Organ-bath studies using the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM)

Effects of (±)-, (+)- and (—)-Celiprolol on the Rat Left Atria and Portal Vein

Differing effects of (±)-celiprolol at β-adrenoceptors have been reported. The effects of (±)-, (+)- and (—)-celiprolol on the contractile responses of the rat left atria and portal vein have therefore been studied. (±)-Celiprolol did not augment the responses of the atria to electrical stimulation and is therefore not an agonist at β1-adrenoceptors. Prolonged treatment with (±)-celiprolol attenuated the contractile activity of the vein and this effect was blocked by ICI 118,551 and is therefore mediated by agonism at β2-adrenoceptors. The pA2 values for (±)-, (+)- and (–)-celiprolol at the β1-adrenoceptors of the atria were 8·0, 6·0 and 8·6, respectively. On the portal vein, (±)- and (–)-celiprolol produced non-parallel rightward shifts of log isoprenaline attenuation curves with a reduction in isoprenaline maximum responses. This effect of (±)-celiprolol on the vein was not due to slowly reversible antagonism, as the inhibitory effect of (±)-celiprolol was readily reversible.

Irreversible binding of a carbostyril-based agonist and antagonist to the beta-adrenoceptor in DDT1 MF-2 cells and rat aorta

1. The chemoreactive ligands 5(2-(((1'-(4'-isothiocyanatophenylamino)thiocarbonyl)-amino) -2-methylpropyl)amino-2-hydroxypropoxy)-3,4-dihydrocarbostyril (DCITC) and 8-hydroxy-5(2-(((1'-(4'-isothiocyanatophenylamino)thiocarbonyl+ ++)amino)-2-methylprop-2-yl)amino-1-hydroxyethyl)-carbostyril++ + (HCITC) were synthesized and shown to be potent irreversible antagonist and agonist ligands, respectively, for the beta-adrenoceptor in DDT1 MF-2 (DDT) cells and the rat isolated aorta. 2. In DDT cell membranes DCITC and HCITC inhibited (-)[125I]-iodocyanopindolol (CYP) binding to the beta-adrenoceptor with IC50 values of 1.1 and 18 nM, respectively. (-)-Isoprenaline inhibited [125I]-CYP binding with an IC50 of 355 nM. Pretreatment of membranes with either chemoreactive ligand produced a time- and concentration-dependent decrease in the beta-adrenoceptor content, indicating irreversible receptor binding. DCITC at concentrations up to 10 microM did not stimulate cyclic AMP accumulation in DDT cells nor did it amplify forskolin-stimulated cyclic AMP accumulation. 3. In the rat isolated aorta, DCITC (0.1 microM) did not affect either the phenylephrine-mediated tissue contraction or the acetylcholine-mediated relaxation. DCITC attenuated the maximal (-)-isoprenaline-mediated relaxation of a phenylephrine contracted aorta in a concentration-dependent manner and shifted the dose-response curves for (-)-isoprenaline to the right. The DCITC-induced decrease in maximal response was not reversed by extensive tissue washing. By use of the operational model of agonism, the calculated dissociation constant for (-)-isoprenaline ws 286 nM and the estimated receptor reserve for this agonist was 23% at the maximal response. 4. HCITC and (-)-isoprenaline stimulated cyclic AMP accumulation in DDT cells with pD2 values (negative logarithm to base 10 of EC50) of 7.95 and 7.97, respectively, and both mediated the same maximal stimulation. In the rat isolated aorta, HCITC produced a concentration-dependent relaxation of the tissue with a pD2 value of 6.62, whereas the pD2 for (-)-isoprenaline was 7.03. However, HCITC produced a greater maximal relaxation of the tissue than (-)-isoprenaline. The HCITC-mediated stimulation of cyclic AMP accumulation and relaxation of the isolated tissue were blocked when the beta-antagonist propranolol was added concurrently. In contrast, once the HCITC-mediated responses were established, the addition of propranolol did not result in any attenuation indicating that HCITC is an irreversible beta-agonist.

Desensitization and functional antagonism by beta-adrenoceptor and muscarinic receptor agonists, respectively: a comparison with receptor alkylation for calculation of apparent agonist affinity

1. Apparent affinity constants (KD) for prenalterol, an agonist of low intrinsic efficacy at beta 1-adrenoceptors in rat left atria, have been determined by use of receptor desensitization and functional antagonism induced by isoprenaline and carbachol, respectively. The values obtained have been compared to those values estimated with the irreversible beta-adrenoceptor antagonist, bromoacetylalprenololmenthane (BAAM). 2. The -log KD values for prenalterol estimated by desensitization or irreversible antagonism ranged from 6.8-7.1 and 6.2-7.1, respectively. 3. Carbachol produced functional antagonism of the response to prenalterol even though it was removed before addition of prenalterol. This effect was mediated by M2-muscarinic receptors. Pretreatment of animals with pertussis toxin did not affect the functional antagonism elicited by carbachol. The apparent KD value obtained after pre-exposure to carbachol (6.8) was similar to those estimated by use of either alkylation with BAAM or desensitization with isoprenaline (see above). 4. It is concluded that acute desensitization or functional antagonism of responses to agonists of low intrinsic efficacy provides a means to estimate apparent KD constants. This approach could be useful to characterize receptors for which an irreversible antagonist may not be available.

Effects of bromoacetylalprenololmenthane (BAAM), an irreversible beta-adrenoceptor antagonist, on the rat isolated left atria and portal vein

1. The effects of treatment with bromoacetylalprenololmenthane (BAAM), an irreversible beta-adrenoceptor antagonist, for 30 min on the contractile responses of the rat left atria and portal vein have been determined. 2. On the electrically driven rat left atria, BAAM at 10(-5) M followed by a 60-min wash-out caused parallel rightward shifts of the isoprenaline response curves with no reduction in the maximal response to isoprenaline. 3. Treatment of the atria with BAAM at 3 x 10(-5) M caused non-parallel rightward shifts of isoprenaline response curves with a reduction in the maximum response to isoprenaline. The KA (dissociation constant) for isoprenaline at beta 1-adrenoceptors was 7.3 x 10(-7) M. Calculation of receptor occupancy demonstrated that in order to produce a maximal response of the rat left atria, isoprenaline had to occupy 12% of the beta 1-adrenoceptors. 4. Treatment of the atria with higher concentrations of BAAM (greater than or equal to 6 x 10(-5) M) caused a non-specific action, the reduction of the response to electrical stimulation. 5. On the rat portal vein, BAAM at 3 x 10(-7)-3 x 10(-6) M followed by a 45-min wash-out had no effect on the spontaneous contractile activity, caused non-parallel rightward shifts of isoprenaline response curves with a depression of the isoprenaline maximum responses. Using data derived from experiments with 3 x 10(-7), 10(-6) and 3 x 10(-6) M BAAM, the KA for isoprenaline at beta 2-adrenoceptor was 6.4, 3.7 and 7.7 x 10(-7) M and the calculated receptor occupancy was that in order to produce a maximal response of the rat portal vein, isoprenaline had to occupy 14, 21 and 12%, respectively of the beta 2-adrenoceptors. 6. The present study with the rat left atria and portal vein has shown that it is possible to determine treatments with BAAM that produce no non-specific actions and consequently BAAM may be used as an experimental tool in KA and receptor occupancy determinations in these tissues.

Affinities of full agonists for cardiac beta-adrenoceptors calculated by use of in vitro desensitization

Positive chronotropic and inotropic responses of guinea-pig isolated spontaneously beating right atria and paced left atria to beta-adrenoceptor agonists were recorded. Cumulative concentration-response curves for the rate and tension responses to isoprenaline, orciprenaline, terbutaline and fenoterol were obtained before incubation of the tissues with isoprenaline (10(-6) M), the tissues were then washed during 1 h to remove isoprenaline before constructing a post-incubation curve to the same agonist. Incubation with isoprenaline for 4 h caused parallel rightwards shifts to the curves, but extending the incubation to 8 h caused additional depression of the rate (85.2 +/- 5.4%) and tension (68.9 +/- 2.3%) maxima. Incubation with isoprenaline for 4 h only shifted the curves for orciprenaline to the right but depressed the post-incubation maximum rate and tension responses to terbutaline (74.8 +/- 1.5 and 33.8 +/- 2.5%) and fenoterol (85.6 +/- 5.5 and 76.0 +/- 5.1%). Thus incubation with isoprenaline for 4 h induced desensitization of the cardiac beta-adrenoceptor which was revealed as a loss in sensitivity to both isoprenaline and other beta-adrenoceptor agonists. The reduced maxima were attributed to a loss of beta-adrenoceptors and exhaustion of the receptor reserve. Those experiments displaying the reduced maxima were used for calculation of dissociation constants (KA) values by the method of Furchgott (1966) for irreversible antagonism. The values for the right atria were 57 (19-170)nM, 29 (8.4-100)microM and 13 (1.9-8.4)microM and for the left atria, 89 (15-510)nM, 25 (9.9-64)microM and 18 (7.7-42)microM for isoprenaline (8 h incubation), terbutaline and fenoterol respectively.(ABSTRACT TRUNCATED AT 250 WORDS)