Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit

1. Using the perforated patch-clamp or whole-cell damp technique, we invest igated the contribution of Ca2+-activated K+ current (I-K(Ca)) and non-sele ctive cation currents (I-NSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (V-m) was -39.2 +/- 0.9 mV (n = 72). It d id not stay at a constant level, but hyperpolarized irregularly, showing sp ontaneous transient hyperpolarizations (STHPs). The mean frequency and ampl itude of the STHPs was 5.6 +/- 1.1. Hz and -7.7 +/- 0.7 mV (n = 12), respec tively. In the voltage-clamp mode, spontaneous transient outward currents ( STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA o r charybdotoxin suppressed both the STHPs and STOCs. The depletion of intra cellular Ca2+ stores by caffeine or ryanodine, and the removal of extracell ular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG(+) caused hyperpolarization o f V-m without affecting STHPs. Removal of extracellular Ca2+ induced a mark ed depolarization of V-m along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The pe rmeability ratio of K+:Cs+:Na+:Li+ was 1.7:1.3:1:0.9, indicating that it is a non-selective cation current (I-NSC). The reversal potential of this cur rent in control conditions was calculated to be -13.9 mV. The current was b locked by millimolar concentrations of extracellular Ca2+ and Mg26. From these results, it was concluded that (i) hyperpolarizing currents a re mainly contributed by Ca2+-activated K+ (K-Ca) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing curre nts are carried through NSC channels.