THE USE OF RECEPTOR DESENSITIZATION TO ANALYZE CCKA AND CCKB GASTRIN RECEPTORS COUPLED TO CONTRACTION IN GUINEA-PIG STOMACH MUSCLE

1 The results of previous studies have been in conflict with respect t o the involvement of specific cholecystokinin (CCKA) and CCKB/gastrin receptors in guinea-pig gastric muscle. Here, in an in vitro, guinea-p ig gastric muscle assay, pentagastrin (PG) and tetragastrin (TG) behav ed as high potency agonists and produced symmetrical concentration-eff ect curves. In contrast, cholecystokinin-octapeptide (CCK-8), while al so behaving as a high potency agonist, produced flat asymmetrical curv es. Unlike recent data reported using this tissue (Boyle et al., 1993) , the CCKA receptor-selective antagonist, devazepide (3, 10, 30 nM) pr oduced a rightward shift of the upper region of the CCK-8 curve render ing it biphasic. The lower phase was abolished by the CCKB/gastrin rec eptor-selective antagonist, L-365260 (300 nM) indicating that the cont ractile effects of CCK-8 in this tissue are mediated by both receptor types. 2 L-365260 produced a concentration-dependent, parallel rightwa rd displacement of PG concentration-effect curves. However, a flat Sch ild plot slope parameter (0.77 +/- 0.06) was; obtained. Therefore, an empirical pA(2) value of 8.64 +/- 0.21 was estimated from the smallest dose ratio. This value is consistent with published values characteri stic of an interaction at CCKB/gastrin receptors. 3 TG (1 mu M) was us ed to densensitize selectively the CCKB/gastrin receptors in the gastr ic muscle assay and thereby expose a population of receptors capable o f responding to subsequent stimulation by CCK-8 but not by PG. The sel ectivity of TG for CCKB/gastrin- over CCKA receptors was demonstrated by its low efficacy compared to CCK-8 in the guinea-pig gallbladder as say, a tissue shown previously to contain a homogeneous population of CCKA receptors. In TG-desensitized gastric muscle, CCK-8 concentration -effect curves were symmetrical and could be displaced in a simple par allel fashion by devazepide at nanomolar concentrations consistent wit h an interaction at CCKA receptors (pK(B) similar to 10). 4 These resu lts indicate that the guinea-pig gastric muscle contains both CCKA- an d CCKB/gastrin receptors and the effects of CCK-8 are mediated via bot h of these receptors. Notwithstanding the complexity of the behaviour of L-365260, it was possible to obtain a reasonable description of the system using a simple 2-receptor model in which the effects of indivi dual receptor activation were assumed to be additive. The absence of a simple competitive interaction of PG with L-365260 may indicate, for example, non-homogeneity of CCKB/gastrin receptors or lack of concentr ation equilibrium between the bath and the receptor biophase.