Biological activity of somatostatin receptors in GC rat tumour somatotrophs: evidence with sst1-sst5 receptor-selective nonpeptidyl agonists

The physiological actions of somatostatin-14 (SRIF: somatotrophin release inhibitory factor) receptor subtypes (sst(1)-sst(5)), which are endogenously expressed in growth cells (GC cells), have not yet been elucidated, although there is evidence that sst(2) receptors are negatively coupled to cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and adenosine 3,5'-cyclic monophosphate (cAMP) accumulation. In addition, both sst(1) and sst(2) receptors are negatively coupled to growth hormone (GH) secretion in GC cells. Here we report on studies concerning the expression, the pharmacology and the functional role of native SRIF receptors in GC cells with the use of five nonpeptidyl agonists, highly selective for each of the SRIF receptors. Radioligand binding studies show that sst(2) and sst(5) receptors are present at different relative densities, while the presence of sst(3) and sst(4) receptors appears to be negligible. The absence of sst(1) receptor binding was unexpected in view of sst(1) receptor functional effects on GH secretion. This suggests very efficient receptor-effector coupling of a low-density population of sst(1) receptors. Functionally, only sst(2) receptors are coupled to the inhibition of [Ca(2+)](i) and cAMP accumulation and the selective activation of sst(5) receptors facilitates the stimulation of adenylyl cyclase activity through G(i/o) proteins. This effect was not observed when sst(2) and sst(5) receptors were simultaneously activated, suggesting that there is a functional interaction between sst(2) and sst(5) receptors. In addition, sst(1), sst(2) and sst(5) receptor activation inhibits GH release, further indicating that SRIF can modulate GH secretion in GC cells through mechanisms both dependent and independent on [Ca(2+)](i) and cAMP-dependent pathways. The present data suggest SRIF-mediated functional effects in GC cells to be very diverse and provides compelling arguments to propose that multiple native SRIF receptors expressed in the same cells are not simply redundant, but contribute to marked signalling diversity.