Cytosolic Ca2+ in melanotrophs: pharmacological insights into regulatory influences of electrical activity and ion channels.

The concentration of intracellular free Ca2+ ([Ca2+]i) was measured in melanotrophs, the characteristic endocrine cells of the pars intermedia of the rat pituitary gland, using the fluorescent Ca indicator fura-2. The resting [Ca2+]i was 211 +/- 8 nM and was little affected by tetrodotoxin (TTX; 5 or 10 microM), which inhibits the spontaneous action potentials that occur in these cells. Removal of extracellular Ca2+ (by chelation with EGTA) or addition of the Ca channel blocker nimodipine (1 microM) produced a rapid fall in [Ca2+]i, which occurred whether TTX was present or not. Excess K+ (60 mM), veratridine (10 or 100 microM) and BAY K 8644 (1 microM) each caused a rapid rise in [Ca2+]i, which was blocked or truncated by EGTA or nimodipine. TTX blocked or truncated the increases in [Ca2+]i induced by veratridine, but not those induced by either excess K+ or BAY K 8644. The results show that manipulations that increase or decrease hormone output increase or decrease [Ca2+]i. Furthermore, the resting [Ca2+]i appears to depend importantly on Ca influx, since it is rapidly and markedly reduced by removal of extracellular Ca2+ or addition of a Ca channel blocker.