[On the mechanism of the positive inotropic effect of triamterene on the myocardium (author's transl)].

The influence of triamterene on the contractile force and the cellular electrolyte content and exchange was studied in isolated, electrically stimulated guinea-pig atria. 1. Triamterene (40 to 200 mumol/l) produces a positive inotropic effect which is most pronounced when the extracellular calcium concentration is reduced (1.2 to 0.6 mmol/l) or the contractile force is diminished by pentobarbital (400 mumol/l). The inotropic action of triamterene was shown to be independent of the rate of stimulation (1--2 Hz). 2. By pretreatment with the adrenergic beta-receptor blocking drugs bupranolol and pindolol or by reserpinization the positive inotropic effect of triamterene was not significantly altered. 3. After incubating the atrial preparations with 45calcium for 30 min the intracellularly exchangeable calcium fraction was significantly enhanced by triamterene (100 mumol/l). If triamterene was added to the bathing medium after the 45calcium exchange process had already reached a steady-state, no further increase in the extent of the intracellular calcium exchange was observed during an additional 30-min period of incubation. Since the cellular total calcium content shows remained unchanged it is postulated that the increase in calcium influx is balanced by a simultaneous elevation of calcium efflux. In addition, triamterene did not show any influence on the intracellular potassium, sodium and magnesium content as well as on the extracellular space volume.