Biophysical properties of Ca²⁺- and Mg-ATP-activated K⁺ channels in pulmonary arterial smooth muscle cells isolated from the rat

A novel class of Ca²⁺-activated K⁺ channel, also activated by Mg-ATP, exists in the main pulmonary artery of the rat. In view of the sensitivity of these "K_Ca,ATP" channels to such charged intermediates it is possible that they may be involved in regulating cellular responses to hypoxia. However, their electrophysiological profile is at present unknown. We have therefore characterised the sensitivity of K_Ca,ATP channels to voltage, intracellular Ca²⁺ ([Ca²⁺]_i) and Mg-ATP. They have a conductance of 245 pS in symmetrical K⁺ and are approximately 20 times more selective for K⁺ ions than Na⁺ ions, with a K⁺ permeability (P_K) of 4.6×10^-13cm s^-1. Ca²⁺ ions applied to the intracellular membrane surface of K_Ca,ATP channels causes a marked enhancement of their activity. This activation is probably the result of simultaneous binding of at least two Ca²⁺ ions, determined using Hill analysis, to the channel or some closely associated protein. This results in a shift of the voltage activation threshold to more hyperpolarized membrane potentials. The activation of K_Ca,ATP channels by Mg-ATP has an EC₅₀ of approximately 50 μM. Although the EC₅₀ is unaffected by [Ca²⁺]_i, channel activation by Mg-ATP is enhanced by increasing [Ca²⁺]_i. One possible interpretation of these data is that Mg-ATP increases the sensitivity of K_Ca,ATP channels to Ca²⁺. It is therefore possible that under hypoxic conditions, where lower levels of Mg-ATP may be encountered, the sensitivity of K_Ca,ATP channels to Ca²⁺ and therefore voltage is reduced. This would tend to induce a depolarising influence, which would favour the influx of Ca²⁺ through voltage-activated Ca²⁺ channels, ultimately leading to increased vascular tone.