The Role of Intracellular Mg<SUP>2+</SUP> in Regulation of Ca<SUP>2+</SUP>-activated K<SUP>+</SUP> Channel in Pulmonary Arterial Smooth Muscle Cells of the Rabbit

The Role of Intracellular Mg<SUP>2+</SUP> in Regulation of Ca<SUP>2+</SUP>-activated K<SUP>+</SUP> Channel in Pulmonary Arterial Smooth Muscle Cells of the Rabbit

<P> Although the Ca<SUP>2+</SUP>-activated K<SUP>+</SUP> (I<SUB>K,Ca</SUB>) channel is known to play an important role in the maintenance of resting membrane potential, the regulation of the channel in physiological condition is not completely understood in vascular myocytes. In this study, we investigated the role of cytoplasmic Mg<SUP>2+</SUP> on the regulation of I<SUB>K,Ca</SUB> channel in pulmonary arterial myocytes of the rabbit using the inside-out patch clamp technique. Mg<SUP>2+</SUP> increased open probability (Po), but decreased the magnitude of single channel current. Mg<SUP>2+</SUP>-induced block of unitary current showed strong voltage dependence but increase of Po by Mg<SUP>2+</SUP> was not dependent on the membrane potential. The apparent effect of Mg<SUP>2+</SUP> might, thus, depend on the proportion between opposite effects on the Po and on the conductance of I<SUB>K,Ca</SUB> channel. In low concentration of cytoplasmic Ca<SUP>2+</SUP>, Mg<SUP>2+</SUP> increased I<SUB>K,Ca</SUB> by mainly enhancement of Po. However, at very high concentration of cytoplasmic Ca<SUP>2+</SUP>, such as pCa 5.5, Mg<SUP>2+</SUP> decreased I<SUB>K,Ca</SUB> through the inhibition of unitary current. Moreover, Mg<SUP>2+</SUP> could activate the channel even in the absence of Ca<SUP>2+</SUP>. Mg<SUP>2+</SUP> might, therefore, partly contribute to the opening of I<SUB>K,Ca</SUB> channel in resting membrane potential. This phenomenon might explain why I<SUB>K,Ca</SUB> contributes to the resting membrane potential where membrane potential and concentration of free Ca<SUP>2+</SUP> are very low.