Comparison of ramp and square-wave exposure to thyrotrophin-releasing hormone or depolarizing K+ on cytosolic Ca2+ concentration and prolactin secretion in GH4C1 cells

Abstract

Abstract The standard method of studying hormone secretion in vitro is to make instantaneous changes in the concentration of stimulators in the medium. However, in vivo the extracellular concentration of such substances changes more gradually; secretion does not occur in square-wave bursts and agonists or antagonists transmitted through the bloodstream are diluted and diffused by plasma or tissue fluid to further decelerate the rate of change in concentration at the cell surface. We have therefore compared in GH4C1 cells the dynamics of changes in cytosolic Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in response to two very different secretagogues, thyrotrophin-releasing hormone (TRH) and depolarizing K+, using a square-wave or ramp exposure for 5 min. The dynamics of hormone secretion were analysed by column perifusion (2 × 106 cells/column). Ca2+ dynamics were monitored by dual excitation microfluorimetry from 20–30 optically isolated cells using the Ca2+ indicator, fura-2. With square-wave exposure, both TRH (0·1–100 nm) and K+ (10–50 mm) induced dose-dependent increases in [Ca2+]i and PRL secretion. Concentrations of TRH >1 nm caused a two-phase increase in [Ca2+]i with an initial high-amplitude first phase and a low-amplitude second phase. Depolarizing K+ induced a sharp increase in [Ca2+]i which peaked within 15 seconds, then declined gradually on a sloping plateau. Both TRH and K+ induced an acute dose-dependent PRL secretory burst peaking within 2·5 min with a subsequent rapid decline. With ramp exposure, high doses of TRH (final concentration 10–100 nm) triggered an acute rise in [Ca2+]i; however, the peaks were clearly lower than those induced by the maximum concentration reached given as a square-wave. TRH (0·1–100 nm) induced PRL secretion in a dose-dependent manner. Ramp depolarizing K+ induced dose-dependent parallel 'ramp' increases in [Ca2+]i concentration and PRL secretion without a 'burst' rise in either. These data suggest that a rise in [Ca2+]i plays a more critical role in K+-induced than in TRH-induced PRL secretion; intracellular transduction systems which do not involve [Ca2+]i appear more important for the latter. Journal of Endocrinology (1994) 142, 145–152