Intercellular communication within the rat anterior pituitary gland. XV. Properties of spontaneous and LHRH-induced Ca2+ transients in the transitional zone of the rat anterior pituitary in situ.

In the transitional zone of the rat anterior pituitary, spontaneous and LHRH-induced Ca2+ dynamics were visualized using fluo-4 fluorescence Ca2+ imaging. A majority of cells exhibited spontaneous Ca2+ transients, while small populations of cells remained quiescent. Approximately 70% of spontaneously active cells generated fast, oscillatory Ca2+ transients that were inhibited by cyclopiazonic acid (10 μm) but not nicardipine (1 μm), suggesting that Ca2+ handling by endoplasmic reticulum, but not Ca2+ influx through voltage-dependent L-type Ca2+ channels, plays a fundamental role in their generation. In the adult rat anterior pituitary, LHRH (100 μg/ml) caused a transient increase in the Ca2+ level in a majority of preparations taken from the morning group rats killed between 0930 h and 1030 h. However, the second application of LHRH invariably failed to elevate Ca2+ levels, suggesting that the long-lasting refractoriness to LHRH stimulation was developed upon the first challenge of LHRH. In contrast, LHRH...