The effects of varying incubation K+ concentration on the inhibitory action of amiodarone on the Mg(2+)-dependent ATP hydrolysis by myocardial Na(+)-K(+)-ATPase (EC 220.127.116.11) were studied in guinea pig heart preparations. In the first part of the study, it was established that the activity of the enzyme increased with growing concentrations up to approximately 20 mM K+. The concentration-response relationships for amiodarone were investigated in incubation media containing 2.5, 5.0 and 10 mM K+ respectively. Amiodarone exhibited similar concentration-dependent inhibitory effects in the range of 0.01 nM-80 microM at 2.5 mM, 0.13-150 microM at 5.0 mM and 0.3-700 microM at 10.0 mM K+. The corresponding IC50 values were 10.4 +/- 3.2 microM, 28.3 +/- 7.6 microM at 5.0 mM and 33.3 +/- 9.2 microM at 10.0 mM K+, respectively. Thus, reduction in the K+ concentration from the "standard" 5.0 to 2.5 mM was accompanied by a significant right-to-left shift in the inhibitory potency of amiodarone, the effective concentrations being shifted from microM into nM ranges. Increasing K+ concentration to 10 mM on the other hand attained opposite but less remarkable effects. The results show that the in vitro inhibition of myocardial Na(+)-K(+)-ATPase activity by amiodarone is related to the K+ concentration of the incubation medium. These effects may be pertinent to the mechanism by which this drug interferes with the electrogenic Na+/K+ pump activity of the enzyme, thereby probably contributing to the mechanism(s) responsible for some of its cardiac actions.