Partial agonists at guinea-pig atrial beta-adrenoceptors display relaxation responses in the guinea-pig ileum independent of beta-adrenoceptor stimulation

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

In vitro study of a novel atypical beta-adrenoceptor agonist, SM-11044

SM-11044 (L-threo-3-(3,4-dihydroxyphenyl)-N-[3-(4-fluorophenyl) propyl] serine pyrrolidine amide hydrobromide) stimulated the relaxation of guinea pig ileum (EC50: 3.0 x 10(-8) M), trachea (EC50: 1.3 x 10(-7) M), lung parenchyma (EC50: 2.1 x 10(-6) M) and the increase in right atrial rate (EC50: 6.9 x 10(-6) M). It also induced lipolysis of rat white adipocytes (EC50: 1.2 x 10(-6) M). Both the relaxant response of ileum and the lipolytic response of adipocytes to SM-11044 were resistant to inhibition by propranolol (10(-6) M), but were antagonized by cyanopindolol (10(-6) M). In contrast, the responses to SM-11044 in trachea, lung parenchyma and right atrium were almost completely abolished by propranolol (10(-6) M). Furthermore, the selectivity of SM-11044 relative to isoproterenol was ileum greater than adipocytes greater than trachea (beta 2) greater than lung (beta 2) greater than atrium (beta 1). These results suggest that SM-11044 is a selective agonist of atypical beta-adrenoceptors that are resistant to antagonism by propranolol but sensitive to cyanopindolol. The receptor binding and adenylate cyclase stimulating activity of SM-11044 were also examined on transfected Chinese hamster ovary cells expressing human beta 1-, beta 2- or beta 3-adrenoceptors. SM-11044 inhibited the binding of [125I]iodocyanopindolol to the three types of receptors in a concentration-dependent manner. The selectivity in terms of Ki values was beta 3 (Ki: 1.3 x 10(-6) M) greater than beta 2 (Ki: 4.1 x 10(-6) M) greater than beta 1 (Ki: 18.1 x 10(-6) M)-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of tertatolol at the “atypical” or beta3-adrenoceptor in guinea-pig ileum

1. Experiments were performed to investigate the possible interaction between tertatolol and the "atypical" beta-adrenoceptor present in guinea-pig ileum. 2. In the electrically field stimulated guinea-pig ileum, both isoprenaline and BRL-37344 produce concentration-dependent inhibition of the "twitch" response. 3. Schild regression analysis of the interaction between isoprenaline and tertatolol indicated two sites of action and yielded an apparent pA2 value for tertatolol of 6.8 at the "atypical" beta-adrenoceptor. 4. BRL-37344 selectively activated the "atypical" beta-adrenoceptor and tertatolol again yielded an apparent pA2 value of 6.7 and 6.8.

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

1. The effects of the irreversible beta-adrenoreceptor antagonist bromoacetylalprenololmenthane (BAAM) were studied in isolated cardiac and uterine preparations from guinea-pigs and rats and in guinea-pig ileal preparations. 2. In the presence of BAAM (0.1-10 microM) concentration-effect curves to (-)-isoprenaline were shifted to the right in a concentration-dependent manner in all cardiac and uterine tissues. Maximum responses to (-)-isoprenaline were unaffected by BAAM except in guinea-pig left atrial and in some guinea-pig uterine preparations; however, the reductions in the maximum responses were not concentration-dependent. The mean pKB values for BAAM in guinea-pig left atria, right atria, rat whole atria and rat uterus were 7.26, 7.24, 6.84 and 7.90 respectively. 3. In guinea-pig ileal preparations, BAAM (0.1-30 microM) relaxed contractions induced by K+, histamine and acetylcholine in a non-beta-adrenoreceptor-related manner since relaxant responses were unaffected by propranolol (0.5 microM). In other tissues higher concentrations of BAAM (30-100 microM) elicited atrial standstill and depressed K+-induced contractions in uterine smooth muscle. 4. Treatment of tissues with BAAM (10-100 microM) followed by extensive wash-out increased the EC50 values for (-)-isoprenaline 21- to 83-fold. The maximum response to the catecholamine was unaffected by BAAM except in guinea-pig left atrial preparations following treatment with 100 microM BAAM. At these concentrations BAAM markedly increased the effective refractory period. 5. Concentration-effect curves for the partial agonist, oxymethylene-isoprenaline (OM-ISO) were shifted to the right 12- to 355-fold after pretreatment of tissues with BAAM (10-30 microM) followed by wash-out. The maximum response to OM-ISO was unaltered in guinea-pig and rat uteri and was reduced to a similar degree as observed with (-)-isoprenaline in guinea-pig left atria. 6. In general, the non-selective beta-adrenoreceptor antagonist BAAM depressed maximum responses to beta-adrenoreceptor agonists only in cardiac preparations and at concentrations which elicited depressant activity. On the basis of the present study, BAAM does not appear to be a suitable irreversible beta-adrenoreceptor antagonist for use in organ bath experiments.

Agonist and antagonist characterization of a putative adrenoceptor with distinct pharmacological properties from the alpha- and beta-subtypes

1. Experiments were done to characterize a putative adrenoceptor which functions to inhibit longitudinal muscle tension development in the guinea-pig ileum. Several phenylethylamine based agonists were investigated: BRL 37344, (-)-isoprenaline, (+)-isoprenaline, noradrenaline, adrenaline, and fenoterol. Propranolol and nadolol were tested as antagonists. Agonist-induced inhibition of the contractile response to histamine was measured under equilibrium conditions with alpha-adrenoceptors and muscarinic cholinoceptors inhibited. 2. Inhibitory responses were obtained to (-)-isoprenaline and BRL 37344 that were resistant to beta-adrenoceptor blockage with propranolol (5 microM) and nadolol (10 microM). These resistant responses were antagonized by much higher concentrations of nadolol (30 to 1000 microM) yielding apparent pA2 values for nadolol of 4.31 with (-)-isoprenaline as the agonist, and 4.68 with BRL 37344 as the agonist. Similar apparent pA2 values for nadolol at the putative adrenoceptor were obtained with noradrenaline (4.79), adrenaline (4.68), and fenoterol (4.38). 3. The order and relative potency of agonists at the putative adrenoceptor was: BRL 37344 (20) greater than (-)-isoprenaline (8) greater than noradrenaline (1) greater than adrenaline (0.5) greater than fenoterol (0.35) greater than (+)-isoprenaline (0.27). 4. The resistance to blockade by propranolol (5 microM), the low affinity of nadolol, and the order and relative potency of agonists, suggest the presence of an adrenoceptor with distinct pharmacological characteristics from currently defined alpha- and beta-adrenoceptors.