness or decreased vessel-wall [Ca²⁺] after chronic hypoxia. In addition, altered VSM cell E_m, myogenic vasoconstriction, and vessel-wall [Ca²⁺] were observed in vessels from CH rats under normoxic conditions, which demonstrates that these deranged responses are independent of the acute effects of hypoxia. Our findings suggest that blunted myogenic reactivity after chronic hypoxia results from a persistent, NO-independent, endothelium-derived hyperpolarizing influence.

Both acute and chronic hypoxic exposure result in blunted agonist-dependent and -independent vasoconstrictor responsiveness. However, these stimuli influence the vasculature via distinct mechanisms (4). This is evident by the persistent nature of blunted vasoconstrictor responsiveness after chronic hypoxia. For example, several studies have demonstrated that vasoreactivity after prolonged hypoxic exposure is attenuated for several hours upon restoration of normoxic conditions in isolated vessel preparations (1, 2, 6, 12, 23, 27) as well as in conscious instrumented rats (4, 9, 16, 21). In contrast, although acute hypoxic exposure results in vasodilation and VSM cell hyperpolarization in certain vascular beds (15); on restoration of normoxia, these effects are rapidly reversed. Our findings demonstrate that myogenic vasoconstriction is blunted in vessels from CH rats (see Figs. 2 and 3) when studied under normoxic conditions. Furthermore, in agreement with a previous report of VSM cell E_m recordings from conduit vessels (6), our results demonstrate that VSM cells in resistance arteries obtained from CH animals are persistently hyperpolarized compared with control animals (see Fig. 1B). Hyperpolarized potentials were recorded under normoxic conditions several hours after the vessels were harvested, which suggests that this response is not due to hypoxia per se but instead results from vascular adaptation to long-term hypoxic exposure. These findings suggest that blunted vasoconstriction and VSM cell hyperpolarization after prolonged hypoxic exposure result from a vascular derangement that is distinct from the effects of acute hypoxia.

The vascular myogenic response is associated with altered VSM cell E_m resulting from mechanical stretch of the vessel wall. For example, depolarization of VSM cells has been observed in isolated vessel preparations in response to increasing intraluminal pressure (10). In addition, dispersed VSM cells also depolarize when stretched, which suggests that mechanosensitivity is an inherent property of these cells (3). Stretch-induced VSM cell depolarization opens VDCC and results in Ca²⁺ influx and increased intracellular [Ca²⁺] (20). Blockade of VDCC inhibits the myogenic response (10, 11, 13, 18), which suggests that pressure-induced vasoconstriction results from [Ca²⁺] influx via these channels. Our findings (Fig. 1B) demonstrate that VSM cell E_m in resistance vessels isolated from both control and CH rats depolarizes as intraluminal pressure is increased. Vessel-wall [Ca²⁺] is also elevated in response to increasing intraluminal pressure for both groups (see Fig. 4). However, compared with control rats, VSM cells in vessels from CH rats are hyperpolarized over the entire range of pressure from which recordings were made (see Fig. 1B). Predictably, VSM cell hyperpolarization in resistance arteries obtained from CH rats correlates with decreased vessel-wall [Ca²⁺] compared with controls (see Fig. 3). This decreased vessel-wall [Ca²⁺] in arteries from CH rats may result from decreased Ca²⁺ influx via VDCCs. Furthermore, vessels from CH rats exhibit less myogenic tone compared with arteries from control rats (see Fig. 3), which suggests that decreased vessel-wall [Ca²⁺] associated with VSM cell hyperpolarization results in blunted myogenic reactivity. These findings suggest that blunted myogenic vasoconstriction after prolonged hypoxic exposure is not a consequence of altered stretch-induced VSM cell depolarization or sensitivity of the contractile apparatus to Ca²⁺ but rather results from tonic endothelium-dependent VSM hyperpolarization and associated decreased vessel-wall [Ca²⁺].

The endothelium may influence the E_m of underlying VSM cells by the release of diffusible factors (8) as well as through direct electrical communication via myoendothelial gap junctions (7, 22, 26). Our findings (see Figs. 6-9) clearly demonstrate that an endothelium-dependent influence is responsible for VSM cell hyperpolarization as well as the associated blunted myogenic vasoconstriction and altered vessel-wall [Ca²⁺] that follow chronic hypoxia. Possible endothelium-derived hyperpolarizing factors include NO, carbon monoxide (CO), prostacyclin, and cytochrome P-450 metabolites of arachidonic acid such as epoxyeicosatrienoic acids (EETs). Although NO elicits VSM cell hyperpolarization (14), a previous study from our laboratory (6) demonstrated that VSM cells in SMAs from CH rats remained hyperpolarized compared with controls during NOS inhibition. Consistent with that observation in conduit vessels, the current study (see Figs. 10-12) demonstrates that myogenic vasoconstriction and Ca²⁺ influx of resistance arteries from CH rats remain blunted in the presence of a NOS inhibitor. These data suggest that an endothelium-derived factor other than NO is responsible for attenuated pressure-induced vasoconstrictor and Ca²⁺ responses following chronic hypoxia. Previous studies from our laboratory suggest that this response may be mediated by heme oxygenase (HO)-derived CO. For example, we reported that the onset of blunted agonist-induced vasoconstriction occurs after 48 but not 24 h of hypoxic exposure and is correlated with increased aortic HO-1 protein levels (16). The presence of a functional hypoxia-response element on the HO-1 promoter suggests that hypoxia per se, rather that hemodynamic influences such as altered shear stress, stimulates increased transcription of this gene (19). Consistently, attenuated vasoconstriction and increased HO-1 levels are both maintained for at least 5 h posthypoxia, and decreased HO-1 levels are associated with partial reversal of blunted reactivity 96 h after relief from hypoxia (16). In addition, we reported that HO inhibition enhances PE-induced vasoreactivity in mesenteric resistance arteries from CH rats (9). Furthermore, a recent study from our laboratory (5) demonstrates that HO inhibition normalizes VSM cell E_m between control and CH groups, which suggests that CO acts as an endothelium-derived hyperpolarizing factor. Considering that the findings of the current study demonstrate that normalization of VSM cell E_m between groups also results in restoration of myogenic reactivity for vessels isolated from CH rats, we conclude that increased CO production may contribute to attenuated pressure-induced vasoconstriction following prolonged hypoxic exposure. Confirmation of this speculated mechanism is the goal of future studies.

In contrast to our findings (Figs. 6 and 7), Toporsian and Ward (23) reported that myogenic responsiveness of diaphragmatic resistance arteries isolated from CH rats remained blunted after removal of the endothelium. These conflicting effects of endothelium removal may be due to differences in the vascular beds that were studied. Alternatively, variations in experimental procedures may also account for the apparent discrepancies. For example, in the earlier report, rats were exposed to hypoxia under normobaric conditions, whereas hypobaric hypoxia was employed for the current study. Although the reason for the differential effect of endothelium removal between the two studies is not clear, our results convincingly demonstrate that an endothelium-dependent factor is responsible for attenuated myogenic responsiveness of small mesenteric arteries after chronic hypoxia.

Maintenance of relatively constant mean arterial pressure is the result of the combined actions of intrinsic vascular properties as well as extrinsic influences such as sympathetic nervous system innervation and circulating vasoactive substances. Previous studies and our findings (Fig. 3) demonstrate that vasoconstriction in response to both intrinsic (myogenic; Refs. 1, 23) and extrinsic (₁-agonists; Refs. 2, 4, 6, 9, 12, 16, 27) stimuli is attenuated after prolonged hypoxic exposure. Although these observations suggest that blood pressure regulation may be compromised after chronic hypoxia, a previous study from our laboratory (9) demonstrates that blood pressure does not differ between control and CH rats. This somewhat surprising finding may result from in vivo mechanisms that compensate for decreased vasoconstrictor sensitivity. For example, blood pressure may be maintained in CH rats by elevated levels of sympathetic tone despite decreased vasoconstrictor sensitivity. Consistent with this possibility, we have observed increased heart rate in CH rats compared with control rats (9). Thus although our findings suggest that an important autoregulatory response is impaired after chronic hypoxia (see Fig. 3), adaptive mechanisms allow for maintenance of blood pressure.

In summary, we have demonstrated that VSM cells in mesenteric resistance arteries isolated from CH rats are persistently hyperpolarized compared with controls. In addition, decreased vessel-wall [Ca²⁺] and blunted myogenic responsiveness are associated with VSM cell hyperpolarization after chronic hypoxia. Removal of the endothelium restored myogenic reactivity to vessels isolated from CH rats, which suggests that an endothelium-derived influence is responsible for altered vascular properties after chronic hypoxia. Taken together, these findings demonstrate that blunted myogenic vasoconstriction after chronic hypoxia results from an endothelium-dependent hyperpolarizing influence.