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) receptors. In contrast to guinea pig pancreas, it is not known whether CCK receptors in rat pancreas are CCK-A receptors, CCK-B (gastrin) receptors, or both. Thus, in the present study, we characterized CCK receptors in rat pancreas at the receptor and mRNA level. 125I-Bolton-Hunter-labeled CCK octapeptide ( 125I-BH-CCK-8), the specific CCK-A and CCK-B (gastrin) receptor antagonists L364,718 and L365,260, and 125I-labeled gastrin-I were utilized to characterize CCK receptors in normal rat pancreas. Additionally, we utilized 32P-labeled cDNA probes of the CCK-A receptor and CCK-B (gastrin) receptor coding regions in order to examine the expression of CCK receptor subtypes in normal rat pancreas at the mRNA level. The dose-inhibition curve of CCK-8 inhibiting binding of 125I-BH-CCK-8 was significantly best fit by a two-site model with a high-affinity site ( K d = 0.68 ± 0.13 n M) and a low-affinity site ( K d = 656 ± 289 n M). L364,718 inhibited binding of 125 I-BH-CCK-8 with high affinity, whereas no high-affinity inhibition for L365,260 to inhibit binding of 125I-BH-CCK-8 was detected. L364,718 was 627 times as potent as L365,260 in inhibiting binding of 125 I-BH-CCK-8. No saturable binding was present for 125 I-labeled gastrin-I. Gastrin-17-I did not inhibit binding of 125 I-BH-CCK-8. The CCK-A receptor mRNA was identified in normal rat pancreas using Northern blot analysis and reverse transcription-polymerase chain reaction. However, no CCK-B (gastrin) receptor mRNA was detected by these techniques. In conclusion, normal 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 present. This study suggests that the rat is a pure in vivo model for studying the biological activity of the CCK-A receptor in the pancreas.