Evidence for a novel intrapituitary autocrine/paracrine feedback loop regulating growth hormone synthesis and secretion in grass carp pituitary cells by functional interactions between gonadotrophs and somatotrophs.

Abstract

Gonadotropin (GTH) and GH released from the pituitary are known to interact at multiple levels to modulate the functions of the gonadotrophic and somatotrophic axes. However, their interactions at the pituitary level have not been fully characterized. In this study, autocrine/paracrine regulation of GH synthesis and secretion by local interactions between gonadotrophs and somatotrophs was examined using grass carp pituitary cells as a cell model. Exogenous GTH and GH induced GH release and GH mRNA expression in carp pituitary cells. Removal of endogenous GTH and GH by immunoneutralization with GTH and GH antisera, respectively, suppressed GH release, GH production, and GH mRNA levels. GH antiserum also blocked the stimulatory effects of exogenous GTH on GH release and GH mRNA levels. In reciprocal experiments, GH release and GH mRNA expression induced by exogenous GH was significantly reduced by GTH antiserum. In addition, exogenous GH was found to be inhibitory to basal GTH release and treatment with GH antiserum elevated GTH secretion at low doses but suppressed GTH production at high doses. These results suggest that local interactions between gonadotrophs and somatotrophs may form an intrapituitary feedback loop to regulate GH release and synthesis. In this model, GTH released from gonadotrophs induces GH release and GH production in neighboring somatotrophs. GH secreted maintains somatotroph sensitivity to GTH stimulation, and at the same time, inhibits basal GTH release in gonadotrophs. This feedback loop may represent a novel mechanism regulating GH release and synthesis in lower vertebrates.