Abstract
Somatostatin (SRIF-14) exerts broad spectrum antisecretory effects by activating the somatostatin 2 (sst(2)) receptor. The rat (r) sst(2) receptor exists in 'long' (sst(2a)) and 'short' (sst(2b)) forms that differ in their C termini, while a single human (h) sst(2a) exists. This study compares the characteristics of recombinant rsst(2a), rsst(2b) and hsst(2a) activation in human epithelia, and with native sst(2) responses in rat colon. Epithelial layers of each clone or rat colon were placed in Ussing chambers and short-circuit current (I (SC)) measured in response to SRIF-14 and chosen analogues. The relative potencies and ability to cause desensitization to SRIF-14 were assessed, and the affinities of the sst(2) antagonist, D-Tyr(8) CYN154806 for hsst(2a), rsst(2a) and native rat colon sst(2) receptors were established. Basolateral SRIF-14 responses were transient in hsst(2a) and rsst(2a) epithelia, but prolonged in rsst(2b)-expressing cells. Activation of rsst(2a) resulted in significant desensitization to SRIF-14 and receptor phosphorylation, whereas the rsst(2b) receptor did neither. Sst(2)-preferred agonists (BIM23190C and BIM23027) reduced I (sc) with similar potency and both caused complete desensitization to SRIF-14. CYN154806 antagonized hsst(2a) and rsst(2a) receptors with pK (B) values of 7.9 and 7.8, respectively. In rat colon mucosa, CYN154806 blocked SRIF-14 responses with a pA (2) value of 8.2, and BIM23190C responses with a pK (B) of 8.4. SRIF-14 caused rapid rsst(2a) receptor phosphorylation and desensitization of epithelial antisecretory responses, neither of which occurred with the rsst(2b) receptor. These mechanisms are most likely to be a prerequisite for sensitivity to sst(2)-analogues with radiotherapeutic potential.
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