Compensatory adaptation of the heart to chronic rate overload: increase in calcium transport ATPase activity of myocardial sarcoplasmic reticulum.

This study was undertaken to test the hypothesis that a compensatory response of the heart to a chronic and continuous, metabolic and heart rate overload was an increase in the calcium sequestering activity of the myocardial sarcoplasmic reticulum. Calcium sequestering activity was estimated by determination of the calcium-dependent ATPase (Ca2+-ATPase) activity of isolated microsomes. Chronic rate overload was modelled by comparing: dysthyroid and control rats; control swine and swine with implanted cardiac pacemakers set at 180 beats/min; and different species of mammals with widely different heart rates. The myocardial sarcoplasmic reticulum Ca2+-ATPase pump activity was significantly increased by 39% for hyperthyroid rats compared to control rats and by 87% for control rats compared to thyroidectomized rats; by 63% for paced swine compared to control swine; and by 43% for rats compared to guinea pigs, by 140% for guinea pigs compared to dogs and by 120% for dogs compared to cows. These data indicate that calcium sequestering activity of myocardial sarcoplasmic reticulum increases in equivalent proportion to the chronotropic demand and that heart rate is a hemodynamic correlate of the sarcoplasmic reticulum Ca2+-ATPase activity.