Calcium channel activity increased by plasma from ischemic hind limbs of rats: role of an endogenous no synthase inhibitor. Jin, Jong-Shiaw, Dixon W. Wilde, R. Clinton Webb, and Louis G. D'alecy. Departments of Physiology, Anesthesiology, and Surgery, University of Michigan Medical School, Ann Arbor, MI 48109-0622 (U.S.A.)
APStracts 2:0445H, 1995.
We tested the hypothesis that an endogenous nitric oxide synthase (NOS) inhibitor released from ischemic hind limbs increases the activity of calcium channels in vascular smooth muscle, thus contributing to the increased contractile response to calcium agonists. Hindlimb ischemia was generated in rats by infra-renal aortic cross-clamping for 5 hours, after which plasma was obtained from femoral vein blood. Incubating naive aortic rings (endothelium intact) for 2 hours in plasma collected from ischemic rats significantly reduced relaxation to acetylcholine in precontracted rings and increased contraction to the calcium channel agonist, Bay K 8644. However, in isolated smooth muscle cells (without endothelium) loaded with fura-2, no difference was noted in Bay K 8644 stimulated [Ca2+]i. The contractile responses to sodium fluoride, serotonin and calcium ionophore A23187 were not different in either ischemic- or control-plasma incubated rings. The augmentation of the contractile response to Bay K 8644 was significantly inhibited by nitroglycerin (10-8 M) and by exposure to calcium-free solution. Nw-nitro-L -arginine (without plasma incubation) pretreated rings also demonstrated hyperresponsiveness to Bay K 8644. The increase in responsiveness to Bay K 8644 exhibited a negative correlation with the maximal relaxation to acetylcholine (R = -0.99) suggesting that the apparent increase in activity of calcium channels is mediated through inhibition of nitric oxide by an endogenous NOS inhibitor on endothelium. 

Received 19 June 1995; accepted in final form 20 September 1995.
APS Manuscript Number H550-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95