The use of receptor desensitization to analyse 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 to the involvement of specific cholecystokinin (CCKA) and CCKB/gastrin receptors in guinea‐pig gastric muscle. Here, in an in vitro, guinea‐pig gastric muscle assay, pentagastrin (PG) and tetragastrin (TG) behaved as high potency agonists and produced symmetrical concentration‐effect curves. In contrast, cholecystokinin‐octapeptide (CCK‐8), while also behaving as a high potency agonist, produced flat asymmetrical curves. Unlike recent data reported using this tissue (Boyle et al., 1993), the CCKA receptor‐selective antagonist, devazepide (3, 10, 30 nm) produced a rightward shift of the upper region of the CCK‐8 curve rendering it biphasic. The lower phase was abolished by the CCKB/gastrin receptor‐selective antagonist, L‐365260 (300 nm) indicating that the contractile effects of CCK‐8 in this tissue are mediated by both receptor types. 2 L‐365260 produced a concentration‐dependent, parallel rightward displacement of PG concentration‐effect curves. However, a flat Schild plot slope parameter (0.77 ± 0.06) was obtained. Therefore, an empirical pA2 value of 8.64 ± 0.21 was estimated from the smallest dose ratio. This value is consistent with published values characteristic of an interaction at CCKB/gastrin receptors. 3 TG (1 μm) was used to densensitize selectively the CCKB/gastrin receptors in the gastric muscle assay and thereby expose a population of receptors capable of responding to subsequent stimulation by CCK‐8 but not by PG. The selectivity of TG for CCKB/gastrin‐ over CCKA receptors was demonstrated by its low efficacy compared to CCK‐8 in the guinea‐pig gallbladder assay, 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 parallel fashion by devazepide at nanomolar concentrations consistent with an interaction at CCKA receptors (pKB ∼ 10). 4 These results indicate that the guinea‐pig gastric muscle contains both CCKA‐ and CCKB/gastrin receptors and the effects of CCK‐8 are mediated via both 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 individual receptor activation were assumed to be additive. The absence of a simple competitive interaction of PG with L‐365260 may indicate, for