Smooth muscle membrane potential (E_m) is the summed effect of hyperpolarizing (i.e., K⁺) and depolarizing conductances. Previously we reported superior mesenteric arteries from rats made hypertensive with N-nitro-L-arginine (LHR) contract more to K⁺ and are depolarized compared to control. We hypothesized a decrease in K⁺ channel function, due to decreased K⁺ channel protein expression, underlies membrane depolarization in LHR vascular smooth muscle. Further, we predicted K⁺ channel blockers would move control E_m (-46 ± 1 mV) towards that in LHR (-37 ± 2 mV) and normalize the sensitivity of K⁺-induced contraction. The relationship between E_m and extracellular K⁺ was less steep in LHR compared to control (23 ± 2 vs. 28 ± 1 mV/log[K⁺]) and contractile sensitivity to K⁺ was increased in LHR (EC₅₀ = 37 ± 1 vs. 23 ± 1 mM). Endothelium removal depolarized control E_m (-41 ± 2 mV) and increased sensitivity to K⁺ (EC₅₀ = 26 ± 1 mM), but did not affect E_m (-36 ± 1 mV) or contraction (EC₅₀ = 21 ± 1 mM) in LHR. Iberiotoxin (10 nM), an inhibitor of Ca²⁺-dependent K⁺ (BK_Ca) channels, depolarized control and LHR E_m to 35 ± 1 and -30 ± 2 mV, respectively; however, effects on K⁺-sensitivity were more profound in LHR (EC₅₀ = 25 ± 2 vs. 15 ± 3 mM). The voltage-dependent K⁺ (Kv) channel blocker 4-aminopyridine (3 mM) depolarized control E_m to the level of LHR (-28 ± 1 vs. 28 ± 1 mV); however, effects on K⁺-sensitivity were greater in LHR (EC₅₀ = 17 ± 4 vs. 4 ± 4 mM). Western blot analysis revealed reduced BK_Ca and KV1.5 channel protein expression in LHR. The findings suggest diminished expression of BK_Ca and KV1.5 channels in arterial smooth muscle contributes to depolarization and enhanced contractile sensitivity to K⁺. These conclusions are supported by direct electrophysiological assessment of BK_Ca and K_v channel function in control and LHR smooth muscle cells (see companion paper, Bratz et al.).