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. Furthermo re, important differences in somatostatin (SRM) action between rats an d sheep have been found previously. Our goal was to compare in male ra t and ram pituitaries the binding characteristics of somatostatin rece ptors and the effect of SRIH and 17 analogues on GH release. Using rad ioautography, SRIH binding was seen to be evenly distributed over the anterior pituitary of both species, In the binding assay, binding site s were three times more concentrated in rats than in sheep, Important interspecies differences in the action of SRM and its analogues were f ound: 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 inhi bit GH release in rats were correlated with somatostatin receptors sub type 2 (sst2) affinities. Our data confirm and extend the quantitative differences between rat and sheep in SRM inhibitory action on GH secr etion and confirm that ligand-binding properties of a given receptor s ubtype cannot be extrapolated across species.