Expression of Ca2+-mobilizing endothelin(A) receptors and their role in the control of Ca2+ influx and growth hormone secretion in pituitary somatotrophs

Abstract

The expression and coupling of endothelin (ET) receptors were studied in rat pituitary somatotrophs. These cells exhibited periods of spontaneous action potential firing that generated high-amplitude fluctuations in cytosolic calcium concentration ([Ca2+](i)). The message and the specific binding sites for ET(A), but not ET(B), receptors were found in mixed pituitary cells and in highly purified somatotrophs. The activation of these receptors by ET-1 led to an increase in inositol 1,4,5-trisphosphate production and the associated rise in [Ca2+](i) and growth hormone (GH) secretion. The Ca2+-mobilizing action of ET-1 lasted for 2-3 min and was followed by an inhibition of action potential-driven Ca2+ influx and GH secretion to below the basal levels. As in somatostatin-treated cells, the ET-1-induced inhibition of spontaneous electrical activity and Ca2+ influx was accompanied by the inhibition of adenylyl cyclase and by the stimulation of inward rectifier potassium current. In contrast to somatostatin, ET-1 did not inhibit voltage-gated Ca2+ channels. During prolonged agonist stimulation a gradual recovery of Ca2+ influx and GH secretion occurred. In somatotrophs treated with pertussis toxin overnight, the ET-1-induced Ca2+mobilizing phase was preserved, but it was followed immediately by facilitated Ca2+ influx and GH secretion. Both somatostatin- and ET-l- induced inhibitions of adenylyl cyclase activity were abolished in pertussis toxin-treated cells. These results indicate that the transient cross-coupling of Ca2+mobilizing ETA receptors to the Gi/Go pathway in somatotrophs provides an effective mechanism to change the rhythm of [Ca2+]i signaling and GH secretion during continuous agonist stimulation.