Developmental and physiological aspects of Ca2+ signaling in agonist-stimulated pituitary gonadotrophs

GnRH-induced Ca2+ signaling was analyzed in gonadotrophs from pituitary glands of neonatal, prepubertal, peripubertal, cycling, and ovariectomized female and adult male rats. In all cases, single cell studies showed three types of Ca2+ responses to increasing GnRH concentrations: subthreshold, baseline oscillatory, and biphasic (an early transient followed by sustained oscillations or a plateau). An increase in the frequency, but not the amplitude, of spiking, and the presence of both baseline oscillatory and biphasic Ca2+ responses were observed in single cells from all groups exposed to low and subsequently to higher GnRH concentrations. Thus, modulation of the frequency of Ca2+ spiking at low to medium GnRH concentrations and suppression of the oscillatory response at high agonist concentrations are characteristics of the gonadotroph that are not affected by age, sex, stage of the estrous cycle, or gonadectomy. Ca2+ influx through plasma membrane channels contributed to both baseline oscillatory and biphasic responses, and both types of Ca2+ signaling were associated with LH release. The sensitivity of gonadotrophs to GnRH stimulation was similar in cells from single and multigland preparations from the same group of donors, but varied according to age and physiological status. Thus, sensitivity was low in neonatal gonadotrophs and progressively increased in postnatal cells. In cycling females, gonadotrophs from proestrous animals were more sensitive than those from estrous and diestrous animals. Ca2+ signaling was more uniform in gonadotrophs from ovariectomized than in those from normal females. During prolonged culture there was an increase in the frequency of Ca2+ spiking in response to 1 nM GnRH over the first 3 days, followed by a gradual decrease over 7-10 days and loss of the response in older cultures. These changes in Ca2+ signaling were accompanied by similar changes in the GnRH-induced secretory responses. These data indicate that frequency-controlled spiking, with suppression of the oscillatory mechanism at high agonist concentrations, is a characteristic of Ca2+ signaling in rat gonadotrophs. In addition, developmental, physiological, and in vitro changes in pituitary sensitivity to GnRH are expressed through modulation of the Ca2+ signal and the consequent secretory response.