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 Ca ²⁺ 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 (100μg/ml) caused a transient increase in the Ca²⁺ level in a majority of preparations taken from the morning group rats killed between 0930h 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 had no effect in most preparations taken from the afternoon group rats euthanized between 1200 h and 1400 h. In the neonatal rat anterior pituitary, LHRH caused a suppression of spontaneous Ca²⁺ transients. Strikingly, the second application of LHRH was capable of reproducing the suppression of Ca²⁺signals, indicating that the refractoriness to LHRH had not been established in neonatal rats. These results suggest that responsiveness to LHRH has a long-term refractoriness in adult rats, and that the physiological LHRH surge may be clocked in the morning. Moreover, LHRH-induced excitation and associated refractoriness appear to be incomplete in neonatal rats and may be acquired during development.