Effects of physical training and thyroxine on rodent cardiac functional and biochemical properties.

The effects of 8 weeks of interval running and of 14 days of thyroxine treatment were studied on selected cardiac functional and biochemical parameters of contraction in both female guinea pigs and rats. Relative to their respective sendentary-control groups, both trained groups demonstrated typical exercise adaptations of resting bradycardia and of increases in skeletal muscle oxidative enzymes and relative heart mass (P less than 0.05). Myosin ATPase, Ca2+ regulated myofibril ATPase, and + dP/dt max were at least two-fold higher in rats compared to guinea pigs (P less than 0.001). Physical training did not augment any of these parameters in either species under the experimental conditions studied. However, biochemical and functional indices of intrinsic cardiac contractility in guinea pigs were increased by injections of thyroxine; whereas, the same treatment regimen was without effect on the rats. These findings suggest that chronic running does not alter the intrinsic cardiac contractility state in either species. Also, exercise doesn't change the thyroid state of the guinea pig to a level sufficient to affect cardiac function.