Differential effects of hyperpolarization and nitric oxide on calcium handling in isolated arterioles

We studied the contribution of nitric oxide (NO) and the putative endothelium-derived hyperpolarizing factor (EDHF) to the dilation and to the calcium response in isolated skeletal muscle arterioles of the hamster (n- 8) upon stimulation with acetylcholine (ACh). Methods: Intracellular free calcium (Ca,) in arteriolar smooth muscle was measured using the Fura2-method. Outer vascular diameters (OD) were determined by videomicroscopy. Changes of OD and Ca( following sodium nitroprusside (SNP) and ACh were tested in preconstricted segments (norepinephrine 0.3mM). The effects of the NO-synthase inhibitor LNNA (30u,M), and of tetrabutylammonium (TBA, which blocks endotheliumdependent hyperpolarization) were compared. Results: ACh (0.01, (M.luM) dilations (12, 51, 64% of control OD) were endothelium dependent. With increasing ACh doses, LNNA was less effective in inhibiting the dilation (60, 32, 0 %). TBA (1 mM) abolished this LNNA insensitive dilation. LNNA did not affect the Ach-induced decreases of Ca, (up to 40%) whereas TBA blocked it completely. SNP (0.1, 1.0, 10u,M) did not affect Ca, in. spite of inducing dose-dependent dilations (8, 27, 40 %). Conclusion: Skeletal muscle arterioles of the hamster release both NO and EDHF. the latter being the predominant dilator at higher concentrations of ACh. EDHF relaxes vascular smooth muscle by a reduction of Ca, whereas NO does not affect calcium homeostasis but rather calcium sensitivity in microvascular smooth muscle.