Glibenclamide does not reverse attenuated vasoreactivity to acute or chronic hypoxia

Effect of K+ATP channel inhibition on total and regional vascular resistance in guinea pig pregnancy

Decreased vascular resistance and vasoconstrictor response during pregnancy enables an increase in cardiac output and regional blood flow to the uterine circulation. We sought to determine whether inhibition of vascular smooth muscle ATP-sensitive potassium (K+ATP) channel activity during pregnancy increased systemic and/or regional vascular resistance and resistance response to ANG II. A total of 32 catheterized, awake, pregnant or nonpregnant guinea pigs were treated with either the K+ATP channel inhibitor glibenclamide (3.5 mg/kg) or vehicle (DMSO) (n = 8/group). In nonpregnant and pregnant animals, glibenclamide raised blood pressure and systemic, uterine, and coronary vascular resistance, diminishing cardiac output and organ blood flow. Glibenclamide produced a greater rise in coronary vascular resistance in the pregnant than nonpregnant groups and increased renal and cerebral vascular resistance in the pregnant animals only. ANG II infusion raised blood pressure and systemic and renal vascular resistance and lowered cardiac output and renal blood flow in vehicle-treated animals. Glibenclamide augmented ANG II-induced systemic vasoconstriction in the nonpregnant and pregnant groups and the rise in uteroplacental vascular resistance in the pregnant animals. We concluded that K+ATP channel activity likely modulates systemic, uterine, and coronary vascular resistance and opposes ANG II-induced systemic vasoconstriction in nonpregnant and pregnant guinea pigs. Pregnancy augments K+ATP channel activity in the uterine, coronary, renal, and cerebral vascular beds and the uteroplacental circulation during ANG II infusion. Thus increased K+ATP channel activity appears to influence regional control of vascular resistance during guinea pig pregnancy but cannot account for the characteristic decrease in systemic vascular resistance and ANG II-induced systemic vasoconstrictor response.

Maintained vasodilatory response to cromakalim after inhibition of nitric oxide synthesis

Activation of vascular smooth-muscle adenosine triphosphate-sensitive potassium channels (KATP channels) causes membrane hyperpolarization, reduced entry of Ca2+ through L-type voltage-gated Ca2+ channels, and subsequent smooth-muscle relaxation. Conversely, opening of endothelial KATP channels elicits hyperpolarization but may induce Ca2+ influx and stimulation of endothelium-derived nitric oxide (EDNO) because these cells appear not to possess L-type Ca2+ channels. We therefore hypothesized that EDNO contributes to KATP channel-mediated vasodilation. To test this hypothesis, we examined vasodilatory responses to the KATP channel opener cromakalim in conscious rats, perfused rat tail artery segments, and isolated perfused rat lungs in the presence or absence of the EDNO synthesis inhibitor Nomega-nitro-L-arginine (L-NNA). Additionally, we compared the effect of cromakalim with the EDNO-dependent dilator bradykinin on NO production and intracellular Ca2+ in cultured rat pulmonary artery endothelial cells. Vasodilatory profiles to cromakalim were unaffected by L-NNA in conscious rats, tail arteries, and isolated lungs. Consistent with these results, cromakalim had no apparent effect on either NO synthesis or Ca2+ levels in cultured endothelial cells. These data suggest a lack of a role for EDNO in contributing to KATP-channel-mediated vasodilation in the rat.

Effects of chronic hypoxia on Ca2+ mobilization and Ca2+ sensitivity of myofilaments in uterine arteries

The effect of chronic hypoxia on free intracellular Ca2+ concentration ([Ca2+]i) and Ca2+ sensitivity of myofilaments during agonist stimulation was examined in uterine arteries obtained from normoxic and chronically hypoxic pregnant sheep maintained at high altitude (3,820 m) for approximately 110 days. Smooth muscle [Ca2+]i was measured simultaneously with muscle contraction in the same intact tissue. Whereas both KCl and 5-HT increased [Ca2+]i and tension simultaneously in the uterine artery, 5-HT produced significantly greater contractile tension (in g) than KCl at a given amount of [Ca2+]i as indicated by the ratio of fura 2 fluorescence intensity induced by excitation at 340 nm to that induced at 380 nm (29.8 +/- 6.9 vs. 16.9 +/- 4.0, P < 0.05). Chronic hypoxia did not change KCl-induced contractions, nor did it affect KCl-mediated increases in [Ca2+]i. In contrast, chronic hypoxia significantly inhibited 5-HT-induced contractions and decreased the 5-HT-stimulated increase in [Ca2+]i (pD2 7.46 +/- 0.18-->6.86 +/- 0.11, P < 0.05, where pD2 is -log half-maximal effective concentration) in uterine arteries. In addition, the slope (g tension/nM [Ca2+]i) of the 5-HT-mediated [Ca2+]i-tension relationship was significantly decreased in chronically hypoxic arteries (0.024 +/- 0.002-->0.013 +/- 0.001, P < 0.01). The results suggest that chronic hypoxia suppresses agonist-mediated Ca2+ homeostasis in uterine arteries by inhibiting Ca2+ mobilization and the agonist-enhanced Ca2+ sensitivity of myofilaments.