N-OMEGA-NITRO-L-ARGININE CONSTRICTS CEREBRAL ARTERIOLES WITHOUT INCREASING INTRACELLULAR CALCIUM LEVELS

The coupling between intracellular Ca2+ concentration ([Ca2+](i)) and smooth muscle cell (SMC) contractility is well known, but recent reports suggest that SMCs can contract without an increase in [Ca2+](i). Penetrating arterioles were isolated from rat cortex, cannulated, and loaded extraluminally with fura 2. We used ratio imaging of corresponding vessel wall areas to estimate and correlate [Ca2+](i) to the vessel diameter during various extraluminal treatments. Control diameter was 45.6 +/- 3.9 (SE) mu m with an estimated [Ca2+](i) of 181 +/- 18 nM. Extraluminal papaverine or pH of 6.8 dilated the vessels to 61.0 +/- 6.6 and 57.8 +/- 5.8 mu m and decreased [Ca2+](i) to 108 +/- 33 and 155 +/- 8 nM, respectively. Alkaline pH of 7.65 or the Ca2+ ionophore ionomycin constricted the arterioles to 34.8 +/- 3.9 and 30.0 +/- 5.8 mu m and increased [Ca2+](i) to 273 +/- 47 and 853 +/- 155 nM, respectively. These results show an inverse relationship between vessel diameter and [Ca2+](i). Blocking the production of endothelium-derived nitric oxide (EDNO) with N-omega-nitro-L-arginine constricted the vessels to 34.4 +/- 3.5 mu m without raising but lowering [Ca2+](i) to 157 +/- 44 nM. The dissociation of vessel tone and estimated [Ca2+](i) after EDNO blocking has not been reported before in cerebral arterioles and may indicate that EDNO regulates vascular tone in a Ca2+-independent manner.