Changes in K(+) conductances and their contribution to membrane depolarization in the setting of an acidic pH environment have been studied in myocytes from aortic smooth muscle cells of spontaneously hypertensive rats (SHR) compared with those from Wistar-Kyoto (WKY) rats. The resting membrane potential (RMP) of aortic smooth muscle at extracellular pH (pH(o)) of 7.4 was significantly more depolarized in SHR than in WKY rats. Acidification to pH(o) 6.5 made this difference in RMP between SHR and WKY rats more significant by further depolarizing the SHR myocytes. Large-conductance Ca(2+)-activated K(+) (BK) currents, which were markedly suppressed by acidification, were larger in aortic myocytes of SHR than in those of WKY rats. In contrast, acid-sensitive, non-BK currents were smaller in SHR. Western blot analyses showed that expression of BK-alpha- and -beta(1) subunits in SHR aortas was upregulated and comparable with those in WKY rats, respectively. Additional electrophysiological and molecular studies showed that pH- and halothane-sensitive two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) (TASK) channel subtypes were functionally expressed in aortas, and TASK1 expression was significantly higher in WKY than in SHR. Although the background current through TASK channels at normal pH(o) (7.4) was small and may not contribute significantly to the regulation of RMP, TASK channel activation by halothane or alkalization (pH(o) 8.0) induced significant hyperpolarization in WKY but not in SHR. In conclusion, the larger depolarization and subsequent abnormal contractions after acidification in aortic myocytes in the setting of SHR hypertension are mainly attributable to the larger contribution of BK current to the total membrane conductance than in WKY aortas.