Investigation of the negative chronotropic action of SK&F 86466 in the pithed normotensive rat

SK&F 86466, 6-chloro-3-methyl-2,3,4,5-tetrahydro-1-H-3-benzazepine, is a potent and selective antagonist of the α₂-adrenoceptor in vitro. This compound produced a small pressor response accompanied by a marked bradycardia when administered i.v. to the pithed normotensive rat. The pressor response was not affected by reserpine treatment, pretreatment with α- or β-adrenoceptor antagonists, atropine, or hexamethonium. The bradycardia was markedly reduced by bilateral vagotomy and pretreatment with atropine and attenuated by hexamethonium. The negative chronotropic action of SK&F 86466 was abolished by a combination of vagotomy and atropine. Mediation of the bradycardia by a baroreceptor reflex was ruled out by the observations that a lack of change in heart rate was associated with the vasopressor response to phenylephrine in the pithed rat pretreated with propranolol. It is concluded that the negative chronotropic action of SK&F 86466 in the pithed rat is mediated indirectly by activation of the cholinergic innervation of the heart.     

A convenient synthesis of [11C]paraquat and other [N‐methyl‐11C]bisquaternary ammonium compounds

[¹¹C]Paraquat was synthesized by the reaction of [¹¹C]methyl triflate with the mono‐triflate salt of 1‐methyl‐[4,4′]bipyridinyl. The product was selectively separated from the precursor by a microcolumn of Chelex 100 ion exchange resin. The method was applied to the synthesis of a variety of [N‐methyl‐¹¹C]bisquaternary ammonium compounds. This is the first reported use of a chelating cation exchange resin for the selective purification of organic dications. Copyright © 2002 John Wiley & Sons, Ltd.     

Neural mechanisms involved in the delay of gastric emptying of liquid elicited by acute blood volume expansion in awake rats

We have previously reported that acute blood volume expansion in awake rats delays the gastric emptying of a liquid meal, using the phenol red method. In this study we attempted to investigate the neural mechanisms involved in this phenomenon. Blood volume expansion, due to Ringer-bicarbonate infusion up to a volume equivalent to 5% of body weight, decreased the gastric emptying of a liquid meal by half (38.2 +/- 1.8 vs 18.7 +/- 3.2%, P < 0.05). The blood volume expansion effect on gastric emptying of liquid was prevented by separate pretreatments, consisting of subdiaphragmatic vagotomy or i.v. injection of hexamethonium (20 mg kg-1) or yohimbine (3 mg kg-1). Intravenous injection of atropine (0.5 mg kg-1), guanethidine (10 mg kg-1), L-NAME (3 mg kg-1), prazosin (1 mg kg-1) or propranolol (2 mg kg-1) did not prevent the blood volume expansion effect on gastric emptying. Bilateral adrenalectomy or coeliac ganglionectomy were also ineffective. The results indicate that blood volume expansion decreases gastric emptying of liquid through vagal-dependent pathways, sensitive to hexamethonium and yohimbine. Evidence for the participation of the peripheral sympathetic nervous system was not found.     

Effects of luminal stimuli on polyamine metabolism in the small intestine of the rat: the role of enteric nerves

The aim of this study was to investigate to what extent polyamine metabolism in the small intestine of the rat is controlled by the enteric nervous system. Polyamine metabolism was followed by measuring the activity of ornithine decarboxylase (ODC) and in some instances also the content of polyamines (putrescine, spermidine and spermine). ODC activity in the intestine was increased when intraluminal pressure was increased and 3 h after placing cholera toxin in the intestinal lumen. Cholera toxin also increased the tissue putrescine content. Atropine or hexamethonium given i.v. did not influence the evoked changes of ODC activity. The pressure induced changes were not decreased by placing lidocaine on the serosal surface. On the other hand, the ODC activity of control segments were decreased by hexamethonium or atropine. The presence of glucose in the intestinal perfusate did not augment tissue ODC activity, neither did the heat stable enterotoxin from Escherichia coli (STa). It is concluded that the effect on polyamine metabolism evoked by luminal pressure or cholera toxin seems not to be mediated via nerves, while nerves seem to influence ODC activity during control conditions. The experiments with enterotoxins suggest that cAMP is the intracellular second messenger controlling intestinal ODC activity.     

The involvement of intramural nerves in cholera toxin induced intestinal secretion

In previous reports we have suggested that nervous reflexes are involved in the pathophysiology of cholera secretion and that these nervous reflexes involve a cholinergic synapse and a neuron with vasoactive intestinal polypeptide (VIP) as neurotransmitter. These proposals were further analyzed in this study. Tetrodotoxin (TTX) and lidocaine applied on the serosal surface inhibited cholera secretion in segments of rat small intestine. Fluid absorption in control rats was not significantly changed. Hexamethonium given i. v. decreased cholera secretion in the cat. No additional inhibition of cholera secretion was observed after giving TTX close i. a. Furthermore, the intestinal secretion evoked by VIP was not influenced by hexamethonium given i. v. or TTX given close i. a. The present observations support the hypothesis of a role for nervous reflexes in cholera secretion. The results suggest that at least a major part of the proposed nervous reflex(es) in cholera have a cholinergic synapse. Furthermore, the VIP-ergic neuron is situated “distal” to the cholinergic neuron in the reflex(es) closer to the effector cells.     

5—羟色胺M受体和胆碱能受体阻断剂在红霉素所致小肠动力紊乱和呕吐中的作用

本实验对８条犬进行小肠肌电测定，并在记录期间分别静注胆碱能神经阻断剂，阿托品和六甲季胺，以及５－羟色胺（５－ＨＴ）Ｍ受体阻断剂，灭吐灵，以观察其对红霉素所致肠道副作用的影响。结果发现阿托品和六甲季胺不仅能明显地抑制空腹时正常传播性肌电综合波（ＭＭＣ）的发生和传播，降低红霉素促进小肠动力的作用，还能减轻红霉素所致饱腹动物的快波数增加，但对呕吐无明显影响。灭吐灵的实验结果显示红霉素的致吐作用是由５－ＨＴＭ受体介导的，与其促进小肠动力的作用无明显因果关系。     

Effect of nicotine on human blood platelet serotonin uptake and efflux

1. 1. Physostigmine administration has been previously shown to decrease the uptake of serotonin in human platelets. In order to test whether uptake could be inhibited as a nicotinic-cholinergic effect, the in vitro effects of nicotine on platelet 5HT uptake and efflux were examined.

2. 2. Nicotine stimulated release of serotonin from human blood platelets, and competitively inhibited human platelet serotonin uptake in a concentration-dependent fashion at in vitro concentrations as low as 20 μM for uptake.

3. 3. The kinetics of the nicotine effects on uptake were different from those of physostigmine. Unlike the effects of physostigmine, nicotine produced different kinetic changes, with an increase in Km and no consistent change in Vmax.

4. 4. The efflux and inhibition of uptake paralleled that previously reported in rat brain in vitro, and was likewise similar to concentrations found previously to augment extracellular amine in other tissue preparations. However, the effects of nicotine in human platelets were not reversible by nicotinic antagonism with hexamethonium.

5. 5. The results distinguish human platelet from rat brain with respect to nicotinic antagonism, and suggest that, at similar concentrations, nicotine may increase extracellular serotonin through differing mechanisms.

The effect of nicotinic and muscarinic receptor blockade on cholera toxin induced intestinal secretion in rats and cats

The effects of hexamethonium (cholinergic nicotinic receptor antagonist) and atropine (cholinergic muscarinic receptor antagonist) on cholera toxin induced secretion were investigated in denervated segments of the small intestine of rats and cats. While there was no effect of atropine, hexamethonium markedly inhibited choleraic secretion and turned it into a net fluid absorption in many animals. This observation further strengthens our hypothesis that the enteric nervous system is involved in cholera secretion.     

Effects of subcutaneous and intracerebroventricular injection of physostigmine on the acute corneal nociception in rats

The present study investigated the effects of subcutaneous (sc) and intracerebroventricular (icv) injections of physostigmine (a cholinesterase inhibitor), atropine (an antagonist of muscarinic cholinergic receptors) and hexamethonium (an antagonist of nicotinic cholinergic receptors) on the acute corneal nociception in rats. Local application of 5 M NaCl solution on the corneal surface of the eye produced a significant nociceptive behavior, characterized by eye wiping. The number of eye wipes was counted during the first 30 s. The sc (0.25, 0.5 and 1 mg/kg) and icv (1.25, 2.5, 5 and 10 μg) injections of physostigmine significantly (p < 0.05) decreased the number of eye wipes. Atropine and hexamethonium at (2 mg/kg, sc and 20 μg, icv) had no effects when used alone, however, atropine, but not hexamethonium prevented the antinociception induced by physostigmine (sc and icv). The results of this study indicate that the central muscarinic, but not nicotinic receptors might be involved in the antinociceptive effect of physostigmine in the acute corneal model of pain in rats.