Growth hormone-releasing factor mobilizes cytosolic free calcium through different mechanisms in two somatotrope subpopulations from porcine pituitary.

Porcine somatotropes can be separated by Percoll density gradient centrifugation into low (LD) and high density (HD) subpopulations that differ ultrastructurally and functionally. Here, we report the effects of growth hormone-releasing factor (GRF) on the cytosolic free calcium concentration ([Ca2+]i) of single LD and HD somatotropes. Resting [Ca2+]i in LD somatotropes was 2-fold higher than in HD cells. GRF induced [Ca2+]i increases in a similar percentage of somatotropes from both subsets. However, amplitude and kinetics of the responses were markedly 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 microM verapamil completely abolished the plateau phase without affecting the initial Ca2+ spike. Conversely, only the plateau phase was preserved in thapsigargin (TG)-treated LD cells. The vast majority of GRF-responsive HD somatotropes exhibited a transient [Ca2+]i peak that returned gradually to baseline (transient-type response). This response was completely blocked by removal of extracellular Ca2+, whereas TG treatment 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 seems to be exclusively dependent on extracellular Ca2+ entry through L-type voltage sensitive Ca2+ channels (VSCC). These findings are the first to demonstrate a differential effect of GRF on Ca2+ mobilization in two somatotrope subpopulations, and suggest the existence of differences in the GRF receptor(s) expressed in each subpopulation and/or in the intracellular signalling pathways activated upon GRF binding.