MODULATION OF ADENOSINE TRIPHOSPHATE-SENSITIVE POTASSIUM CHANNEL AND VOLTAGE-DEPENDENT CALCIUM-CHANNEL BY ACTIVIN-A IN HIT-T15 CELLS

The ATP-sensitive potassium channel(K-ATP channel) determines the membrane potential of pancreatic beta-cells and plays a critical role in the regulation of insulin secretion. The present study was conducted to investigate the effect of activin A, a member of the transforming growth factor-beta supergene family, on the K-ATP channel in HIT-T15 clonal hamster insulinoma cells. In an excised inside-out patch, ATP-sensitive currents with a single channel conductance of approximately 20 picosiemens were observed. In an outside-out-patch, currents with identical unitary conductance were also observed. In either case, the currents were augmented by diazoxide and blocked by glibenclamide, verifying that they were K-ATP channel currents. When K-ATP channel currents were monitored in an outside-out patch, activin A added to the bath solution inhibited K-ATP channel currents. Upon removal of activin A, the K-ATP channel currents were restored, suggesting that the inhibition was not due simply to spontaneous disappearance of channel activity (run-down). The K-ATP channel activity was markedly reduced after the addition of activin A and was reversed by diazoxide. Besides the inhibition of K-ATP channel, activin A increased, in a perforated patch, the amplitude of the inward Ba2+ current in response to a depolarizing pulse from -40 to +10 mV. Under the current clamp condition, activin A induced gradual depolarization, followed by a burst of action potentials. Activin-mediated action potentials were accompanied by an elevation of the cytoplasmic free calcium concentration. These results indicate that activin A causes depolarization of the plasma membrane by inhibiting the activity of the K-ATP channel. In addition, activin A directly modulates the voltage-dependent calcium channel and augments calcium entry.