The present study evaluates3H-ouabain binding site (Na,K-ATPase) concentration in left ventricular myocardium of dogs with heart failure induced by tachycardia as a result of ventricular pacing. Samples of left ventricle were obtained from 10 dogs exposed to pacing of 240 beats/min for 3 to 4 weeks and eight sham-operated controls. Na,K-ATPase was quantified using vanadate facilitated3H-ouabain binding to intact samples. At time of sacrifice paced dogs showed clinical signs of heart failure, a significant 257% increase in left ventricular end diastolic pressure and a significant 46% decrease in left ventricular dP/dt compared with control. There was no significant change in left ventricular mass.3H-ouabain binding concentration was significantly reduced by 16%. Evaluation of3H-ouabain binding kinetics revealed no significant difference between myocardium from paced and control dogs: Equilibrium binding conditions were at the various concentrations used obtained after similar incubation time; nonspecific uptake and retention of3H-ouabain was 0.9-0.8% of total uptake and retention obtained in the standard assay; apparent dissociation constant (KD) was 6.5×10-8-6.6×10-8mol/l; loss of specifically bound3H-ouabain during washout at 0°C occurred with a half-life time (T3/2) of 120 and 121h. Hence, total3H-ouabain binding site concentration in left ventricular myocardium was (mean±SEM) 1110±56 and 1317±68 pmol/g wet weight, 8.54±0.43 and 10.05±0.52 pmol/mg protein, and the total amount of3H-ouabain binding sites in the entire left ventricle 121±6 and 162±8 nmol in paced (n=10) and control (n=8) dogs (p<0.05), respectively. In conclusion, the present study reports a significant reduction in left ventricular myocardium3H-ouabain binding site concentration in tachycardia induced heart failure. This observation supports the concept of a relationship between Na,K-ATPase concentration and contractile capacity and may be of pathophysiological importance in tachycardia and heart failure.