Inosine : a naturally occurring cardiotonic agent.

For many years, Inosine was considered to be a simple metabolite of adenosine which was devoid of any cardiovascular effects. This theoretical ineffectiveness can be explained in the light of recent studies by the use of inadequate doses. In fact, higher doses of inosine, a non-toxic nucleoside, have demonstrated, experimentally, a cardiovascular activity and the pharmacological profile of this naturally occurring substance has been defined. Like adenosine, inosine is a potent coronary vasodilator. The vasodilatation induced by inosine is only partly due to increased metabolic demands. Inosine has a direct action on coronary artery relaxation independent of the inotropic effect. It alters the balance between oxygen supply and demand which is reflected by an intramyocardial redistribution of oxygen in favour of the sub-endocardial zones. Inosine acts on the coronary circulation like a "regulator of myocardial nutrition", unlike adenosine, which can be thought of as a "coronary vasoregulator". This dissociation between the two nucleosides is apparently due to different vascular sites of action. The positive inotropic action of inosine, which has been demonstrated in both healthy and pathological myocardium in all of the experimental animal species studied, is not due to stimulation of the cardiac beta-adrenergic receptors, as beta-blockers do not antagonize the positive inotropic effect of inosine. This increase in myocardial contractile dynamics is evident in infarcted as well as healthy areas of myocardium. The inotropic and coronary vasodilator effects of inosine are not associated with any modification of the chronotropic function. Inosine is not arrhythmogenic, even at high doses. Furthermore, it does not affect atrioventricular conduction. It has been demonstrated that inosine is capable of antagonizing ouabain induced arrhythmias. Various clinical studies confirm the positive inotropic action of inosine, without any alteration in the post-load, the pre-load or the heart rate. The positive inotropic action of inosine can therefore be considered to be selective. Together with these haemodynamic effects, it has been shown that the addition of inosine to cardioplegic solutions improves the functional recovery of the myocardium, by increasing the quantity of energy-rich phosphates. Similar beneficial results have been obtained in renal transplantation, both experimentally and in clinical studies. The mechanism of action of inosine remains unknown. Are the haemodynamic effects of this compound due to its metabolic effects? Are there specific myocardial purinergic receptors? (ABSTRACT TRUNCATED AT 400 WORDS)