Hyperthyroid dog left ventricle has the same oxygen consumption versus pressure-volume area (PVA) relation as euthyroid dog.

We studied the effects of hyperthyroidism on the relation between O2 consumption (Vo2) and the pressure-volume area (PVA) of the left ventricle (LV) in dogs. PVA is a measure of the total mechanical energy generated per beat of LV. Dogs were treated by daily intramuscular injection of 0.3 or 1.0 mg/kg L-thyroxine over 2-5 weeks. Hyperthyroid dogs had a 40 times higher serum T4, a 40% higher sinus heart rate, and a 35% higher LV Emax (an index of ventricular contractility) than euthyroid dogs. Hyperthyroid dog hearts had linear Vo2-PVA relations like euthyroid dog hearts. The regression line was Vo2 = A x PVA + B, where A was 2.30 (dimensionless) and B was 0.53 J/beat per 100 g LV. B was significantly increased with dobutamine and decreased with propranolol, whereas A was not significantly changed by them. These A and B values were comparable to euthyroid data. Hyperthyroidism did not significantly affect myosin Ca-ATPase activity and V3-type myosin predominance, but increased the speed of the force transient response to length perturbation by 20%-70%, suggesting similar increases in crossbridge cycling rate. We conclude that in spite of accelerated crossbridge cycling rate the Vo2-PVA relation was not altered by hyperthyroidism in dogs.