Multiple Ca2+ channel-dependent components in growth hormone secretion from rat anterior pituitary somatotrophs.

Abstract

The involvement of L-type Ca(2+) channels in both 'basal' and 'stimulated' growth hormone (GH) secretion is well established; however, knowledge regarding the involvement of non-L-type Ca(2+) channels is lacking. We investigated whether non-L-type Ca(2+) channels regulate GH secretion from anterior pituitary (AP) cells. To this end, GH secretion was monitored from dissociated AP cells, which were incubated for 15min with 2mm K(+) ('basal' secretion) or 60mm K(+) ('stimulated' secretion). The role of non-L-type Ca(2+) influx was investigated using specific channel blockers, including ω-agatoxin-IVA, ω-conotoxin GVIA or SNX-482, to block P/Q-, N- or R-type Ca(2+) channels, respectively. Our results demonstrate that P/Q-, N- and R-type Ca(2+) channels contributed 21.2±1.9%, 20.2±7.6% and 11.4±1.8%, respectively, to 'basal' GH secretion and 18.3±1.0%, 24.4±5.4% and 14.2±4.8%, respectively, to 'stimulated' GH secretion. After treatment with a 'cocktail' that comprised the previously described non-L-type blockers, non-L-type Ca(2+) channels contributed 50.9±0.4% and 45.5±2.0% to 'basal' and 'stimulated' GH secretion, respectively. Similarly, based on the effects of nifedipine (10μM), L-type Ca(2+) channels contributed 34.2±3.7% and 54.7±4.1% to 'basal' and 'stimulated' GH secretion, respectively. Interestingly, the relative contributions of L-type/non-L-type Ca(2+) channels to 'stimulated' GH secretion were well correlated with the relative contributions of L-type/non-L-type Ca(2+) channels to voltage-gated Ca(2+) influx in AP cells. Finally, we demonstrated that compartmentalisation of Ca(2+) channels is important for GH secretion. Lipid raft disruption (methyl-β-cyclodextrin, 10mm) abrogated the compartmentalisation of Ca(2+) channels and substantially reduced 'basal' and 'stimulated' GH secretion by 43.2±3.4% and 58.4±4.0%, respectively. In summary, we have demonstrated that multiple Ca(2+) channel-dependent pathways regulate GH secretion. The proper function of these pathways depends on their compartmentalisation within AP cell membranes.