Reduced 3H-ouabain binding site (Na,K-ATPase) concentration in ventricular myocardium of dogs with tachycardia induced heart failure.

Abstract

The present study evaluates 3H-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 facilitated 3H-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 of 3H-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 of 3H-ouabain was 0.9-0.8% of total uptake and retention obtained in the standard assay; apparent dissociation constant (KD) was 6.5 x 10(-8)-6.6 x 10(-8) mol/l; loss of specifically bound 3H-ouabain during washout at 0 degrees C occurred with a half-life time (T1/2) of 120 and 121 h. Hence, total 3H-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 of 3H-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 myocardium 3H-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.