Decreased myocardial function and myosin ATPase in hearts from diabetic rats.

The effect of diabetes on cardiac function was determined in isolated rat hearts. Diabetes was induced by injection of alloxan (doses ranged from 37.5 to 60 mg/kg body wt), and the heart were removed and perfused in the working heart preparation. Doses of alloxan ranging from 37.5 to 42 mg/kg did not consistently alter cardiac function even though serum glucose was elevated and serum thyroid hormones were reduced. Injection of 45 mg/kg of alloxan caused a large increase in serum glucose and a larger decrease in thyroid hormones. In this case, ventricular function was more consistently depressed after 1-2 wk. Function was not altered 48 h after injection of 60 mg kg of alloxan. However, when animals were given 60 mg/kg of alloxan and then maintained on insulin for 7 days, depressed cardiac function developed within 4 days after the insulin treatment was stopped. The decline in function involved a decrease in heart rate peak systolic pressure, and left ventricular +dP/dt. It was associated with greatly reduced serum thyroid hormones (both T3 and T4) and lower ventricular Ca2+-activated myosin ATPase activity. Fasting of rats for 4 days also resulted in decreased serum T3 and T4, depressed cardiac function (although heart rate was unchanged), and lower Ca2+-activated myosin ATPase activity.