Intracellular free calcium concentration in rat anterior pituitary cells as indicated by fura-2: effect of arginine-vasopressin

Abstract

Arginine-vasopressin (AVP) stimulates adrenocorticotropin and beta-endorphin release from corticotrophs of the anterior pituitary gland through mechanisms which are not initiated by an elevation of the cellular levels of adenosine-3',5'-cyclic-monophosphate. In the present study the effect of AVP on the cytoplasmic concentrations of free calcium ions in rat anterior pituitary cells was examined. Cytosolic free Ca2+ concentrations were monitored directly using the new, intracellularly trapped fluorescent indicator fura-2. In cells incubated in medium containing 1.3 mmol/l Ca2+, AVP (100 nmol/l) caused an immediate elevation of the cytoplasmic Ca2+ concentration by about 50 nmol/l (P less than 0.001). The intracellular Ca2+ levels remained elevated during the observation period of 2-3 min. This effect of AVP was blocked by a specific vasopressin antagonist. By contrast, the glucocorticoid dexamethasone did not affect the AVP-induced elevation of cytosolic Ca2+ concentration. When the cells were incubated in Ca2+-free medium (Ca2+ omitted, EGTA 2 mmol/l), the AVP-induced as well as the K+ depolarization-induced increase in free cytoplasmic Ca2+ were abolished, whereas the ionophore ionomycin evoked a rapid transient elevation of free Ca2+. The increase in cytoplasmic Ca2+ concentration induced by AVP was preserved in medium containing the calcium channel blockers Mg2+ (Mg2+ 31.2 mmol/l; Ca2+ 1.3 mmol/l) or nifedipine (1 mumol/l). The potassium-evoked calcium signal was blocked by Mg2+ (31.2 mmol/l). We conclude that vasopressin induces a rapid rise in the cytoplasmic concentration of free calcium ions in corticotrophs. Vasopressin may mobilize calcium through mechanisms that neither are glucocorticoid-sensitive nor involve the influx of extracellular calcium through voltage-dependent calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)