Characterization of a newly found stretch-activated K(Ca,ATP) channel in cultured chick ventricular myocytes

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.