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 memb rane potential of pancreatic beta-cells and plays a critical role in t he regulation of insulin secretion. The present study was conducted to investigate the effect of activin A, a member of the transforming gro wth factor-beta supergene family, on the K-ATP channel in HIT-T15 clon al hamster insulinoma cells. In an excised inside-out patch, ATP-sensi tive currents with a single channel conductance of approximately 20 pi cosiemens were observed. In an outside-out-patch, currents with identi cal unitary conductance were also observed. In either case, the curren ts were augmented by diazoxide and blocked by glibenclamide, verifying that they were K-ATP channel currents. When K-ATP channel currents we re monitored in an outside-out patch, activin A added to the bath solu tion inhibited K-ATP channel currents. Upon removal of activin A, the K-ATP channel currents were restored, suggesting that the inhibition w as not due simply to spontaneous disappearance of channel activity (ru n-down). The K-ATP channel activity was markedly reduced after the add ition of activin A and was reversed by diazoxide. Besides the inhibiti on of K-ATP channel, activin A increased, in a perforated patch, the a mplitude of the inward Ba2+ current in response to a depolarizing puls e from -40 to +10 mV. Under the current clamp condition, activin A ind uced gradual depolarization, followed by a burst of action potentials. Activin-mediated action potentials were accompanied by an elevation o f the cytoplasmic free calcium concentration. These results indicate t hat activin A causes depolarization of the plasma membrane by inhibiti ng the activity of the K-ATP channel. In addition, activin A directly modulates the voltage-dependent calcium channel and augments calcium e ntry.