Effect of L-NAME, cPTIO and endothelium denudation on myogenic tone of arterioles

1Timothy V. Murphy, 2*Neela Kotecha & 1Michael A. Hill, 1Department of Physiology and Pharmacology, School of Medical Sciences, University of New South Wales, Sydney NSW 2052, Australia and 2Division of Biosciences, School of Medical Sciences, RMIT University, Plenty Road, Bundoora Vic. 3083, Australia.

Myogenic tone in arterioles is the level of constriction observed in response to transmural pressure. In many studies removal of the vascular endothelium does not alter the level of arteriolar myogenic tone, suggesting no role for endothelium-derived nitric oxide (NO) or other vasoactive agents (Davis et al., 1999). Some investigators have observed a constrictor effect of the nitric oxide synthase (NOS) inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) on arterioles possessing myogenic tone, which has been interpreted as evidence for a contribution of spontaneously-released NO. The aim of this study was to investigate this seemingly paradoxical constrictor effect of L-NAME on arterioles possessing myogenic tone.

First-order cremaster muscle arterioles (passive diameter 160 ± 4 µm, n = 38) from Sprague-Dawley rats were isolated, cannulated, superfused and pressurized in the absence of luminal flow. Arterioles were placed on the stage of an inverted microscope and internal diameter recorded using an electronic video caliper. Membrane potential recordings were made using glass microelectrodes. Arterioles maintained at an intra-luminal pressure of 50 or 120 mmHg constricted to 52.5 ± 2.5 % (n = 14) or 41.5 ± 2.1 % (n = 15) respectively of the corresponding passive diameters. L-NAME (100 µM) caused a constriction of 15.9 ± 2.8% at 50 mmHg (n = 5) or 14.8 ± 2.6% (n = 5) at 120 mmHg. The L-NAME-induced constriction was prevented by L-arginine (1 mM). L-NAME did not cause significant smooth muscle membrane depolarization in the pressurized arterioles. At 50 mmHg membrane potential was -38.8 ± 2.4 mV, in the presence of L-NAME -34.8 ± 2.7 mV (n = 5; P>0.05, paired t-test). At 120 mmHg the corresponding values were -28.3 ± 0.9 mV and -25.3 ± 1.8 mV (n = 4, P>0.05, paired t-test). Removal of the vascular endothelium by passage of an air bolus through the lumen did not significantly alter the level of myogenic tone. L-NAME caused constriction in these vessels however, to a similar extent as that observed in arterioles with an intact endothelium (50 mmHg, 18.3 ± 2.3%; 120 mmHg, 18.6 ± 1.5%, n = 4 for both). The role of K+-channels was investigated using the K+-channel antagonist TEA (1 mM). In endothelium-intact arterioles at 70 mmHg, TEA alone constricted the arterioles but did not inhibit L-NAME-induced constriction. L-NAME or the NO scavenger cPTIO (100 µM) inhibited dilator responses to acetylcholine, however cPTIO neither induced constriction of the arterioles nor altered the constriction caused by L-NAME.

In arterioles with myogenic tone, the NOS inhibitor L-NAME caused a constriction which was endothelium- and NO-independent, but involved an L-arginine binding site. The L-NAME-induced constriction was not mediated by inhibition of K+-channels or alterations in membrane potential. The conclusion of this study is to caution against the exclusive ascribing of the effects of L-NAME to NOS inhibition in vascular preparations with myogenic tone.

Davis, MJ et al. (1999). Physiol. Rev. 79, 387-483.