The rate of increase not the amplitude of cytosolic Ca2+ regulates the degree of prolactin secretion induced by depolarizing K+ or hyposmolarity in GH4C1 cells.

Depolarizing K+ and medium hyposmolarity caused striking rises in both cytosolic free Ca2+ concentration [( Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells, which were completely blocked by removal of medium Ca2+. However, the increase in [Ca2+]i and PRL secretion induced by hyposmolarity was clearly slower than that induced by K+. Although there was a good correlation between the zenith of PRL secretion and [Ca2+]i induced by various intensities of K+ or hyposmolarity, the regression slopes were significantly different between the K(+)-and hyposmolarity-induced changes (P less than 0.01). There was a good correlation between the maximum rate of change in PRL secretion and that of the increase in [Ca2+]i when the data from the 2 secretagogues were combined (r = 0.994, P less than 0.001, N = 9). We suggest that the rate of increase in [Ca2+]i may be more important than the amplitude of [Ca2+]i in stimulating PRL secretion.