Ventricular function and Na+,K(+)-ATPase activity and distribution with chronic supraventricular tachycardia

The molecular and cellular mechanisms responsible for the dilated cardiomyopathy associated with chronic supraventricular tachycardia are not well understood. The purpose of this study was to examine Na+,K(+)-ATPase activity and distribution in a pacing induced model of dilated cardiomyopathy.

Left ventricular function and Na+,K(+)-ATPase activity and distribution were examined in two groups of pigs: (1) atrially paced for 3 weeks (supraventricular tachycardia, 240 beats.min-1); (2) sham operated controls.

10 Yorkshire male swine (23-25 kg) were randomly assigned to the control group or the supraventricular tachycardia group.

Left ventricular function was examined using simultaneous pressure echocardiography. Na+,K(+)-ATPase activity was determined in tissue homogenates by measuring the rate of p-nitrophenol-phosphate (pNPP) hydrolysis. Changes in content and distribution of Na+,K(+)-ATPase were examined immuno-histochemically in tissue sections. Left ventricular fractional shortening decreased significantly with supraventricular tachycardia as compared to controls, at 15 (SEM 3)% v 31(3)%, respectively p less than 0.05. Supraventricular tachycardia resulted in a significant increase in end diastolic dimension [5.0(0.3) cm v 3.5(0.2) cm, respectively p less than 0.05] and pressure [22(4)mm Hg v 6(2)mm Hg, respectively p less than 0.05]. Maximal Na+,K(+)-ATPase activity (microgram pNPP.mg-1 protein.h-1) was significantly lower with supraventricular tachycardia than in controls, at 0.45(0.12) v 0.64(0.06), respectively p less than 0.05. In the presence of 7 microM digitalis, Na+,K(+)-ATPase activity was inhibited by 68% in control and by 45% in supraventricular tachycardia homogenates (p less than 0.05). In control sections all left ventricular myocytes showed a uniform immunostaining pattern along the sarcolemma for Na+,K(+)-ATPase, whereas a focal loss of staining was observed in myocytes from the supraventricular tachycardia group.

The congestive cardiomyopathy produced by supraventricular tachycardia was associated with a reduction in sarcolemmal Na+,K(+)-ATPase activity and changes in enzyme distribution. The findings also suggest a reduction in digitalis sensitivity with chronic supraventricular tachycardia. These alterations in Na+,K(+)-ATPase activity may be one potential mechanism responsible for the depressed left ventricular function associated with chronic supraventricular tachycardia.