Thyrotropin-releasing hormone-induced rise in cytosolic calcium and activation of outward K+ current monitored simultaneously in individual GH3B6 pituitary cells

Thyrotropin-releasing hormone (TRH) acts on pituitary cells to raise the cytosolic free Ca2+ concentration (Ca2+]i) and causes simultaneously a transient hyperpolarization of the plasma membrane. The combination of the microfluorimetric monitoring of Ca2+]i with electrophysiological recordings obtained using the patch clamp technique in its whole cell configuration, allows the analysis of the correlation between changes in Ca2+]i and the alterations in ionic currents at the plasma membrane. It was shown that in the absence of hormone stimulation, a depolarization-induced change in steady state Ca2+]i, as well as the internal perfusion with Ca2+ at microM levels at constant membrane potential led to the activation of outward K+ current. TRH stimulation resulted in a marked but transient rise in Ca2+]i; concomitantly, there was an increase in membrane conductance and an enhancement of outward current. During the time course of an individual response, an excellent correlation between the changes in Ca2+]i and those in conductance or current was observed. The relative changes of current and conductance during the TRH response were consistent with the activation of a single type of ionic current, the apparent reversal potential of which coincided with the equilibrium potential for K+. A strong correlation between the TRH-induced changes in Ca2+]i and K+, conductance was demonstrated in a large number of cells with varied kinetic features: significant correlation coefficients were found both for the transition time from basal to maximal values (r = 0.85, p less than 0.001) as well as for the total duration of the responses (r = 0.68, p less than 0.002). It is concluded that during the early phase of TRH action, the hormone-induced rise in Ca2+]i is the principal cause of enhanced K+ channel activation.