Effect of some arrhythmogenic agents upon the acetylcholine content of the rabbit atria

Positive chronotropic and inotropic effects of higenamine and its enhancing action on the aconitine-induced tachyarrhythmia in isolated murine atria

Aconitine and higenamine are the components of aconite root. We investigated the cardiac effects of these compounds on murine right and left atria and the interaction of higenamine with aconitine on the rate of spontaneously beating right atria. Higenamine increased the rate (EC50 = 38 nM) and the force of contraction (EC50 = 97 nM), the maximal responses being comparable with those of isoproterenol. The positive chronotropic effect of higenamine was antagonized by propranolol (30-300 nM) and practolol (10 nM-3 microM), but not by butoxamine (1 microM), indicating that it was a beta 1-adrenoceptor-mediated action. The positive chronotropic effect of higenamine was not changed by pretreatment with reserpine (4 mg/kg, i.p., 4 hr). Aconitine (0.16-0.25 microM) induced tachyarrhythmia in right atria was attenuated by quinidine (1 microM), atropine (8.6 microM) and AF-DX 116 (8.6 microM), suggesting that aconitine activates sodium channels and muscarinic receptors. Higenamine (2.5 nM) and dobutamine (1 nM) did not cause chronotropic effects by themselves, but enhanced the aconitine-induced tachyarrhythmia. These results indicate that higenamine is a beta 1-adrenoceptor full agonist in murine atria and that the aconitine-induced tachyarrhythmia is augmented by the beta 1-adrenergic action of higenamine.

Cellular electrophysiology of digitalis toxicity

The toxic effects of digitalis are attributable in part to poisoning of the enzyme Na+-K+ ATPase and in part to the interactions of digitalis with the sympathetic and parasympathetic nervous systems. Additional modifiers of the toxic effects of digitalis include the concentrations of ions such as K+ and Ca2+, the age of the subject and the extent and type of cardiac disease. At the cellular electrophysiologic level, digitalis toxicity is seen as a depolarization of the membrane with the occurrence--individually or simultaneously--of abnormalities of impulse initiation (including delayed afterdepolarizations and abnormal automaticity) and abnormalities of conduction. The afterdepolarizations result in triggered arrhythmias that differ partially in their characteristics of onset and termination from automatic and reentrant arrhythmias. The cellular electrophysiologic basis for these arrhythmias induced by toxic concentrations of digitalis and their implications with respect to arrhythmogenesis in the in situ heart are explored in detail.