The influence of potassium ion (K+) on digoxin-induced inhibition of porcine cerebral cortex Na+/K+-ATPase
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The in vitro influence of potassium ion modulations, in the concentration range 2mM - 500mM, on digoxin-induced inhibition of porcine cerebral cortex Na+/K+-ATPase activity was studied. The response of enzymatic activity in the presence of various K+ concentrations to digoxin was biphasic, thereby, indicating the existence of two Na+/K+-ATPase isoforms, differing in the affinity towards the tested drug. Both isoforms showed higher sensitivity to digoxin in the presence of K+ ions below 20mM in the medium assay. The IC50 values for high/low isoforms 2.77 x 10(-6) M/8.56 x 10(-5) Mand 7.06 x 10(-7) M/1.87 x 10(-5) Mwere obtained in the presence of optimal (20mM) and 2mMK(+), respectively. However, preincubation in the presence of elevated K+ concentration (50 - 500mM) in the medium assay prior to Na+/K+-ATPase exposure to digoxin did not prevent the inhibition, i.e. IC50 values for both isoforms was the same as in the presence of the optimal K+ concentration. On the contrary, addition of... 200mMK(+) into the medium assay after 10 minutes exposure of Na+/K+-ATPase to digoxin, showed a time-dependent recovery effect on the inhibited enzymatic activity. Kinetic analysis showed that digoxin inhibited Na+/K+-ATPase by reducing maximum enzymatic velocity (V-max) and K-m, implying an uncompetitive mode of interaction.