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8.70. 3 Overall, the data could be accounted for by assuming the variable expression of two receptor subtypes across the assays. The rat stomach appeared to express a single receptor characterized by a low affinity constant for L-365,260 (pKB-7.5). The guinea-pig gastric muscle and mouse stomach data could be explained by the presence of this receptor and a second one characterized by a high affinity constant for ). The activity of the two proposed receptor subtypes was consistent between experiments in the guinea-pig and the high affinity receptor appeared to be predominant. In contrast, the mouse stomach data could only be simulated by assuming that the proportion and absolute number of each subtype varied significantly between the replicate experiments. 4 The L-365,260 affinity estimates at the inferred receptor subtypes were indistinguishable from those obtained in a corresponding analysis of the behaviour of L-365,260 in CCKB/gastrin receptor radioligand binding experiments in guinea-pig gastric gland and mouse and rat cerebral cortex preparations.

No abstract

James Black Foundation, 68 Half Moon Lane, Dulwich, London SE24 9JE, England 1 The previously described complex behaviour of the CCK B /gastrin receptor antagonist, L-365,260, in radioligand binding assays could be explained by a variable population of two binding sites. We have investigated whether other CCK B /gastrin receptor ligands (PD134,308, PD140,376, YM022 and JB93182) can distinguish between these sites. 2 In the mouse cortex assay, Hill slopes were not dierent from unity and the ligand pK I values did not dier when either [ I]-BH-CCK-8S or [3 H]-PD140,376 was used as label as expected for a single site (G 2 ). 3 In the rat cortex, where previous analysis of replicate (n=48) L-365,260 data indicated the presence of two CCK B /gastrin sites (G 1 and G 2 ), the competition data for PD134,308, PD140,376, YM022 and JB93182 could be explained by a homogeneous population of CCK B /gastrin sites because the Hill slope estimates were not signi®cantly dierent from unity. However, the estimated anity values for JB93182 and YM022 were signi®cantly higher and that for PD134,308 was signi®cantly lower than those obtained in the mouse cortex when the same radioligand was used. In view of our previous data obtained with L-365,260, the rat cortex data were also interpreted using a two-site model. In this analysis, SR27897 expressed *9 fold, PD134,308 *13 fold and PD140,376 *11 fold selectivity for the G 2 site. In contrast, JB93182 expressed *23 fold and YM022 *4 fold selectivity for the G 1 site. If the two-site interpretation of the data is valid then, because of its reverse selectivity to L-365,260, JB93182 has been identi®ed as a compound which if radiolabelled could provide a test of this receptor subdivision.

Lane, London SE5 9NU and 2 James Black Foundation, 68 Half Moon Lane, Dulwich, London SE24 9JE1 Left atrial strips from transgenic (TG4) mice with cardiac-specific overexpression (B200-fold) of the b 2 adrenoceptor (b 2 AR) were isolated, and their isometric force of contraction (F c ) in response to electrical stimulation was measured. 2 The bAR agonist isoprenaline elicited negative inotropic responses in all left atrial strips; in 6/11 preparations, it also had a small positive inotropic effect. This 'up-phase' was observed from 0.1 to 10 nM, with the 'down-phase' occurring at higher concentrations. Both phases were mediated by b 2 AR, as shown by their sensitivity to the b 2 AR antagonist ICI-118,551 (100 nM; pA 2 8.6070.07, 8.4570.19, for 'up-phase' and 'down-phase,' respectively), but not the b 1 AR antagonist CGP-20712A (100 nM). Conversely, nontransgenic littermate preparations responded to isoprenaline treatment solely by an increase in F c , which was b 1 AR-mediated. 3 Pretreatment of left atrial strips with either 10 nM isoprenaline or 1 mM 8-bromo-cAMP significantly attenuated the TG4 'up-phase', while having no effect on either the TG4 'down-phase' or the littermate controls' responses. B. pertussis toxin treatment of the animals prevented isoprenaline's negative inotropic effects in TG4 preparations, but had no effect in littermate controls. 4 The findings imply that the responses of TG4 left atrium to isoprenaline are because of b 2 AR coupling to G s and G i proteins, consistent with the model of Daaka et al., in which protein kinase A phosphorylation of the b 2 AR causes a switch from G s to G i protein coupling.