Autoradiographic location of beta-adrenoceptor subtypes in cat colon smooth muscle

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.

Postjunctional alpha 1- and beta-adrenoceptor effects of noradrenaline on electrical slow waves and phasic contractions of cat colon circular muscle

1. The postjunctional excitatory and inhibitory effects of noradrenaline and selective alpha 1- and beta-adrenoceptor agonists on electrical and mechanical activity of cat colon muscle strips were studied by microelectrode recordings and isometric force measurements. Experiments were performed in the presence of tetrodotoxin (0.5 microM) or atropine (0.5 microM). 2. Circular muscle cells near the submucosal border had a mean resting membrane potential of -76.1 +/- 1.2 mV and exhibited electrical slow waves at frequencies of 4-6 cycles min-1. The mean values of electrical slow wave components were: upstroke potential, -40.7 +/- 1.2 mV; plateau potential, -43.7 +/- 0.8 mV; and duration, 4.9 +/- 0.4 s. Electrical slow waves were in phase with rhythmic contractions of the circular muscle layer. Muscle cells near the myenteric border had a mean testing membrane potential of -51.1 +/- 5.5 mV and did not exhibit electrical slow waves. 3. Noradrenaline (1 microM) increased the duration of electrical slow waves. This effect was inhibited by prazosin (1 microM) and potentiated by propranolol (5 microM), indicating activation of alpha 1- and beta-adrenoceptors. Also, when alpha 1-adrenoceptors were irreversibly blocked by phenoxybenzamine (1 microM), noradrenaline decreased the duration of electrical slow waves. Phenylephrine (1 microM), a selective alpha 1-adrenoceptor agonist, and isoprenaline (1 microM), a beta-adrenoceptor agonist, increased or decreased the duration of electrical slow waves, respectively. 4. Phenylephrine (0.01-5 microM) caused a linear increase in the area of electrical slow waves and phasic contractions but did not affect resting membrane potential or resting muscle tension. Higher concentrations of phenylephrine (5-50 microM) depolarized the resting membrane potential (2-6 mV) and increased muscle tone. 5. Nitrendipine or verapamil (each at 5 microM) reduced the amplitude of the upstroke potential and nearly abolished the plateau phase of the electrical slow waves. In the presence of L-type Ca2+ antagonists, noradrenaline (1-10 microM) or phenylephrine (1-100 microM) had no effect on electrical slow waves and phasic contractions. This indicates that the effects of noradrenaline and phenylephrine involve the influx of extracellular Ca2+ through voltage-dependent L-type Ca2+ channels. 6. Ryanodine, an alkaloid that depletes intracellular Ca2+ stores nearly abolished phasic contractions. In muscle strips, pretreated with ryanodine (10 microM for 30 min), phenylephrine (1 microM) increased and isoprenaline (1 microM) decreased the duration of electrical slow waves but neither was able to reverse the ryanodine-suppressed phasic contractions. This suggests that adrenoceptor effects on electrical slow waves are coupled to contractions via Ca2+ release from ryanodine-sensitive intracellular stores. 7. In summary, noradrenaline activates postjunctional alpha 1- and beta-adrenoceptors. Activation of alpha 1-adrenoceptors increases the magnitude of electrical slow waves and phasic contractions, whereas activation of beta-adrenoceptors decreases them. The alpha 1-adrenoceptor mediated effects on electrical slow waves and phasic contractions require the influx of Ca2+ through voltage-gated L-type Ca2+ channels. Phasic contractions also involve Ca2+ release from ryanodine-sensitive intracellular stores.

Contractions mediated by alpha 1-adrenoceptors and P2-purinoceptors in a cat colon circular muscle

1. The postjunctional excitatory and inhibitory effects of adrenoceptor and purinoceptor agonists and antagonists were studied in circular smooth muscle strips of cat colon. 2. In the presence of tetrodotoxin (0.5 microM), noradrenaline caused contraction or relaxation of circular smooth muscle at resting tension or with raised tone, respectively. The noradrenaline-evoked contractions were potentiated and the noradrenaline-evoked relaxations were antagonized by propranolol (1 microM), suggesting beta-adrenoceptor involvement. 3. At resting tension, noradrenaline, adrenaline and the selective alpha 1-adrenoceptor agonist, phenylephrine, caused concentration-dependent contractile responses, with EC50 values of 1.8 +/- 0.2 microM, 1.9 +/- 0.4 microM and 4.3 +/- 1.7 microM, respectively. The EC50 values and the amplitude of maximal responses were not significantly different from one another. Clonidine (0.1-500 microM), a selective alpha 2-adrenoceptor agonist, was not effective. 4. Prazosin (0.1-9 microM), competitively antagonized the contractile effects of noradrenaline with an estimated pA2 value of 6.93 and a slope of 1.07 +/- 0.03. The Kb values, estimated from a single shift (0.1 microM prazosin) of the concentration-response curves to noradrenaline, adrenaline and phenylephrine were 92.8 +/- 9.3 nM, 108.7 +/- 6.4 nM and 18.4 +/- 3.1 nM, respectively. 5. At resting tension, adenosine 5' triphosphate (ATP, 5-1000 microM), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP, 0.05-50 microM), beta,gamma-methylene adenosine 5'-triphosphate (beta,gamma-MeATP, 0.5-100 microM), and 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 1-500 microM) caused concentration-dependent contractions with EC50 values of 60.5 +/- 15.9 microM, 0.7 +/- 0.1 microM, 7.6 +/- 0.1 microM and 25.3 +/- 12.8 microM, respectively. The maximal responses to alpha, beta-MeATP and beta,gamma-MeATP were greater than maximal responses to 2-MeSATP and ATP.6. Suramin (50-500 microM), competitively antagonized the contractile responses of alpha,beta-MeATP with an estimated pA2 value of 4.92 and a slope of 1.08 +/- 0.04. The Kb values, estimated from a single shift(1I00 microM suramin) of the concentration-response curves to ATP, alpha,beta-MeATP, beta,gamma-MeATP and 2MeSATPwere 52.3 +/- 20.2 microM, 25.2 +/- 4.5 microM, 21.7 +/- 11.0 microM and 11.6 +/- 2.7 microM, respectively.7. At resting tension, reactive blue 2 (100 microM), a selective antagonist of the P2Y-purinoceptor, and 8-(p-sulphophenyl)-theophylline (8-SPT) (1 microM), a selective antagonist of the PI-purinoceptor, did not antagonize the contractile responses to alpha,beta-MeATP (0.5-5 microM). Contractile responses to ATP(50-500 microM) were not altered by 8-SPT (I microM) but were potentiated by reactive blue 2 (100 microM).8. With raised tone, ATP and 2-MeSATP caused a relaxant effect. This effect of ATP was not altered by either tetrodotoxin (TTX) (0.5 microM) or suramin (100 microM), but was antagonized by reactive blue 2(100 microM) and 8-SPT (1 microM), suggesting that inhibitory P2y- and P1-purinoceptors are involved. In contrast, alpha,beta-MeATP and Beta,gamma-MeATP caused only contractions. This contractile effect of alpha,beta-MeATPwas resistant to TTX (0.5 microM) and antagonized by suramin (100 microM).9. In summary, cat colon circular muscle contains postjunctional alpha 1-adrenoceptors and P2X purinoceptors which mediate contractions and P2y- and PI-purinoceptors which mediate relaxation.Postjunctional alpha2-adrenoceptors appear not to be present.