Vascular smooth muscle cell membrane depolarization after NOS inhibition hypertension

Abstract

Nitric oxide (NO) synthase (NOS) inhibition with N ω-nitro-l-arginine (l-NNA) produces l-NNA hypertensive rats (LHR), which exhibit increased sensitivity to voltage-dependent Ca2+ channel-mediated vasoconstriction. We hypothesized that enhanced contractile responsiveness after NOS inhibition is mediated by depolarization of membrane potential ( E m) through attenuated K+ channel conductance. E m measurements demonstrated that LHR vascular smooth muscle cells (VSMCs) are depolarized in open, nonpressurized (−44.5 ± 1.0 mV in control vs. −36.8 ± 0.8 mV in LHR) and pressurized mesenteric artery segments (−41.8 ± 1.0 mV in control vs. −32.6 ± 1.4 mV in LHR). Endothelium removal or exogenous l-NNA depolarized control VSMCs but not LHR VSMCs. Superfused l-arginine hyperpolarized VSMCs from both the control and LHR groups and reversedl-NNA-induced depolarization (−44.5 ± 1.0 vs. −45.8 ± 2.1 mV). A Ca2+-activated K+channel agonist, NS-1619 (10 μM), hyperpolarized both groups of arteries to a similar extent (from −50.8 ± 1.0 to −62.5 ± 1.2 mV in control and from −43.7 ± 1.1 to −55.6 ± 1.2 mV in LHR), although E m was still different in the presence of NS-1619. In addition, superfused iberiotoxin (50 nM) depolarized both groups similarly. Increasing the extracellular K+ concentration from 1.2 to 45 mM depolarized E m, as predicted by the Goldman-Hodgkin-Katz equation. These data support the hypothesis that loss of NO activation of K+ channels contributes to VSMC depolarization inl-NNA-induced hypertension without a change in the number of functional large conductance Ca2+-activated K+ channels.