Ca2+ and Mg-ATP activated potassium channels from rat pulmonary artery

Abstract

The hyperpolarizatlon-activated current (I_f) was recorded from single myocytes dissociated from rabbit sinoatrial node. Although I_f is usually carried by both Na⁺ and K⁺, removal of the minor K⁺ component from physiological saline suppresses inward component. This inward Na⁺ current through I_f channel increases on raising the extracellular K⁺ concentration. The Na⁺ conductance relative to K⁺ conductance (P_Na/P_K), as measured from the reversal potential, increases and saturates near 5 mM K⁺. This effect is different from the current increase caused by raising the concentration of carrier ion K⁺, which saturates at 70 mM with a half-maximal value (K_1/2) of 10 mM. It is suggested that the I_f channel has multiple, interactive binding sites for cation permeation.