Modulation of myogenic responsiveness by carbon dioxide in rat
diaphramgatic arterioles: role of the endothelium.
Nagi, Mohamed M., and Michael E. Ward.
Divisions of Pulmonary and Critical Care Medicine, Royal Victoria
Hospital and the Meakins-Christie Laboratores, McGill University,
Montreal, Quebec, Canada
APStracts 3:0434H, 1996.
The effect of hypercapnia on the myogenic response was determined in
arterioles (80 to 100 [mu]m 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
to 200 mmHg and had no flow through their lumen. In five separate
groups of vessels (n=7 per group) the PCO2 of 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 the 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 PCO2 equal to 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 de-endothelialized
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.
Received 10 May 1996; accepted in final form 24 September 1996.
APS Manuscript Number H421-6.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996