N(ω)-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) μm 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 μm 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 μm 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(ω)-nitro-L-arginine constricted the vessels to 34.4 ± 3.5 μm 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.