Intracellular Mg(2+) hyperpolarizes rabbit coronary artery smooth muscle cells by differential modulation of...

We investigated the role of intracellular Mg(2+) ([Mg(2+)](i)) in the regulation of membrane potential ( V(m)) in rabbit coronary artery smooth muscle cells. V(m), membrane currents and intracellular Ca(2+) ([Ca(2+)](i)) were measured using standard patch-clamp and microfluorometry techniques. When [Ca(2+)](i) was increased by caffeine, V(m) depolarized at low [Mg(2+)](i) (0.1 mM), but hyperpolarized at high [Mg(2+)](i) (> or =1.2 mM). Effects of [Mg(2+)](i) on caffeine-induced currents were investigated. [Mg(2+)](i) selectively facilitated the activation of Ca(2+)-activated K(+) currents ( I(K,Ca)), while Ca(2+)-activated Cl(-) currents ( I(Cl,Ca)) were unaffected. Simultaneous recording of [Ca(2+)](i) and I(K,Ca) at different [Mg(2+)](i) showed that [Mg(2+)](i) increased the Ca(2+) sensitivity of I(K,Ca). [Ca(2+)](i) also inhibited voltage-dependent K(+) (K(V)) currents, although this effect was significant only at low [Mg(2+)](i). These results imply that the relative contributions of I(K,Ca), I(Cl,Ca) and K(V) currents to V(m) during an increase in [Ca(2+)](i) are affected by [Mg(2+)](i): at low [Mg(2+)](i), activation of I(Cl,Ca) and inhibition of K(V) currents depolarized V(m); at high [Mg(2+)](i) the activation of I(K,Ca) predominated, resulting in hyperpolarization of V(m). In conclusion, [Mg(2+)](i) hyperpolarizes V(m) by selective facilitation of I(K,Ca) and may thus possibly contributes to the relaxation of the coronary artery.