With the use of the patch-clamp technique, five kinds of stretch-activated (SA) ion channels were identified on the basis of their single-channel conductances and ion selectivities in cultured chick ventricular myocytes. Because a high-conductance K⁺-selective channel predominated among these channels, we concentrated on characterizing its properties mostly using excised inside-out patches. With 145 mM KCl solution in the pipette and the bath, the channel had a conductance of 199.8 ± 8.2 pS (n = 22). The ion selectivities among K⁺, Na⁺, Ca²⁺, and Cl as estimated from their permeability ratios were P_Na/P_K = 0.03, P_Ca/P_K = 0.025, and P_Cl/P_K = 0.026. The probability of the channel being open (P_o) increased with the Ca²⁺ concentration in the bath ([Ca²⁺]_b; dissociation constant K_d = 0.51 µM at +30 mV) and membrane potential (voltage at half-maximal P_o = 39.4 mV at 0.35 µM [Ca²⁺]_b). The channel was blocked by gadolinium, tetraethylammonium, and charybdotoxin from the extracellular surface and, consequently, was identified as a Ca²⁺-activated K⁺ (K_Ca) channel type. The channel was also reversibly activated by ATP applied to the intracellular surface (K_d = 0.74 mM at 0.10 µM [Ca²⁺]_b at +30 mV). From these data taken together, we concluded that the channel is a new type of K_Ca channel that could be designated as an "SA K_Ca,ATP channel." To our knowledge, this is the first report of K_Ca channel in heart cells.

patch clamp; stretch-activated channel; calcium-activated potassium channel; adenosine 5'-triphosphate-activated channel

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