Analysis of the CCKB receptor antagonism of virginiamycin in guinea-pig ileum longitudinal myenteric plexus.

1. Virginiamycin, a macrolide reported to bind selectively to CCKB/gastrin receptors has been studied in a functional test, namely cholecystokinin-induced contraction of guinea-pig ileum myenteric plexus (LMMP). 2. Virginiamycin (1-10 microM) antagonized the selective CCKB agonist cholecystokinin tetrapeptide (CCK-4). The antagonism appeared not to be competitive as the highest concentration (10 microM) caused a reduction of its maximal effect. An apparent pA2 of 6.64 +/- 0.06 (s.e.) could be estimated if this depression was ignored. The selective CCKB antagonist, L-365,260 (0.01-0.3 microM) antagonized competitively the CCK-4 induced contraction and a pKB of 8.60 +/- 0.16 (s.e.) was estimated. 3. The combined dose-ratio analysis for virginiamycin, tested at 3 and 10 microM in association with 0.03 and 0.1 microM L-365,260, respectively, resulted in observed log dose-ratios of 1.39 and 1.53. That was consistent with both antagonists acting on the same receptor in LMMP. 4. These data, represent the first evidence of the antagonism of virginiamycin in a functional assay and they support the hypothesis of homogeneity between CCKB receptors in the CNS and in peripheral tissues.