nitroarginine and Bradycardia

1. Cardiac natriuretic peptides act on cardiopulmonary chemoreceptor afferents to enhance the von Bezold-Jarisch reflex (BJR). Activity of the natriuretic peptide particulate guanylyl cyclase receptor is essential for full expression of the BJR. Whether natriuretic peptides act directly on cardiac afferents or they require another intermediate factor(s) for their effects on the BJR is unknown. Endogenous candidates tested as possible intermediates in the present study were prostanoids and nitric oxide (NO), plausible endogenous chemical mediators of cardiac chemoreflex activity. 2. Dose-dependent BJR bradycardia was evoked by the 5-HT(3) receptor agonist, phenylbiguanide (range 5-89 μg/kg), in conscious instrumented adult sheep (n = 6). The influence of B-type natriuretic peptide (BNP; the most potent of the natriuretic peptides) on the BJR was assessed before and after blockade of prostanoids (using indomethacin, 1 mg/kg per h i.v.) or nitric oxide (using N-nitro-l-arginine (NOLA), 3 mg/kg bolus, then 3 mg/kg per h infusion i.v.). 3. On their own, indomethacin and NOLA did not significantly alter the BJR, showing that prostanoids and NO are not essential endogenous mediators of the BJR. As shown in previous studies, BNP (10 pmol/kg per min i.v.) infusion enhanced the BJR by 85 ± 36%, P < 0.05. When the production of either prostanoids or nitric oxide was inhibited, BNP still enhanced the BJR. 4. The present study provides evidence that BNP does not require the activity or influence of prostaglandins or NO for its sensitising effects on the BJR. We propose that natriuretic peptides act directly on cardiac afferents, in synergy with 5-HT(3) agonists, to facilitate the BJR.

Topics: Animals; Awareness; Biguanides; Bradycardia; Chemoreceptor Cells; Heart; Indomethacin; Natriuretic Peptide, Brain; Nitric Oxide; Nitroarginine; Prostaglandins; Reflex; Serotonin Receptor Agonists; Sheep

Acute systemic blockade of nitric oxide (NO) production by nonselective inhibitors of NO synthase (NOS) isoforms, including N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA), has been shown to produce a long-lasting pressor response in conscious and anaesthetised animals. The present study was undertaken to clarify whether the renin-angiotensin system contributes to the development of this pressor response to L-NNA. Systemic blood pressure and heart rate were continuously monitored in dogs anaesthetised with pentobarbital. Plasma renin activity in the blood obtained from a femoral artery and a renal vein was measured by use of radioimmunoassay. The acute pressor response produced by the intravenous administration of L-NNA was accompanied by reduced renin activity in both systemic and renal vascular beds. Captopril, an angiotensin converting enzyme inhibitor, counteracted the pressor response to L-NNA, whereas candesartan, an angiotensin AT1-receptor antagonist, had no apparent effect on it. The counteraction by captopril of the L-NNA-induced pressor response was likely to be attributable to enhancement by captopril of depressor responses to bradykinin, as HOE-140, a bradykinin B2 receptor antagonist, neutralised the effect of captopril. These results suggest that the pressor response acutely produced by the intravenous injection of a NOS inhibitor is not mediated by the renin-angiotensin system in anaesthetised dogs.

Topics: Animals; Antihypertensive Agents; Arginine; Benzimidazoles; Biphenyl Compounds; Blood Pressure; Bradycardia; Bradykinin; Captopril; Dogs; Dose-Response Relationship, Drug; Enzyme Inhibitors; Female; Heart Rate; Injections, Intravenous; Male; Nitroarginine; Renin; Renin-Angiotensin System; Stereoisomerism; Tetrazoles

Nitric oxide (NO) has an important role in controlling heart rate and contributes to the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Based on evidence of NO overproduction in cholestasis and also on the existence of bradycardia in cholestatic subjects, this study aimed to evaluate the chronotropic effect of epinephrine in isolated atria of cholestatic rats and determine whether alterations in epinephrine-induced chronotropic responses of cholestatic rats are corrected after systemic inhibition of NO synthase (NOS) with N(G)-nitro-L-arginine (L-NNA). Male Sprague-Dawley rats were used. Cholestasis was induced by surgical ligation of the bile duct under general anesthesia and sham-operated animals were considered as control. The animals were divided into three groups, which received either L-arginine (200 mg/kg/day), L-NNA (10 mg/kg/day) or saline. One week after the operation, a lead II ECG was recorded from the animals, then spontaneously beating atria were isolated and chronotropic responses to epinephrine were evaluated in a standard oxygenated organ bath. The results showed that plasma gamma-glutamyl transpeptidase and alanine aminotransferase activity was increased by bile-duct ligation, and that L-aginine treatment partially, but significantly, prevented the elevation of these markers of liver damage. The results showed that heart rate of cholestatic animals was significantly less than that of sham-operated control rats in vivo and this bradycardia was corrected with daily administration of L-NNA. The basal spontaneous beating rate of atria in cholestatic animals was not significantly different from that of sham-operated rats in vitro. Meanwhile, cholestasis induced a significant decrease in chronotropic effect of epinephrine. These effects were corrected by daily administration of L-NNA. Surprisingly L-arginine was as effective as L-NNA and increased the chronotropic effect of epinephrine in cholestatic rats but not in sham-operated animals. Systemic NOS inhibition corrected the decreased chronotropic response to adrenergic stimulation in cholestatic rats, and suggests an important role for NO in the pathophysiology of heart rate complications in cholestatic subjects. The opposite effect of chronic L-arginine administration in cholestasis and in control rats could be explained theoretically by an amelioration of cholestasis-induced liver damage by chronic L-arginine administration in bile duct-ligated rats

Topics: Alanine Transaminase; Animals; Arginine; Bradycardia; Cholestasis; Dose-Response Relationship, Drug; Enzyme Inhibitors; Epinephrine; gamma-Glutamyltransferase; Heart Atria; Heart Rate; In Vitro Techniques; Male; Nitric Oxide; Nitroarginine; Rats; Rats, Sprague-Dawley; Tyrosine; Vasoconstrictor Agents

This study focuses on the potential role for nitric oxide on the actions of the parasympathetic innervation to the heart. Earlier, we showed that the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) reduced the bradycardia induced by stimulation of vagal efferent motor fibres and that these effects are reversible through administration of the NOS substrate L-arginine. In the present study, we show that D-arginine does not reverse the effects of the inhibitors and confirm that they are reversed by L-arginine. Another NOS inhibitor, NG-nitro-L-arginine (L-NOARG), produced similar effects which were not reversed by L-arginine. In an examination of the effect of increasing NO availability with the NO donor sodium nitroprusside the vagally induced bradycardia was enhanced at all frequencies tested. In a separate series, the effects of NOS inhibitors and NO donors on the dromotropic actions of the vagus were examined. The NOS inhibitor L-NAME, reduced the increase in atrio-ventricular conduction delay normally induced by efferent vagal stimulation at all frequencies tested both in the 'paced' and 'unpaced' heart. Further, sodium nitroprusside enhanced this delay. Overall the study indicates that NO has an important facilitatory role on both the chronotropic and dromotropic actions of the vagus nerve on the heart and that NO may be a rate-limiting factor in the cardiac responses to vagal stimulation.

Topics: Animals; Arginine; Blood Pressure; Bradycardia; Electric Stimulation; Enzyme Inhibitors; Ferrets; Heart; Heart Rate; NG-Nitroarginine Methyl Ester; Nitric Oxide; Nitroarginine; Nitroprusside; Vagus Nerve

The pressor effect of NG-nitro-L-arginine (L-NNA), a potent inhibitor of nitric oxide (NO) synthesis, was studied in pentobarbital anesthetized rats. Iv injections of L-NNA from 0.25 to 8 mg/kg caused bradycardia and a dose-dependent increase in mean arterial pressure (MAP) with a maximal response of 43 +/- 5 mmHg and ED50 value of 1.3 +/- 0.2 mg/kg. The time course of the response to the injection of a single dose of L-NNA was also determined. Peak response was reached 60 min after the injection of a single dose (4 mg/kg, iv) and the effect lasted greater than 5 h. The rising phase of the pressor response was accompanied by slight bradycardia while the recovery phase was associated with significant tachycardia. Iv injections of L-arginine (12.5-200 mg/kg) caused transient dose-dependent reductions in MAP. The pressor effect of L-NNA (4 mg/kg, iv bolus) was dose-dependently attenuated by L-arginine. The results show that L-NNA is an efficacious and long-acting pressor agent and are consistent with the hypothesis that endogenous NO plays an important role in the regulation of blood pressure.

Topics: Anesthesia; Animals; Arginine; Blood Pressure; Bradycardia; Dose-Response Relationship, Drug; Heart Rate; Nitric Oxide; Nitroarginine; Pentobarbital; Rats; Rats, Inbred Strains