Actions of growth-hormone-releasing hormone on rat pituitary cells: intracellular calcium and ionic currents

Actions of growth-hormone-releasing hormone (GHRH) on single rat anterior pituitary cells were studied using indo-1 fluorescence to monitor changes in intracellular calcium, [Ca2+]i, and perforated-patch recording to measure changes in membrane potential and ionic currents. GHRH elevated [Ca2+]i in non-voltage-clamped cells by a mechanism that was dependent upon extracellular Na+ and Ca2+ and was blocked by the dihydropyridine Ca(2+)-channel blocker, nitrendipine. Resting cells had a fluctuating membrane potential whose a mean value depolarized by 9 mV in response to GHRH. The membrane-permeant cAMP analogue, 8-(4-chlorophenylthio)cAMP, mimicked the action of GHRH on membrane potential. Under voltage clamping, GHRH activated a small inward current (1-5 pA). Two types of response could be distinguished. The type I response had an inward current that was largest at more negative potentials (-90 mV), and the type II response had inward current that was larger at more positive potentials (-40 to -70 mV). Both types of response were reversible and blocked by removal of extracellular Na+. These results suggest that the rise in [Ca2+]i produced by GHRH in non-voltage-clamped cells results from the activation via cAMP of a Na(+)-dependent conductance, which depolarizes the cell and increases the Ca2+ influx through voltage-gated Ca2+ channels.