CHARACTERIZATION OF CHOLECYSTOKININ RECEPTORS AND MESSENGER-RNA EXPRESSION IN RAT PANCREAS - EVIDENCE FOR EXPRESSION OF CHOLECYSTOKININ-A RECEPTORS BUT NOT CHOLECYSTOKININ-B (GASTRIN) RECEPTORS

It has been previously demonstrated that guinea pig pancreas possesses both cholecystokinin-A (CCK-A) receptors and CCK-B (gastrin) receptor s. In contrast to guinea pig pancreas, it is not known whether CCK rec eptors in rat pancreas are CCK-A receptors, CCK-B (gastrin) receptors, or both. Thus, in the present study, we characterized CCK receptors i n rat pancreas at the receptor and mRNA level. I-125-Bolton-Hunter-lab eled CCK octapeptide (I-125-BH-CCK-8), the specific CCK-A and CCK-B (g astrin) receptor antagonists L364,718 and L365,260, and I-125-labeled gastrin-I were utilized to characterize CCK receptors in normal rat pa ncreas. Additionally, we utilized P-32-labeled cDNA probes of the CCK- A receptor and CCK-B (gastrin) receptor coding regions in order to exa mine the expression of CCK receptor subtypes in normal rat pancreas at the mRNA level. The dose-inhibition curve of CCK-8 inhibiting binding of I-125-BH-CCK-8 was significantly best fit by a two-site model with a high-affinity site (K-d = 0.68 +/- 0.13 nM) and a low-affinity site (K-d = 656 +/- 289 nM). L364,718 inhibited binding of I-125-BH-CCK-8 with high affinity, whereas no high-affinity inhibition for L365,260 t o inhibit binding of I-125-BH-CCK-8 was detected. L364,718 was 627 tim es as potent as L365,260 in inhibiting binding of I-125-BH-CCK-8. No s aturable binding was present for I-125-labeled gastrin-I. Gastrin-17-I did not inhibit binding of I-125-BH-CCK-8. The CCK-A receptor mRNA wa s identified in normal rat pancreas using Northern blot analysis and r everse transcription-polymerase chain reaction. However, no CCK-B (gas trin) receptor mRNA was detected by these techniques. In conclusion, n ormal rat pancreas expresses two classes of CCK receptors, i.e., high- affinity receptor and low-affinity receptor. All of the CCK receptors are CCK-A receptors and no CCK-B (gastrin) receptors appear to be pres ent. This study suggests that the rat is a pure in vivo model for stud ying the biological activity of the CCK-A receptor in the pancreas. (C ) 1995 Academic Press, Inc.