Pharmacological characterization of alpha adrenoceptors in the rat gastric fundus.

Alpha receptors on the intramural cholinergic neurons and on the smooth muscle cells are involved in the inhibitory effect of catecholamines on rat gastric fundus motility. The pharmacological characteristics of these alpha receptors were assessed using longitudinal muscle strips of the rat gastric fundus, contracted to a similar degree by electrical stimulation and by methacholine. All alpha agonists studied (norepinephrine, phenylephrine, methoxamine, clonidine, UK-14,304 and B-HT 920) concentration-dependently inhibited the stimulation-induced contractions. Norepinephrine, phenylephrine, methoxamine and clonidine also inhibited the methacholine-induced contractions, but for the same concentration of agonist, the inhibition was less pronounced than during electrical stimulation-induced contractions; UK-14,304 and B-HT 920 inhibited the methacholine-induced contractions only in a concentration of 10(-4) M. The effect of clonidine and UK-14,304 on electrical stimulation-induced contractions was antagonized competitively by the alpha antagonists rauwolscine and yohimbine (slope in the Schild plot not different from 1). The effect of norepinephrine and phenylephrine on methacholine-induced contractions was antagonized by the alpha antagonists prazosin, corynanthine and yohimbine; against phenylephrine, the antagonism was competitive (slope in the Schild plot not different from 1). It is concluded that the muscular alpha receptors in the rat gastric fundus are of the alpha-1-type. On the postganglionic cholinergic neurons, alpha-2-like receptors are present; it is not yet clear whether the pronounced effect of alpha-1 agonists on the cholinergic neuron activity is due to interaction with these receptors.

Pharmacological characterization of the postjunctional beta-adrenoceptors in the rat gastric fundus.

In order to characterize the postjunctional beta-adrenoceptors in the rat gastric fundus, we studied the influence of beta-agonists and beta-antagonists on methacholine-contracted fundus strips. The mixed beta-agonist isopropylnoradrenaline and the beta 2-selective agonist fenoterol had a concentration-dependent relaxing effect and at higher concentrations completely inhibited the methacholine-induced tone. The reputedly beta 1-selective agonist prenalterol only produced about 50% inhibition and another reputedly beta 1-selective agonist, tazolol, had almost no relaxing effect. The beta-antagonists propranolol (beta 1 + beta 2), practolol (beta 1), H35/25 (beta 2) and ICI 118,551 (beta 2) all shifted the concentration-response curves for isopropylnoradrenaline and fenoterol in a parallel way to the right, but the slope of the Schild plot was not significantly different from 1 only for the antagonism of isopropylnoradrenaline by H35/25. The relaxing effect of prenalterol was only clearly antagonized by ICI 118,551. The results suggest that postjunctional beta 1- and beta 2-adrenoceptors are present in the rat gastric fundus.

Effects of dopamine and apomorphine on the response of rabbit isolated atria to sympathetic nerve stimulation.

In isolated rabbit atria, dopamine (10(-6) M to 3 X 10(-6) M), in the presence of cocaine and atropine, inhibits the chronotropic responses to electrical stimulation of the sympathetic nerves without influencing the responses to isoprenaline. The inhibitory effect of dopamine is antagonized by cis-flupenthixol in a concentration (2.5 X 10(-6) M) that does not antagonize the inhibitory effect of clonidine. Phentolamine, in a concentration (10(-5) M) that antagonizes clonidine, does not influence the inhibitory effect of dopamine. Apomorphine (10(-6) M) also produces an inhibition of the nerve stimulation-induced chronotropic responses that can be prevented by cis-flupenthixol. These data suggest that prejunctional dopamine receptors are present in rabbit atria.

The effects of partial hypoblast removal on the cell morphology of the epiblast in the chick blastoderm.

The hypoblast of early-primitive-streak-stage chick blastoderms was partially removed. This experiment provokes a reaction in the epiblast which curls up and becomes even at its ventral surface. The basal lamina underlying the epiblast is also dependent upon the presence of hypoblast. During culture after partial hypoblast removal, active hypoblast wound healing is observed. Where the hypoblast underlies the epiblast again, the effects of the removal disappear and normal development proceeds. The results suggest that the normal epiblast morphology is dependent upon the presence of hypoblast. This influence of hypoblast on epiblast is thought to be concerned with the morphology of the epiblast and not directly with its morphogenesis.