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

Abstract

1 Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca²⁺-activated K⁺ current (I_K(Ca)) and non-selective 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 did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5·6 ± 1·1 Hz and -7·7 ± 0·7 mV (n= 12), respectively. 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 or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca²⁺ stores by caffeine or ryanodine, and the removal of extracellular Ca²⁺ also abolished STHPs and STOCs. 4 Replacement of extracellular Na⁺ with NMDG⁺ caused hyperpolarization V_m of without affecting STHPs. Removal of extracellular Ca²⁺ induced a marked depolarization of V_m along with the disappearance of STHPs. 5 The ionic nature of the background inward current was identified. The permeability 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 current in control conditions was calculated to be -13·9 mV. The current was blocked by millimolar concentrations of extracellular Ca²⁺ and Mg²⁺. 6 From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca²⁺-activated K⁺ (K_Ca) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.