Signal transduction in smooth muscle -: Selected contribution:: NO released to flow reduces myogenic tone of skeletal muscle arterioles by decreasing smooth muscle Ca2+ sensitivity

To clarify the contribution of intracellular Ca2+ concentration ([Ca2+](i)) dependent and -independent signaling mechanisms in arteriolar smooth muscle (aSM) to modulation of arteriolar myogenic tone by nitric oxide (NO), released in response to increases in intraluminal flow from the endothelium, changes in aSM [Ca2+](i) and diameter of isolated rat gracilis muscle arterioles (pretreated with indomethacin) were studied by fluorescent videomicroscopy. At an intraluminal pressure of 80 mmHg, [Ca2+](i) significantly increased and myogenic tone developed in response to elevations of extracellular Ca2+ concentration. The Ca2+ channel inhibitor nimodipine substantially decreased [Ca2+](i) and completely inhibited myogenic tone. Dilations to intraluminal flow (that were inhibited by N-omega-nitro-L-arginine methyl ester) or dilations to the NO donor S-nitroso-N-acetyl-DL-penicillamine (that were inhibited by the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one) were not accompanied by substantial decreases in aSM [Ca2+](i). 8-Bromoguanosine cGMP and the cGMP-specific phosphodiesterase inhibitor zaprinast significantly dilated arterioles yet elicited only minimal decreases in [Ca2+](i). Thus flow-induced endothelial release of NO elicits relaxation of arteriolar smooth muscle by a cGMP-dependent decrease of the Ca2+ sensitivity of the contractile apparatus without substantial changes in the pressure-induced level of [Ca2+](i).