Uncoupling of calcium mobilization and entry pathways in endothelin-stimulated pituitary lactotrophs

In cells expressing Ca2+-mobilizing receptors, InsP3-induced Ca2+ release from intracellular stores is commonly associated with extracellular Ca2+ influx. Operation of these two Ca2+ signaling pathways mediates thyrotropin-releasing hormone (TRH) and angiotensin II (AII)-induced prolactin secretion from rat pituitary lactotrophs. After an initial hyperpolarization induced by Ca2+ mobilization from the endoplasmic reticulum (ER), these agonists generated an increase in the steady-state firing of action potentials, further facilitating extracellular Ca2+ influx and prolactin release. Like TRH and AII, endothelin-1 (ET-1) also induced a rapid release of Ca2+ from the ER and a concomitant spike prolactin secretion during the first 3-5 min of stimulation. However, unlike TRH and AII actions, Ca2+ mobilization was not coupled to Ca2+ influx during sustained ET-1 stimulation, as ET-1 induced a long-lasting abolition of action potential firing. This lead to a depletion of the ER Ca2+ pool, a prolonged decrease in [Ca2+]i, and sustained inhibition of prolactin release. ET-1-induced inhibition and TRH/AII-induced stimulation of Ca2+ influx and hormone secretion were reduced in the presence of the L-type Ca2+ channel blocker, nifedipine. Basal [Ca2+]i and prolactin release were also reduced in the presence of nifedipine. Furthermore, TRH-induced Ca2+ influx and secretion were abolished by ET-1, as TRH was unable to reactivate Ca2+ influx and prolactin release in ET-1-stimulated cells. Depolarization of the cells during sustained inhibitory action of ET-1, however, increased [Ca2+]i and prolactin release. These results indicate that L-type Ca2+ channel represents a common Ca2+ influx pathway that controls basal [Ca2+]i and secretion and is regulated by TRH/AII and ET-1 in an opposite manner. Thus, the receptor-mediated uncoupling of Ca2+ entry from Ca2+ mobilization provides an effective control mechanism in terminating the stimulatory action of ET-1. Moreover, it makes electrically active lactotrophs quiescent and unresponsive to other calcium-mobilizing agonists.