SUBSTANCE-P INCREASES INTRACELLULAR CA2+ IN INDIVIDUAL RAT PITUITARY LACTOTROPHS, SOMATOTROPHS, AND GONADOTROPHS

The present study has investigated transients in the intracellular cal cium concentration [Ca2+](i) in response to substance P (SP) in single pituitary cells. SP raised [Ca2+](i) in three subtypes of pituitary c ells: lactotrophs, somatotrophs, and gonadotrophs. In all three cell s ubtypes the [Ca2+](i) response to SP was amplitude-modulated and a con centration of 100 nM was necessary to elicit well pronounced two phase d [Ca2+](i) transients. The first phase was associated with increased generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) in all thre e cell types. In lactotrophs, the second phase, but not the first, was blunted by depletion of extracellular Ca2+(Ca2+ free EGTA incubation buffer) and by addition of dopamine (1 mu M). In somatotrophs, the sec ond phase of the SP-induced [Ca2+](i) response was inhibited by deplet ion of extracellular Ca2+ and by addition of somatostatin (100 nM), wh ile the first phase was unaffected by this treatment. In gonadotrophs: the second phase, but not the first, was inhibited by the Ca2+ channe l blocker methoxyverapamil and depletion of extracellular Ca2+. SP was compared with other agonists having an action on lactotrophs, somatot rophs or gonadotrophs. These experiments demonstrated that SP was a we aker agonist in terms of maximal [Ca2+](i) response than thyrotropin-r eleasing hormone (TRH) (in lactotrophs), growth hormone-releasing hexa peptide (in somatotrophs) and GnRH (in gonadotrophs). On the basis of these results it is concluded that SP exerts direct Ca2+ mobilizing ef fects in single lactotrophs, somatotrophs, and gonadotrophs derived fr om male peripubertal rats. The first phase in SP-induced [Ca2+](i) tra nsients is likely to be brought about by inositol 1,4,5-trisphosphate- mediated Ca2+ release from internal stores while the second phase refl ects an influx of calcium through voltage-gated calcium channels. (C) 1997 Elsevier Science Ireland Ltd.