FURTHER EVIDENCE FROM FUNCTIONAL-STUDIES FOR SOMATOSTATIN RECEPTOR HETEROGENEITY IN GUINEA-PIG ISOLATED ILEUM, VAS-DEFERENS AND RIGHT ATRIUM

1 Somatostatin (SRIF) causes a concentration-dependent inhibition of neurotransmission in guinea-pig ileum and vas deferens as well as negative inotropy in guinea-pig isolated right atrium. The SRIF receptors mediating these effects have now been further characterized by use of the peptides BIM-23027, BIM-23056 and L-362855, reported as selective for the recombinant SRIF receptor types, sst(2), sst(3) and sst(5), respectively. 2 BIM-23027 was a highly potent agonist at causing an inhibition of neurotransmission in the guineapig ileum (EC(50) value 1.9 nM), being about 3 times more potent than SRIF (EC(50) value 6.8 nM). In contrast, in both guinea-pig vas deferens and right atrial preparations, BIM-23027 was a relatively weak agonist being at least 30-100 times weaker than SRIF. In guinea-pig atria, BIM-23027 (3 mu M) antagonized the negative inotropic action of SRIF(28) (apparent pK(B) = 5.9 +/- 0.1) but had no effect on the negative inotropic action of cyclohexyladenosine. 3 The inhibitory effect of BIM-23027 in the guinea-pig ileum was readily desensitized. Prior exposure to BIM-23027 (0.3 mu M) markedly attenuated the inhibitory effect of SRIF but had no effect on the inhibitory action of clonidine suggesting that BIM-23027 and SRIF act via a common receptor mechanism. 4 L-362855 caused a concentration-dependent inhibition of neurotransmission in both the guinea-pig ileum and vas deferens as well as causing negative inotropy in the guinea-pig atrium but was at least 30-100 times weaker than SRIF. In guinea-pig isolated atria, L-362855 (3 mu M) did not antagonize the negative inotropic action of SRIF(28). 5 BIM-23056 in concentrations up to 1 mu M was inactive as an agonist in guinea-pig isolated ileum, vas deferens and atrium and did not antagonize the inhibitory actions of SRIF in any of these preparations. 6 The results from this study support our previous contention that the sst(2) receptor type mediates inhibition of neurotransmission by SRIF in the guinea-pig ileum. The SRIF receptor type mediating inhibition of neurotransmission in the guinea-pig vas deferens appears different, but similar, to that mediating negative inotropy in the atrium. However the characteristics of these latter receptors appear different from that of the recombinant sst(2), sst(3) and sst(5) receptors for SRIF described for rat and man.