We have examined statistically the actions of thyrotropin-releasing hormone (TRH) and Bay K 8644, an L-type Ca(2+)-channel agonist, on the frequency and shape of cytosolic Ca2+ spikes in individual GH4C1 rat pituitary cells. TRH induced a brief (0-40 s) suppression of Ca2+ spikes followed by a period (40-200 s) of increased spike frequency. TRH treatment reduced the rate of rise and amplitude of Ca2+ spikes, and increased the rate of fall, relative to spontaneous spikes before treatment. TRH had no significant effect on the correlation between spike amplitude and the spike decay time constant tau, suggesting that the increased rate of fall was due to enhanced Ca2+ extrusion and not to decreased Ca(2+)-induced Ca2+ release. Bay K rapidly (t1/2 = 9-13 s) induced a 2-fold increase in the rate of rise of spikes with no change in the total rise time, leading to an increase in spike amplitude. It increased by 2-fold the fall time of spikes, as predicted solely by the previously observed relationship between spike amplitude and fall time. Bay K therefore appeared to increase the number of Ca2+ channels participating in each spike event without altering the kinetics of channel activation or deactivation, and without influencing Ca2+ extrusion. After addition of Bay K, the interval between spikes gradually (t1/2 approximately 100 s) increased, whereas the rate of rise remained constant and maximal. To explain these actions of TRH and Bay K, we postulate that a fraction of L-type Ca2+ channels are inactivated during each spike and must be re-activated in order to participate in a subsequent spike. We conclude further that the changes in spike frequency and profiles induced by these secretagogues are most consistent with a model in which TRH induces increases in both Ca2+ influx and efflux while Bay K induces a large increase in Ca2+ influx but has little effect on efflux.