Mechanical responses to catecholamines in isolated strips of the guinea-pig stomach muscle

Effects of catecholamines on isolated human colonic smooth muscle

1. The effects of catecholamines and some adrenoceptor agonists and antagonists on isolated preparations of human colonic smooth muscle obtained from surgical resections were examined. 2. Strips of circular smooth muscle displayed rhythmic myogenic spontaneous contractions which were inhibited by catecholamines with an order of potency of isoprenaline (1.0) > noradrenaline (0.32) > adrenaline (0.2). Phentolamine (0.7 microM) significantly shifted the noradrenaline concentration-response curve (CRC) to the right but had no significant effect on isoprenaline or adrenaline. Propranolol (1 microM) significantly shifted the isoprenaline to the right but had no significant effect on noradrenaline or adrenaline. 3. Salbutamol (30 microM) had no inhibitory effect on the spontaneous activity and ICI 118,551 (1 microM) had no effect on inhibitory responses to isoprenaline. Betaxolol (1 microM) significantly shifted the CRC to isoprenaline to the right. BRL 37344 had no effect on spontaneous activity. 4. Responsiveness of circular strips to catecholamines was not affected by age of the patient and no consistent differences between males and females were shown. 5. Strips of taenia coli exhibited little or no spontaneous phasic activity. Noradrenaline and isoprenaline relaxed KCl-induced tone. The effects of noradrenaline and isoprenaline were antagonized by propranolol but not by phentolamine. BRL 37344 had no effect on KCl-induced tone. 6. In conclusion, catecholamines relaxed spontaneous activity of human colon circular smooth muscle through an action on both alpha- and beta-adrenoceptors. The alpha-adrenoceptors were of the alpha 1-subtype. The beta-adrenoceptor-mediated relaxation appeared to be primarily beta 1. In taenia coli, catecholamines relaxed KCl-induced tone via beta-adrenoceptors only.

Receptors involved in mechanical responses to catecholamines in the circular muscle of guinea-pig stomach treated with meclofenamate

1. In circular muscle strips of the fundus and corpus of guinea-pig stomach, mechanical responses to catecholamines were studied mainly in the presence of a prostaglandin biosynthesis inhibitor, meclofenamate. 2. Normal preparations developed considerable muscle tone, and adrenaline (10-100 microM) in the presence of 3-5 microM propranolol produced a multiphasic response, generally consisting of transient relaxation and contraction, followed by slow relaxation and then contraction. Responses to phenylephrine were similar to those of adrenaline. 3. Meclofenamate (0.3 microM) nearly abolished the muscle tone and under this condition, both adrenaline and phenylephrine produced a simple contraction. This response was strongly inhibited by prazosin, but only weakly by yohimbine. 4. When muscle tone was maintained by prostaglandin E2 (10 nM) in the presence of meclofenamate, phenylephrine (30 microM) produced transient relaxation followed by slow contraction in most preparations. These were strongly inhibited by prazosin. Adrenaline produced a similar response, but the relaxation was only partially reduced by prazosin. The remaining relaxation was more dominant in the middle fundic region and this was considered to be mediated through beta-adrenoceptors. 5. It is concluded that in the circular muscle of the fundic region of guinea-pig stomach, endogenous prostaglandins are involved in maintaining muscle tone and in modifying the response to catecholamines and that both contraction and relaxation are mediated by alpha 1-adrenoceptors.

Different innervation mechanisms between the lesser and greater curvature of guinea pig antrum

Transmural stimulation (TS) produced a frequency-dependent contraction of the longitudinal muscles from the lesser curvature of the guinea pig antrum, which was abolished by atropine. On the other hand, a response to TS of the strips from the greater curvature was biphasic: a rapid contraction followed by a relaxation, which was abolished by tetrodotoxin. By pretreatment with atropine, rapid contraction of the biphasic response evoked by TS in the greater curvature was abolished and relaxation was augmented. Relaxation to TS of the greater curvature was not affected by prazocine, yohimbine, phentolamine, propranolol, theophylline, apamin, alpha-chymotrypsin, and vasoactive intestinal polypeptide receptor antagonist. Different innervation mechanisms were suggested to be present in the longitudinal muscles between the lesser curvature (innervated with excitatory cholinergic neurons) and the greater curvature (innervated with excitatory cholinergic neurons and nonadrenergic inhibitory neurons) of the guinea pig antrum.

Mechanical and electrical responses to alpha-adrenoceptor activation in the circular muscle of guinea-pig stomach

1 In the circular muscle of the corpus region of the guinea-pig stomach, the effects of catecholamines on mechanical activity were studied with simultaneous recording of membrane potential by use of intracellular microelectrodes. In order to investigate responses mediated through alpha-adrenoceptors, the beta-adrenoceptors were blocked by propranolol (10(-6) M) in most experiments. 2 Adrenaline (less than 10(-6) M) produced a monophasic contraction with little change in membrane potential. At higher concentrations (greater than 10(-5) M), adrenaline usually produced an early transient contraction followed by a slow tonic contraction. During the early phase, the membrane was hyperpolarized and slow waves were reduced in amplitude. 3 The phasic contractions superimposed on the late slow phase of the contractile response were higher in frequency and larger in amplitude than in controls before adrenaline application. These changes were associated with an increase in the amplitude and frequency of slow waves, and with a spike-like component appearing on the top of each slow wave. 4 Adrenaline-induced changes in mechanical and electrical activity were inhibited by prazosin, but largely unaffected by yohimbine. In some preparations, there was a transient weak inhibition of phasic contraction before muscle tone was increased by adrenaline, and this became more pronounced in the presence of yohimbine. 5 Phenylephrine, selective for alpha 1-receptors, produced responses very similar to those of adrenaline, whereas agonists selective for alpha 2-receptors, clonidine and B-HT 920, produced only a small slow contraction without any clear change in membrane potential. 6 It is concluded that in the circular muscle of guinea-pig stomach, the contractile response and changes in the electrical slow wave activity produced by adrenaline are both mediated mainly through the activation of alpha-receptors.

[Effects of precursors and metabolites of catecholamines on motricity of isolated rat duodenum. Particular reference to dopamine]

Studying the effects, on the isolated rat duodenum motricity, of ten compounds precursors or metabolites of catecholamines, the following results were obtained: The direct metabolites of epinephrine and norepinephrine (metanephrine, normetanephrine), are either ineffective at concentrations below 5 X 10(-6) M, or weakly inhibitory at higher concentrations. Such inhibitory effects are prevented by alpha- and beta-blockers. 3-methoxy, 4-hydroxyphenylglycol and vanylmandelic acid have no significant effect. The catecholamine precursor, dopamine, the related compounds DOPA, 3 methoxytyramine, and to a lesser extent, 3-O methyl DOPA and homovanillic acid, have excito-motor effects at concentrations ranging mainly from 10(-7) M to 10(-5) M. At higher concentrations, the same compounds frequently exhibit inhibitory effects. The excito-motor effects might be due to a serotoninergic mechanism, since they are suppressed by the serotoninergic blocking agents methysergide and cyproheptadine. Furthermore, in the case of DOPA, we were able to establish a relationship between the excito-motor effects and duodenal serotonin stores. As for the inhibitory effects, they may be prevented by using alpha and beta blocking agents. Dihydroxyphenylacetic acid has no effect on the isolated rat duodenum motricity. The fact that dopamine and related compounds may have excitomotor effects at some concentrations, correlated with some physiopathological observations in man and animal allows some considerations about the eventual role of dopamine on intestinal motricity.