Species differences between male rat and ram pituitary somatostatin receptors involved in the inhibition of growth hormone secretion.

The sheep is a valuable model in which to study GH neuroregulation as its pattern of GH secretion is very close to that in humans. Furthermore, important differences in somatostatin (SRIH) action between rats and sheep have been found previously. Our goal was to compare in male rat and ram pituitaries the binding characteristics of somatostatin receptors and the effect of SRIH and 17 analogues on GH release. Using radioautography, SRIH binding was seen to be evenly distributed over the anterior pituitary of both species. In the binding assay, binding sites were three times more concentrated in rats than in sheep. Important interspecies differences in the action of SRIH and its analogues were found: they inhibited GH at lower concentrations in rats than in sheep. Seven peptides displayed greater inhibitory ability in sheep than in rats while three were more potent in rats. Agonistic potencies to inhibit GH release in rats were correlated with somatostatin receptors subtype 2 (sst2) affinities. Our data confirm and extend the quantitative differences between rat and sheep in SRIH inhibitory action on GH secretion and confirm that ligand-binding properties of a given receptor subtype cannot be extrapolated across species.