Discrimination between CCK receptors of guinea-pig and rat brain by cyclic CCK8 analogues.

The properties of high affinity CCK8 binding sites of guinea-pig and rat brain cortex were compared using [3H]pCCK8. Large differences were observed, with the KD value being significantly higher in the rat (KD = 1.25 nM) than in guinea-pig brain (KD = 0.18 nM). Both sites exhibited different specificities for various CCK8 analogues, the selectivity factors KI rat/KI guinea-pig varied from 0.9 for CCK4 to 64 for cyclic CCK8-related compounds. Significant differences in the inhibition of [3H]pCCK8 binding by monovalent and nucleotides cations were also observed. These results could be explained by a difference in receptor environment or by a species difference in the proportion of CCK8 receptor-subtypes.