Substance P increases intracellular Ca 2+ in individual rat pituitary lactotrophs, somatotrophs, and gonadotrophs

Abstract

The present study has investigated transients in the intracellular calcium concentration [Ca 2+] i in response to substance P (SP) in single pituitary cells. SP raised [Ca 2+] i in three subtypes of pituitary cells: lactotrophs, somatotrophs, and gonadotrophs. In all three cell subtypes the [Ca 2+] i response to SP was amplitude-modulated and a concentration of 100 nM was necessary to elicit well pronounced two phased [Ca 2+] i transients. The first phase was associated with increased generation of inositol 1,4,5-trisphosphate (Ins(1,4,5) P 3) in all three cell types. In lactotrophs, the second phase, but not the first, was blunted by depletion of extracellular Ca 2+ (Ca 2+ free EGTA incubation buffer) and by addition of dopamine (1 μM). In somatotrophs, the second phase of the SP-induced [Ca 2+] i response was inhibited by depletion of extracellular Ca 2+ and by addition of somatostatin (100 nM), while the first phase was unaffected by this treatment. In gonadotrophs, the second phase, but not the first, was inhibited by the Ca 2+ channel blocker methoxyverapamil and depletion of extracellular Ca 2+. SP was compared with other agonists having an action on lactotrophs, somatotrophs or gonadotrophs. These experiments demonstrated that SP was a weaker agonist in terms of maximal [Ca 2+] i response than thyrotropin-releasing hormone (TRH) (in lactotrophs), growth hormone-releasing hexapeptide (in somatotrophs) and GnRH (in gonadotrophs). On the basis of these results it is concluded that SP exerts direct Ca 2+ mobilizing effects in single lactotrophs, somatotrophs, and gonadotrophs derived from male peripubertal rats. The first phase in SP-induced [Ca 2+] i transients is likely to be brought about by inositol 1,4,5-trisphosphate-mediated Ca 2+ release from internal stores while the second phase reflects an influx of calcium through voltage-gated calcium channels.