Role of alpha- and beta-adrenergic activation in ventricular fibrillation death of corticoid-pretreated rats

Cardiotoxic Effects in Rats and Rabbits Treated with Terbutaline Alone and in Combination with Aminophylline

Studies were conducted to determine the myocardial effects of a selective beta-adrenoceptor agonist, terbutaline, administered alone and in combination with aminophylline in the 4-to 5-month-old, 500-600 g (heavy) rat and the rabbit, using electrocardiographic and histopathological methods. Terbutaline given at high (5.0 mg/kg) and low (0.1 mg/kg) doses was not arrhythmogenic in the heavy rat; however, dose-dependent myocardial lesions were observed. Terbutaline given at the same doses to heavy rats pretreated with aminophylline was arrhythmogenic and produced severe cardiac lesions. Rats administered aminophylline at a dose of 18.75 mg/kg had plasma theophylline levels of 15-22 μg/ml; these concentrations are similar to the recommended human therapeutic levels, i.e., 10-20 μg/ml. The administration of terbutaline in conjunction with aminophylline did not seem to affect the plasma levels of theophylline. No arrythmias were detected in rabbits given terbutaline alone or in combination with aminophylline and no deaths occurred.

The effects of inflammation on heart rate and rhythm in a canine model of cardiac surgery

BACKGROUND

Heart rate (HR) and rhythm disturbances are common after cardiac surgery. This study tests the hypothesis that the inflammation caused by cardiac surgery is an underlying mechanism for postoperative changes in HR, rhythm, and HR variability (HRV).

METHOD AND RESULTS

Normal canines (n = 6 per group) were divided into 4 groups: (1) anesthesia, (2) sternotomy and pericardiotomy, (3) atriotomy, and (4) corticosteroids combined with an atriotomy. Continuous electrocardiographic recordings were done preoperatively and for 3 postoperative days. Electrophysiologic testing was done at the initial and terminal surgeries. C-reactive protein level was assessed at each study day, and tissue myeloperoxidase activity was assessed at the terminal study. Measurements of HRV were determined daily to detect changes in autonomic tone. Postoperatively, the HR increased in the pericardiotomy (P = .0005) and atriotomy (P = .001) groups and HRV decreased in both the groups. No significant change occurred in either the HR or HRV in the anesthesia (P = .52) and steroid (P = .16) groups. HRV (triangular index) on postoperative day 3 was correlated with the tissue myeloperoxidase levels (r = -.83; P = .0004). Autonomic blockade with atropine and esmolol resulted in an HR and HRV that were not significantly different between groups. Atrial premature beats occurred postoperatively in the all the groups except the anesthesia group and were independent of the degree of inflammation.

CONCLUSION

Cardiac surgery increases the postoperative HR by reducing HRV, mostly because of a reduction in vagal tone. Furthermore, the magnitude of these changes is dependent on the degree of inflammation and is normalized by corticosteroids.

Electrophysiologic Study of the Effects of Aminophylline and Metaproterenol on Canine Myocardium

Aminophylline and beta-adrenergic agonists are widely used in the treatment of obstructive lung diseases. It has been suggested that combined aminophylline and beta-agonist therapy may promote the development of atrial and ventricular arrhythmias. The effects of these agents in combination on myocardial conduction and tissue refractoriness have not been documented. We evaluated the electrophysiologic effects of intravenous aminophylline and inhaled metaproterenol on canine myocardium. Aminophylline produced significant decreases from baseline in the AH interval (85 +/- 6.5 [SD] to 63 +/- 4.1 ms [p less than 0.02]), Wenckebach cycle length (WCL) (226 +/- 8.7 to 182 +/- 5.8 ms [p less than 0.02]), and ventricular effective refractory period (VERP) (166 +/- 6.0 to 148 +/- 4.9 ms [p less than 0.01]). Metaproterenol produced similar results, except metaproterenol significantly decreased the atrial effective refractory period (AERP) from 152 +/- 6.6 to 130 +/- 3.2 ms (p less than 0.02), an effect not seen with aminophylline alone. Metaproterenol also produced significantly greater reductions in AH interval and WCL, as well as a greater increase in heart rate than aminophylline did. When compared with aminophylline alone, combined metaproterenol and aminophylline therapy produced significantly greater reductions in the AH interval (63 +/- 4.1 versus 48 +/- 1.2 ms for combined therapy [p less than 0.01]), HV interval (32 +/- 1.2 versus 28 +/- 2.0 ms for combined therapy [p less than 0.02]), WCL (182 +/- 5.8 versus 150 +/- 7.1 ms for combined therapy [p less than 0.02]), and VERP (148 +/- 4.9 versus 132 +/- 2.0 ms for combined therapy [p less than 0.02]). We conclude that both aminophylline and metaproterenol significantly enhance AV nodal and His-Purkinje conduction. Metaproterenol produced significant changes in both atrial and ventricular tissue refractoriness. Metaproterenol produced significantly greater changes than aminophylline alone, and inhaled metaproterenol combined with intravenous aminophylline produced greater changes in AV nodal and His-Purkinje conduction and ventricular refractoriness than did aminophylline alone in a canine model.

The arrhythmogenicity of theophylline. A multivariate analysis of clinical determinants

OBJECTIVE

to determine the relation of serum theophylline concentrations (STC) to the occurrence of cardiac arrhythmias in a population of hospitalized patients.

STUDY DESIGN

A cross-sectional study of 100 patients receiving theophylline who had electrocardiograms at the time STC had been obtained. Categorization was based on STC without knowledge of clinical status: group 1 (25) had STC less than 2.5 mg/L; group 2 (25), STC greater than or equal to 2.5 less than 10 (6.2 +/- 0.4) mg/L; group 3 (25), STC greater than or equal to 10 less than or equal to 20 (15.8 +/- 0.4) mg/L; and group 4 (25), STC greater than 20 (27.8 +/- 1.1) mg/L.

SETTING

A university-affiliated teaching hospital.

RESULTS

Two-thirds of the patients were receiving theophylline in an oral form, with a similar distribution found in all groups. Groups were not different with respect to age, clinical diagnoses, electrocardiographic patterns, arterial pH or gases, serum calcium, or treatment with beta-agonists. Although fewer patients in group 1 were receiving steroids than those in the other groups, and serum potassium was lower in group 3 than in group 1 (4.0 +/- 0.6 vs 4.5 +/- 0.5 mEq/L, p less than 0.05), on multivariate analysis, serum potassium was not an independent predictor of arrhythmia, and treatment with steroids or digoxin was, at most, only weakly associated with the presence of arrhythmia. Heart rate was directly related to STC (r = 0.64, p less than 0.01), and STC was the strongest independent predictor of arrhythmia in this population. Although only 20 percent of patients in group 1 or 2 had arrhythmias, 48 percent of group 3 (p less than 0.05) and 56 percent of group 4 (p less than 0.05) had this finding. Moreover, the odds ratio of group 3 having an arrhythmia compared with group 1 was 3.7, 1.5 to 11.7, p less than 0.01. Multifocal atrial tachycardia was found in 8 percent of the patients in group 3 and 16 percent in group 4 but in none of the patients in groups 1 or 2. Two patients with multifocal atrial tachycardia died suddenly within 24 hours of having their STC measurement; none had concomitant ventricular ectopy.

CONCLUSION

Theophylline causes tachycardia and serious arrhythmias even at STC considered to be therapeutic. Multifocal atrial tachycardia, an arrhythmia associated with use of this drug, may herald sudden cardiac death.

Cardiovascular effect and stimulus-dependent inhibition of superoxide generation from human neutrophils by tibenelast, 5,6-diethoxybenzo(b)thiophene-2-carboxylic acid, sodium salt (LY186655)

Tibenelast (LY186655), 5,6,-diethoxybenzo(b)thiophene-2-carboxylic acid, sodium salt, is an orally active anti-anaphylactic compound in guinea pigs, and has been shown to prevent bronchospasm in moderately severe asthmatic patients. Pharmacological studies with tibenelast demonstrated that it is a selective phosphodiesterase (PDE) inhibitor in that it is moderately active against the lung and stomach enzyme while being a very weak inhibitor of the heart enzyme. The compound was without cardiovascular effects at anti-anaphylactic doses. In contrast to theophylline, tibenelast did not have a direct inotropic effect in the cat papillary muscle system. The concentration that inhibited 50% of the enzymatic activity (IC50) for tibenelast was 20- to 30-fold lower for neutrophil PDE than for PDE of other tissues. It was 100 times more potent than aminophylline in inhibiting superoxide generation from platelet-activating factor (PAF)-primed polymorphonuclear leukocytes (PMNL) challenged with chemotactic factor, N-formyl-methionyl-leucyl-phenylalanine. However, tibenelast was less effective in the tumor necrosis factor-primed system, and did not inhibit superoxide generation during phagocytosis or when other soluble stimuli, such as phorbo-12-myristate-13-acetate or the calcium ionophore A23187, were used. Furthermore, tibenelast did not inhibit enzymes involved in arachidonic acid metabolism. These results suggest that tibenelast probably inhibits superoxide release from PMNL via a selective inhibition on PDE.

Salbutamol induced hypokalaemia: the effect of theophylline alone and in combination with adrenaline

1. We have previously shown that salbutamol induced hypokalaemia, like adrenaline induced hypokalaemia, is the result of stimulation of a membrane bound beta 2-adrenoreceptor linked to Na+/K+ ATPase. We have also demonstrated that adrenaline induced hypokalaemia is potentiated by therapeutic concentrations of theophylline. 2. In a single-blind study of 14 normal volunteers, we infused salbutamol in doses used in clinical practice and examined the effects of the addition of theophylline alone or combined with (-)-adrenaline on plasma potassium levels, heart rate and blood pressure. The combinations studied were (i) salbutamol + vehicle control adrenaline infusion + placebo theophylline; (ii) salbutamol + vehicle control adrenaline infusion + theophylline; (iii) salbutamol + adrenaline + theophylline. 3. In a randomised, balanced placebo controlled design oral slow release theophylline or placebo was given for 9 days. Subjects were studied twice on the active limb (days 7 and 9) and once on the placebo limb (day 9) and the procedure was identical on each of the 3 study days except for the solutions administered. 4. Theophylline increased salbutamol induced hypokalaemia and in some individuals profound hypokalaemia (less than 2.5 mmol l-1) was observed with these relatively low doses of salbutamol and theophylline. Adrenaline did not further increase the magnitude of the fall in potassium observed. Combining theophylline with salbutamol increased the tachycardia resulting from the salbutamol infusion. Salbutamol infusion caused a fall in diastolic and rise in systolic blood pressure on all 3 study days and this was not altered by either theophylline or adrenaline alone or together.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal hypertensive hypertrophy in the rat: a substrate for arrhythmogenicity

We investigated the ability of ouabain to produce lethal arrhythmias in rats with myocardial hypertrophy resulting from chronic renal hypertension. A gradual pressure overload was produced in female Wistar rats by left renal artery stenosis (two kidney, one clip, Goldblatt hypertension). Hypertension (systolic blood pressure greater than 150 mm Hg) developed within three weeks after clipping of the left renal artery and blood pressure continued to increase for the next five weeks. At ten weeks after the onset of hypertension animals were anesthetized with sodium pentobarbital (40 mg/kg) and artificially ventilated with room air while ECG was continually monitored and recorded. Continuous infusion of ouabain was maintained (0.7 mg/kg/min) through the inferior vena cava. Body weight and heart rate of control animals (C) was not significantly different from hypertensive (H) values, while systolic blood pressure in animals hypertensive for ten weeks was considerably greater (187 +/- 8.4 mm Hg) than their age-matched normotensive counterparts (123 +/- 6.0 mm Hg). Heart weight in hypertensive animals was elevated by 69% +/- 2.5 by time of study. Serological evaluation of both groups of animals revealed no significant differences in electrolytes and blood gases while significant differences were noted in glucose, BUN and creatinine. The average time to the first premature ventricular contraction was significantly shorter in H animals (3.5 +/- 0.2 min) when compared to C rats (6.0 +/- 0.2 min). The average time to ventricular tachycardia, ventricular fibrillation and death were also significantly shorter in H rats when compared to C animals (7.5 +/- 0.6 vs. 13.5 +/- 0.3; 13.5 +/- 0.5 vs. 21.0 +/- 0.5; 15.6 +/- 0.4 vs. 24.0 +/- 0.6 min). Thus, the hypertensive hypertrophied myocardium displays an increased propensity for lethal cardiac arrhythmias due to ouabain.

DOCA-salt induced myocardial sensitization to ventricular fibrillation by isoprenaline in rats. Role of the autonomic nervous system

The possible role of the autonomic nervous system in the development of myocardial sensitization to isoprenaline produced by DOCA-salt pretreatment was evaluated in bilaterally unanaesthetized rats subject to chronic ganglionic blockade with chlorisondamine or hexamethonium and to treatment with 6-hydroxydopamine. Isoprenaline produced ventricular fibrillation and death in rats pretreated for 6 days with DOCA (20 mg pellet, s.c.) and 0.9% saline. The incidence of mortality was dose dependent and was 76.6% with 150 microgram/kg of isoprenaline. Adrenalectomy did not alter the mortality rate. The combination of adrenalectomy with concurrent chronic ganglionic blockade significantly decreased the incidence of mortality and delayed the time of death. In contrast, 6-hydroxydopamine treatment and adrenalectomy did not alter the incidence of mortality nor time of death. No differences in myocardial noradrenaline turnover were detected in rats susceptible and nonsusceptible to death in ventricular fibrillation. We conclude that myocardial sensitization to isoprenaline induced by DOCA-salt requires a neurogenic component, which is not, however, manifested by changes in myocardial noradrenaline turnover.

Corticosterone secretion in the rat after DSP-4 treatment

Functioning of the hypothalamo-pituitary-adrenocortical axis was assessed in rats treated with DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine), a selective noradrenergic neurotoxin. Although adrenal weights were slightly increased 10 days after DSP-4 injection, basal plasma corticosterone levels were unaltered compared to those of control animals. Moreover, the elevation of corticosterone levels induced by the stress of ether inhalation did not appreciably differ from that in control rats. These various results suggest that DSP-4 treatment has no marked effect on the normal functioning of the hypothalamo-pituitary-adrenocortical axis.

Lack of functional influence by prenalterol through alpha 1-adrenoceptors in rat myocardium

Some of the cardiac properties of the partial beta-agonist prenalterol may indicate a contribution from alpha 1-adrenergic stimulation. We therefore studied whether prenalterol interacted with alpha 1-adrenoceptors in rat myocardium. Combination with propranolol did not reveal an alpha 1-adrenergic inotropic effect of prenalterol in papillary muscles. Neither did prenalterol block the alpha 1-adrenergic response to phenylephrine. In myocardial cells, prenalterol inhibited 3H-prazosin binding to alpha 1-adrenoceptors only at very high concentrations. Prenalterol thus exhibited no functionally important interactions with alpha 1-adrenoceptors in myocardium, and its properties cannot be accounted for in terms of contribution from alpha 1-adrenergic effects.

Enhanced arrhythmogenic activity of beta-adrenoceptor stimulants in desoxycorticosterone-pretreated rats

The arrhythmogenic activity of four beta-adrenoceptor stimulants, metaproterenol, salbutamol, isoetharine, and dobutamine, was determined in adult, Wistar rats pretreated for 3-4 weeks with desoxycorticosterone (DOCA) and 1% saline as drinking fluid. Doses (SC) of these stimulants which were well tolerated in untreated animals elicited severe ventricular dysrhythmias, often leading to death in ventricular fibrillation. The LD50 in mg/kg was as follows: metaproterenol, 0.28; salbutamol, 0.60; isoetharine, 2.3; and dobutamine, 4.8. The relative potency of metaproterenol, salbutamol, and isoetharine correlates well with their ability to stimulate beta 1-adrenoceptors. It is suggested that DOCA-saline pretreatment in rats may be used as a model for the rapid screening of drugs affecting beta-adrenoceptors.