Inhibitory Control of Growth Hormone Secretion by Somatostatin in Rat Pituitary GC Cells: sst₂ but Not sst₁ Receptors Are Coupled to Inhibition of Single-Cell Intracellular Free Calcium Concentrations

Abstract

Rat pituitary tumor cells (GC cells) exhibit spontaneous oscillations of intracellular free calcium concentration ([Ca²⁺]_i) that allow continuous release of growth hormone (GH). Of the somatostatin (SRIH) receptor subtypes (sst receptors) mediating SRIH action, sst₁ and sst₂ receptors are highly expressed by GC cell membranes. In the present study, the effects of sst₁ or sst₂ receptor activation on single-cell [Ca²⁺]_i were investigated in GC cells by confocal fluorescence microscopy. In addition, the effects of sst₁ or sst₂ receptor activation on GH secretion were also studied. Our results demonstrate that SRIH decreases [Ca²⁺]_i baseline and almost completely blocks Ca²⁺ transients through activation of sst₂ but not of sst₁ receptors. In contrast, SRIH effectively inhibits GH secretion through activation of both sst₁ and sst₂ receptors. Blocking Ca²⁺ transients is less efficient than SRIH to inhibit GH release. The cyclic octapeptide, CYN-154806, antagonizes sst₂ receptors at [Ca²⁺]_i since it abolishes the sst₂ receptor-mediated inhibition of [Ca²⁺]_i without affecting single-cell Ca²⁺ signals. On the other hand, CYN-154806 alone potently inhibits GH secretion through the involvement of pertussis toxin-sensitive G proteins. In conclusion, the present results demonstrate that SRIH inhibition of GH release in GC cells involves mechanisms either dependent or independent on SRIH modulation of [Ca²⁺]_i. The implications of CYN-154806 inhibition of GH secretion are discussed.

The β isoenzyme of Ca2+/calmodulin-dependent kinase type II as possible mediator of somatostatin functions in pituitary tumour cells

“…In somatotropinomas, sst 2 is the main receptor inhibiting voltage-gated Ca 2+ current (Yang et al 2007) and it has also been suggested to decrease GH release through the inhibition of the voltage-dependent Ca 2+ channels (Florio et al 2003). These evidences are confirmed by previous studies performed in rat anterior pituitary GC cells, an adenoma cell line which exhibits pacemaker activity and concomitant spontaneous oscillations of [Ca 2+ ] i and GH secretion (Cervia et al 2002b). In GC cells, sst 2 activation is responsible for the [Ca 2+ ] i decrease and the block of Ca 2+ transients which, in turn, results in the sst 2 -mediated decrease of GH secretion (Petrucci et al 2000;Cervia et al 2002aCervia et al ,b, 2003.…”

“…CaMKII activation and GH secretion are reported to be correlated also in rat GH 3 tumor pituitary cells (Kanasaki et al 2002). Homologous normal and tumoral pituitary cells, including GC cells, display the same type of secretory mechanisms in vitro (Cervia et al 2002b;Stojilkovic et al 2005). Thus, CAMKIIβ activity may be required for GH production also in normal somatotropes of mammals, as demonstrated in fishes .…”

“…On the other hand, experimental approaches based on RNA interference have been used in tumour pituitary cells to examine signal molecules/receptors involved in SRIF functions (Theodoropoulou et al 2006;Ben-Shlomo et al 2007). In different experimental models (Nie et al 2007;Hao et al 2008;Wheeler et al 2008), including neuroendocrine cells (Basavappa et al 1999) transients which have been demonstrated as an effective manner to maintain GH exocytosis (Cervia et al 2002b;Dominguez et al 2007). In neurons, exocytosis seems to depend on multiple factors, including the activation of CaMKII signalling (which causes an increase of [Ca 2+ ] i ) and proceeds via delayed fusion pore opening (Kolarow et al 2007).…”

“…Using published protocols (Cervia et al 2002b(Cervia et al , 2003, GH was detected and quantified by means of a commercially available EIA kit (SPI Bio, Montigny Le Bretonneux, France) utilizing the double-antibody sandwich technique, according to the manufacturer's recommended procedure. Briefly, GC cells (transfected or untransfected) were detached from 100-mm dishes, and seeded in new dishes at a concentration of 10 7 cells.…”

Expression, pharmacology, and functional role of somatostatin receptor subtypes 1 and 2 in human macrophages

“…Calcium and activated G proteins are known to regulate the exocytosis of different factors, i.e., chemokines, synergistically from the activated macrophages, although a Ca 2ϩ -independent component may also be involved [76]. SRIF receptor coupling to Ca 2ϩ homeostasis is well established [2] as well as the fact that SRIF receptors may regulate exocytosis through mechanisms dependent and independent on Ca 2ϩ [2,35,51]. Whether the SRIF system couples to Ca 2ϩ homeostasis in human macrophages remains to be established.…”