Unitary current through the inward rectifier K+ channel cloned from rabbit heart--comparison with the native K+...

Recently, we have cloned a cDNA for a putative cardiac inward rectifier K+ channel (RBHIK1) from rabbit cardiac muscles. However, it's single channel characteristics have remained unknown. Therefore, we investigated the single channel characteristics of the RBHIK1 channel expressed in Xenopus oocytes by the cell attached patch clamp configuration, and compared them with those of the native Ik1 channel of the freshly-isolated ventricular myocytes under similar temperature conditions. In patch clamp experiments with 145 mmol/l K+ in the pipette at room temperature (20-22 degrees C), both the RBHIK1 currents and the native Ik1 showed a strong inward rectifying property. The single channel conductance of the RBHIK1 channel was 17.8 +/- 0.47 pS (n = 4), and that of the native Ik1 channel was 23.5 +/- 0.29 pS (n = 5). The activities of the cloned channel were sensitive to the putative K+ channel blockers (TEA, Cs+ and Ba2+). The open and closed time histograms at -140 mV could be fitted by a single exponential both in the RBHIK1 channel and the native Ik1 channel. Although the closed-time histogram of the native Ik1 channel was fitted by a sum of two exponential curves, that of the RBHIK1 channel was fitted by a single exponential curve. The sublevel corresponding to two-thirds of the unitary current was observed both in the RBHIK1 channel and the native Ik1, but it was more frequently detected in the RBHIK1 channel. Amplitude histogram constructed at -140mV in the RBHIK1 channel exhibited three peaks, which indicated closed, full-open, and 2/3 sublevel state, respectively. Unitary current was calculated to be 2.5 pA and sublevel of the unitary current was 1.68 pA. These characterization in the single channel activities of the RBHIK1 channel will help to study the molecular regulation of the Ik1 channel in cardiac cells.