GROWTH HORMONE-RELEASING FACTOR MOBILIZES CYTOSOLIC-FREE CALCIUM THROUGH DIFFERENT MECHANISMS IN 2 SOMATOTROPE SUBPOPULATIONS FROM PORCINE PITUITARY

Porcine somatotropes can be separated by Percoll density gradient cent rifugation into low (LD) and high density (HD) subpopulations that dif fer ultrastructurally and functionally. Here, we report the effects of growth hormone-releasing factor (GRF) on the cytosolic free calcium c oncentration ([Ca2+](i)) of single LD and HD somatotropes. Resting [Ca 2+](i) in LD somatotropes was 2-fold higher than in HD cells. GRF indu ced [Ca2+](i) increases in a similar percentage of somatotropes from b oth subsets. However, amplitude and kinetics of the responses were mar kedly different. In all responsive LD somatotropes, GRF evoked a rapid initial peak followed by a sustained plateau (plateau-type response). Blockade of extracellular Ca2+ entry by 3 mM EDTA, 2 mM CoCl2, or 100 mu M verapamil completely abolished the plateau phase without affecti ng the initial Ca2+ spike. Conversely, only the plateau phase was pres erved in thapsigargin (TG)-treated LD cells. The vast majority of GRF- responsive HD somatotropes exhibited a transient [Ca2+](i) peak that r eturned gradually to baseline (transient-type response). This response was completely blocked by removal of extracellular Ca2+, whereas TG t reatment had no effect. Taken together, our results indicate that the response of LD somatotropes to GRF depends on mobilization of Ca2+ of both extra-and intracellular origin, whereas that of HD somatotropes s eems to be exclusively dependent on extracellular Ca2+ entry through L -type voltage sensitive Ca2+ channels (VSCC). These findings are the f irst to demonstrate a differential effect of GRF on Ca2+ mobilization in two somatotrope subpopulations, and suggest the existence of differ ences in the GRF receptor(s) expressed in each subpopulation and/or in the intracellular signalling pathways activated upon GRF binding.