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

Young Min Bae, Myoung Kyu Park *, Suk Ho Lee, Won-Kyung Ho and Yung E. Earm

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.

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.

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.

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.

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⁺.

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.

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