Action of beta-receptor blocking sympatholytics and of catecholamine depleting agents on CaCl2-induced arrhythmias in rats

Novel Method to Assess Cardiac Electrophysiology in the Rat

The rat continues to be an important tool to assess cardiac electrophysiologic (EP) effects of test agents and to study the distribution/role of ion channels in cardiovascular diseases. However, no data have been described that accurately measure discrete cardiac EP parameters in rats in vivo. Therefore, we developed a method to assess cardiac EP in rats and then profiled several ion channel agents. Briefly, rats were instrumented with endocardially placed electrodes to assess cardiac refractoriness and conduction. Administration of class I agents resulted in a dose-dependent slowing of ventricular conduction. The potassium channel blocker 4-aminopyridine caused significant increases in atrial and ventricular refractoriness. An IKr blocker had little or no effect on atrial and ventricular refractoriness but significantly increased AV nodal refractoriness. Additionally, an IKs blocker had little effect on rat cardiac EP. The L-type blocker diltiazem caused a dose-dependent delay in AV node conduction and an increase in AV node refractoriness. Overall, this study provides normative data that describe the roles of Na, Ca, and K channels in rat cardiac electrophysiology, in vivo. Furthermore, the model provides a method to assess changes in cardiac electrophysiology in the setting of disease by using well-established rat models of induced or genetic cardiovascular disease.

Cardiotonic and antidysrhythmic effects of oleanolic and ursolic acids, methyl maslinate and uvaol

The cardiotonic and antidysrhythmic effects of four triterpenoid derivatives, namely oleanolic acid (OA), ursolic acid (UA), and uvaol (UV), isolated from the leaves of African wild olive (Olea europaea, subsp. africana) as well as methyl maslinate (MM) isolated from the leaves of Olea europaea (Cape cultivar) were examined. The derivatives showed low toxicity on brine shrimp test. They displayed significant, dose-response vasodepressor effect and sinus bradicardia, most prominent for OA and MM. The derivatives acted as beta-adrenergic antagonists, blocking the effect of adrenaline and isoprenaline. The established positive inotropic and dromotropic effects were most distinctive for OA and MM. The antidysrhythmic effects were evaluated on CaCl2- and adrenaline-induced chemical arrhythmias, and on ischemia-reperfusion arrhythmia. OA and UA displayed antidysrhythmic effects on both types of chemical arrhythmia; OA and UV in dose 40 mg/kg conferred significant antidysrhythmic activity on ischemia and reperfusion arrhythmias. The effect was comparable to that of propranolol and suggestive of beta-adrenergic antagonistic activity. On the basis of the vasodepressor, cardiotonic and antidysrhythmic effects of these compounds, it was concluded that OA and UV isolated from wild African olive leaves, or crude extract containing all components, can provide a cheap and accessible source of additive to conventional treatment of hypertension, complicated by stenocardia and cardiac failure.

Comparison of verapamil and bepridil, two slow channel inhibitors, in protection against calcium-induced arrhythmias

Verapamil and bepridil share the common property of antagonizing the slow inward calcium-mediated current, but bepridil has some additional antiarrhythmic properties. The efficacy of these two compounds against CaCl2-induced arrhythmias has been compared in rats. CaCl2 was administered i.v. by continuous infusion until death (25 mg X kg-1 X min-1 or 40 mg X kg-1 X min-1) or by bolus injection (160 mg X kg-1). Bepridil (5, 10 mg X kg-1) or verapamil (2.5, 5 mg X kg-1) were injected 10 min before CaCl2. Bepridil (10 mg X kg-1) or verapamil (5 mg X kg-1) prolong the survival time during CaCl2 infusion. After pretreatment, the injection of 160 mg X kg-1 CaCl2 is less toxic: 25% of animals are protected by bepridil (5 mg X kg-1), 41% by bepridil (10 mg X kg-1) or verapamil (5 mg X kg-1). At death the myocardial Ca2+ level is not different in controls and pretreated animals, thus, the ratio myocardial Ca2+/total injected Ca2+ is significantly lowered by bepridil (10 mg X kg-1) or verapamil (5 mg X kg-1). The efficacy of the two drugs on this model appears related solely to inhibition of slow inward current despite the additional antiarrhythmic profile of bepridil.

The role of the local anaesthetic properties of beta-adrenoceptor blocking agents in antagonizing CaCl2-induced arrhythmias in the rat

1 Eight beta-adrenoceptor blocking agents were compared for their ability to antagonize arrhythmias induced by bolus injections of CaCl2 in rats. 2 Only those beta-blockers possessing local anaesthetic properties were effective in antagonizing this type of arrhythmia and there was a highly significant correlation between antiarrhythmic and local anaesthetic properties. 3 Effects on adrenoceptors seem to play only a minor role in the genesis of this type of arrhythmia.