Down-regulation of Na+ pump α2 isoform in isoprenaline-induced cardiac hypertrophy in rat:: Evidence for increased receptor binding affinity but reduced inotropic potency of digoxin

Cardiac hypertrophy in rats induces a down-regulation of Na+,K+-ATPase alpha(2) isoform, although its functional consequences are poorly understood. Using a mathematical modeling approach that allows differentiation between effects elicited at the receptor and postreceptor level, we studied uptake, receptor binding kinetics, and positive inotropism of digoxin in single-pass Langendorff-perfused hearts of vehicle- and isoprenaline-pretreated rats (2.4 mg/kg per day over 4 days). Digoxin outflow concentration and left ventricular developed pressure data were measured for three consecutive doses (15, 30, and 45 mu g) in the absence and presence of the reverse mode Na+/Ca2+ exchange inhibitor 2-[2-[4-(4-nitrobenzyloxyl)phenyl]ethyl isothiourea methansulfonate] (KB-R7943) (0.1 mu M) in perfusate. In hypertrophied hearts, 1) the amount of alpha(2) receptors was reduced to 52% of control levels; 2) the digoxin binding affinity was increased 12-fold due to a decrease in dissociation rate constants of alpha(1) and alpha(2) receptors, and 3) the inotropic responsiveness to digoxin was attenuated on the stimulus-response level, where the coupling ratio of stimulus to response was reduced to 38% of control values. Only in the lowest dose level (15 mu g) was this decrease in inotropic potency counterbalanced by the increase in receptor affinity. The Na+,K+-ATPase isoform shift was not responsible for the diminished inotropic effect of digoxin. Coadministration of KB-R7943 significantly reduced cellular response generation at higher digoxin doses to the same limiting stimulus-response relationship in both the vehicle and isoprenaline group.