M.C. Bernardini

Differential Affinities of AF-DX 116, Atropine and Pirenzepine for Muscarinic Receptors of Guinea Pig Gastric Fundus, Atria and Urinary Bladder: Might Atropine Distinguish among Muscarinic Receptor Subtypes?

The pA2 values and the Schild plots of the antimuscarinic drugs AF-DX 116, atropine and pirenzepine for muscarinic receptors of isolated guinea pig gastric fundus (acid secretion) and atrial and urinary bladder preparations (contractile force) obtained from the same animals were calculated against bethanechol as the agonist. The antimuscarinic drugs concentration-dependently shifted the concentration-response curves to bethanechol to the right without any change in the maximum response. The analysis of data based on Schild plots was consistent with a simple competitive antagonism, since regression slopes did not differ significantly from unity. The pA2 values indicated a significantly higher affinity of AF-DX 116 and atropine for atrial muscarinic receptors with respect to those of the gastric mucosa or urinary bladder. By contrast, in the case of pirenzepine the pA2 values for the three tissues did not differ significantly. These results suggest that each examined tissue apparently contains homogeneous population of acetylcholine muscarinic (M2) receptors. The pA2 values found for AF-DX 116 and atropine suggest, however, that the putative M2 subtype of atrial muscarinic receptor differs from both those of the gastric fundus and those of the urinary bladder.

Phaclofen-Sensitive GABA-B Receptors Do Not Mediate Excitatory Effects of Baclofen on Gastric Secretion

The present study investigates the effects of centrally or peripherally administered baclofen on gastric acid secretion from pylorus-ligated rats. The influence of baclofen on basal acid secretion from isolated guinea-pig gastric fundus was also evaluated. At all doses employed, intracerebroventricular baclofen significantly decreased acid secretion, this inhibitory action being antagonized by intracerebroventricular phaclofen. Intravenous baclofen induced both gastric inhibitory and excitatory responses at low and high doses, respectively. Intracerebroventricular phaclofen prevented the inhibitory effect, while neither intracerebroventricular nor intravenous phaclofen modified the stimulant action of parenteral baclofen. Both central and parenteral muscimol did not influence gastric acid secretion. Moreover, baclofen or muscimol were without effect on basal acid secretion from isolated guinea-pig gastric fundus, whereas bethanechol caused a marked and concentration-dependent stimulant action. The present results provide further evidence for the inhibitory role of central phaclofen-sensitive GABA-B receptors in the regulation of acid secretion. In addition they indicate that the hypersecretory effect exerted by parenteral baclofen may depend upon the activation of putative peripheral non-A and phaclofen-insensitive GABA-B receptors. Finally, peripheral GABA receptors do not appear to be significantly involved in the direct control of gastric secretion.

Fructose-1,6-diphosphate reduces acute ECG changes due to doxorubicin in isolated rat heart.

Doxorubicin (DXR) (0.17×10−4) induces an acute cardiotoxicity in isolated rat heart; there is a progressive widening of the SαT segment, with a decrease in force derivatives and in the coronary flow. Concurrent perfusion with fructose-1,6-diphosphate (FDP) (10−5–10−4) dose-dependently reduces the SαT enlargement but fails to affect the reduction in force derivatives and coronary flow. The target of cardiac protection by FDP might be the ionic mechanisms underlying the action potential configuration.

α2-Adrenoceptor-mediated inhibitory and excitatory effects of detomidine on rat gastric acid secretion

Abstract— The effects of the selective α2‐adrenoceptor agonist detomidine on gastric acid secretion from pylorus‐ligated and stomach‐perfused rats have been investigated. In pylorus‐ligated rats i.p. injection of detomidine markedly inhibited acid secretion, this effect being prevented by yohimbine or idazoxan. Under the same conditions, idazoxan significantly increased secretion in a dose‐independent fashion. In non‐vagotomized and vagotomized stomach‐perfused rats i.p. detomidine stimulated acid secretion: this excitatory effect was antagonized by idazoxan. The present results suggest that both inhibitory and excitatory gastric secretory effects of detomidine are mediated by α2‐adrenoceptors on cholinergic and adrenergic nerves, respectively. The stimulant activity of idazoxan on gastric secretion from pylorus‐ligated rats may be interpreted in terms of increased excitatory vagal tone following the blockade of inhibitory α2‐adrenoceptors.

Excitatory and inhibitory cholinergic effects of yohimbine on isolated guinea-pig small intestine.

The interaction of yohimbine with the cholinergic intestinal system was investigated in the isolated guinea-pig ileum using a wide range of drug concentrations from 3 x 10(-13) to 2 x 10(-4) g/ml. Low concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) caused dose-dependent contractions of the ileal longitudinal muscle, which were potentiated by eserine 1 x 10(-8) g/ml and prevented by tetrodotoxin 1 x 10(-6) g/ml or by atropine 1 x 10(-12) g/ml; methysergide and diphenydramine were ineffective up to 3 x 10(-7) g/ml dose. Submaximal stimulatory responses evoked by twitch stimulation or by acetylcholine were significantly potentiated by the same concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) and blocked by atropine 1 x 10(-12) g/ml. By contrast, high concentrations of yohimbine (1 x 10(-6) to 2 x 10(-4) g/ml) displayed dose-dependent inhibitory effects on cholinergic responses. The stimulant effect of yohimbine seems to be indirect and mediated by the increase in the release of acetylcholine, while the inhibitory action may be due to a molecular interaction with the muscarinic receptors allowing non-specific receptor blockade.

GABA-A-mediated gastrin release induced by baclofen in the isolated vascularly perfused rat stomach.

In order to investigate the role of peripheral GABA-B receptors, the effects of the putative GABA-B agonist baclofen on immunoreactive gastrin release from an isolated vascularly perfused rat stomach preparation were examined. The vascular infusion of baclofen at graded concentrations induced a dose-dependent increase in gastrin release; this was unaffected by the GABA-B antagonist delta-aminovaleric acid, but was fully prevented by the selective GABA-A antagonist bicuculline as well as by atropine or tetrodotoxin. These results suggest that the stimulant effects of baclofen are mediated by nervous cholinergic structures, associated with GABA-A receptors, and indicate that this GABA-B agonist must be regarded as a partial agonist of peripheral GABA-A receptors.

Peripheral opioid receptors mediate gastrointestinal secretory and motor effects of dermorphin N-terminal tetrapeptide (NTT) in the dog

Dermorphin N-terminal-tetrapeptide-amide (NTT) increased both basal and pentagastrin- or histamine-induced secretion in conscious dogs chronically implanted with both gastric fistulae and Heidenhain pouches. These excitatory effects were significantly prevented by the opioid receptor antagonists naloxone and N-methyl-levallorphan-methanesulphonate. In conscious dogs fitted with electrodes and strain-gauges in different parts of gastrointestinal tract, a premature phase III of the migrating myoelectric complex (MMC) in the duodeno-jejunum was triggered by NTT, while the activity of the antrum was not significantly modified. Further, the peptide enhanced the contractile activity of both proximal and distal portions of the colon, including a long-lasting period of increased muscle tone on the distal colon. Either naloxone or N-methyl-levallorphan-methanesulphonate completely prevented motor effects of NTT on gastrointestinal tract. It is concluded that NTT displays significant opiate-like activity on gastric acid secretion and intestinal motility of the dog by activating peripheral mu opioid receptors.