Cortisol rapidly suppresses intracellular calcium and voltage-gated calcium channel activity in prolactin cells of the tilapia (Oreochromis mossambicus)

Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia ( Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in Ca-45(2+) and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium (Ca-i(2+)) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca2+-sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced Ca-i(2+) within seconds ( P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces Ca-i(2+). Under depolarizing [K+], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca2+ channels, stimulates PRL release in a concentration-dependent fashion ( P < 0.01). Cortisol ( and somatostatin) blocks BAY K 8644-induced PRL release ( P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in Ca-i(2+) that likely involve an attenuation of Ca2+ entry through L-type voltage-gated Ca2+ channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.