Pharmacological characterization of cholecystokinin receptors mediating contraction of human gallbladder and ascending colon

Abstract

Cholecystokinin (CCK) produces contractions of gallbladder and colon in a number of different species. Although the effects of CCK on the human gallbladder are relatively well documented, the CCK receptors in the human colon have not been clearly characterised. Therefore, in this study, the CCK receptors in the human gallbladder and colon were compared using pharmacological techniques. Contraction of specimens of the human tissue was measured using in vitro organ bath bioassay. The effect of selective concentrations of CCK₁ and CCK₂ receptor antagonists (L-364,718 and JB93182, respectively) was determined on agonist concentration–effect (E/[A]) curves obtained by cumulative dosing with sulphated CCK. The CCK₁ antagonist L-364,718 produced a rightward shift of the CCK-8S [E/[A] curve in the human gallbladder (pA₂=9.15±0.26) and ascending colon (pA₂=9.20±0.33). In both tissues, the CCK₂ receptor antagonist, JB93182, had no effect on the CCK E/[A] curves. In addition, in the colon, pentagastrin responses were inhibited by L-364,718 but unaffected by JB93182. These data indicate that the CCK-induced contraction of the human colon and gallbladder smooth muscle is mediated solely through the CCK₁ receptor subtype, and the antagonist affinity estimates are consistent with those previously obtained in experiments on animal tissue.

Introduction

Cholecystokinin (CCK) receptors were originally classified into two subtypes, CCK₁ and CCK₂, on the basis of differences in agonist rank potency orders and through the use of receptor-selective antagonists [1]. Both of these receptor subtypes can mediate the contraction of gastrointestinal (GI) tissues. However, the relative contribution of each subtype varies between GI tract tissues. For example, both CCK₁ and CCK₂ receptors mediate the contraction of the guinea pig ileum [2], [3], [4], whereas the guinea pig gallbladder contraction is mediated solely by CCK₁ receptors [5]. In the human GI tissue, the use of CCK receptor-selective antagonists to characterise the receptor subtypes responsible for mediating these actions of CCK and gastrin has yielded conflicting results in terms of the antagonist affinity values estimated. In one study, the antagonists lorglumide, loxiglumide and L-364,718 were found to be significantly less potent in the human gallbladder and colon than in the guinea pig ileum (e.g. K_B for L-364,718 was ∼10,000-fold lower on human isolated in vitro bioassay) [6], suggesting possible species differences in the pharmacology of these receptors. In contrast, other studies have reported no differences in the affinity values estimated for the antagonists lorglumide and 2-NAP on human and animal gallbladder preparations [7], [8].

The aim of this study was to characterise the CCK receptors mediating contraction of the human gallbladder and colon using standard bioassay methods and a combination of CCK receptor-selective agonists and antagonists. In performing a bioassay of the human colon tissue, a wide variation in the responsiveness of individual muscle strips to gastrin/cholecystokinin was encountered, a problem which was first reported by Bennett et al. [9]. We also report the results of an attempt to reduce this variation by keeping colon muscle preparations under organ culture conditions for 24 h prior to performing the bioassays.