Vascular smooth muscle cell membrane depolarization after NOS inhibition hypertension

Nitric oxide (NO) synthase (NOS) inhibition with N-omega-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 reversed L-NNA-induced depolarization (-44.5 +/- 1.0 vs. -45.8 +/- 2.1 mV). A Ca2+-activated K+ channel agonist, NS-1619 (10 muM), 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 Em 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 in L-NNA-induced hypertension without a change in the number of functional large conductance Ca2+-activated K+ channels.