Modulation of myogenic responsiveness by CO2 in rat diaphragmatic arterioles: role of the endothelium

Abstract

The effect of hypercapnia on the myogenic response was determined in arterioles (80- to 100-microm internal diameter) isolated from the diaphragms of rats killed by decapitation. All arterioles were exposed to step changes in intraluminal pressure over a range of 10-200 mmHg and had no flow through their lumen. In five separate groups of vessels (n = 7 per group), PCO2 of the superfusing buffer was adjusted to 40, 60, 80, 90, or 100 mmHg. In three further groups of vessels (n = 7 per group), the endothelium was removed by low-pressure air perfusion (2 ml at 20 mmHg) and PCO2 of the superfusing buffer was adjusted to 40, 80, or 100 mmHg. In endothelium-intact vessels, increasing PCO2 to 80 mmHg enhanced the myogenic response, as reflected by a negative slope of the pressure-diameter relationship (slope = -0.164 +/- 0.03 vs. 0.004 +/- 0.02 for vessels at PCO2 = 40 mmHg, P < 0.05). With a PCO2 of 100 mmHg, dilation accompanied increasing intraluminal pressure and the slope of the pressure-diameter curve was positive (0.154 +/- 0.03, P < 0.05 for difference from vessels at PCO2 = 40 mmHg). In deendothelialized vessels, the curve was shifted upward in a parallel manner during exposure to increased PCO2 levels. Moderate hypercapnia (PCO2 < 80 mmHg) elicits endothelium-dependent enhancement of myogenic tone. Severe hypercapnia (PCO2 > 80 mmHg) inhibits myogenic tone through a direct effect on vascular smooth muscle and through endothelium-dependent inhibitory mechanisms.

The myogenic response: established facts and attractive hypotheses

“…An important factor regulating local blood flow is the PO # . Thus, a moderate elevation of PCO # resulted in an increase, and a severe elevation of PCO # produced a decrease, of the strength of the myogenic response in rat diaphragmatic arterioles, although this was not related to the accompanying changes in pH [95]. In contrast, in cat cerebral arteries a decrease in PO # reduced the membrane depolarization accompanying the myogenic response [93], and in afferent arterioles in the intact vascular bed of an in-vitro kidney preparation, hypoxia inhibited the myogenic response almost completely [94].…”

The myogenic response: established facts and attractive hypotheses

“…Another closely related factor is the PCO # , which induced more complex effects. Thus, a moderate elevation of PCO # resulted in an increase, and a severe elevation of PCO # produced a decrease, of the strength of the myogenic response in rat diaphragmatic arterioles, although this was not related to the accompanying changes in pH [95]. Thus, only a few studies report on the interaction of local factors with the myogenic response.…”

20-Hydroxyeicosatetraenoic acid is a vasoconstrictor in the newborn piglet pulmonary microcirculation

“…To determine whether the vascular actions of 20-HETE are endothelium dependent, we disrupted the endothelium of some PRA by intraluminal perfusion of 2-10 ml of air, followed by a 10-min perfusion with Krebs buffer, as previously described (1,8,18). Only air-denuded vessels that constricted with KCl but either failed to dilate or constricted in response to ACh or A-23187 were studied further.…”