Increased heart rate prevents the isomyosin shift after cardiac transplantation in the rat.

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Abstract
The heterotopically transplanted rat heart undergoes significant atrophy and a shift from V1 to V3 isomyosin. The purpose of this study was to pace the cardiac isograft and determine whether an increase in heart rate would attenuate the changes in cardiac mass and isoenzyme distribution. Nonpaced transplanted hearts were compared with hearts in which pacing was initiated at 7 Hz, 24 hours after transplantation, and continued for 7 days. There was a 29% decrease in myosin ATPase activity and a 22% decrease in alpha-myosin in the nonpaced isograft; both decreases were completely prevented by pacing. The decrease in cardiac mass was also significantly attenuated. Pacing did not alter intrinsic heart rate, systolic pressure, dP/dt, or norepinephrine concentration in the isograft. These results suggest that the adaptation in both cardiac mass and isoenzymes may be related to the rate or the rate-pressure product in the transplanted paced heart independent of left ventricular pressure, tissue catecholamines, or neural activity.