Two strains of rats were obtained by selective breeding: the IR strain, resistant to isoprenaline-induced myocardial lesions and the IS strain, sensitive to this damage. The IR rats grew more slowly, the weight of their adipose tissue was higher and the weight of m. soleus was less than that of the IS rats. The IR rats had a higher content of triglycerides in the serum and a lower isoprenaline-stimulated lipolytic activity of adipose tissue in vitro. The basal NEFA level in the serum and its rise after the administration of isoprenaline in vivo did not differ between the strains. The IR rats had a higher content of glycogen in the heart and in the muscle. After the administration of isoprenaline the glycogen content decreased more slowly in IR rats. The findings indicate a considerable importance of the glycogen stores in the heart for the resistance of myocardium to damage.

The effect of an increased ambient temperature on the incidence of acute myocardial necrosis caused by two subcutaneous injections of isoproterenol (80 mg/kg) 24 h apart has been investigated. Raising the temperature of the microclimate in the cages by 3–5 °C considerably increased the incidence of cardiac necrosis and the mortality after isoproterenol. Increasing the number of animals per cage enhanced the necrogenic effect of isoproterenol on heart muscle due to a rise in cage temperature. Exposing rats for 3–4 h after isoproterenol administration to a raised ambient temperature increased their sensitivity to isoproterenol. The same exposure made 20 h after isoproterenol administration had no such effect. Shaving the animals decreased their sensitivity to isoproterenol at an increased ambient temperature. The relation between the necrogenic effect of catecholamines and thermoregulation is discussed.

For the quantitative evaluation of myocardial damage induced by isoprenaline (ISO) a method based on the uptake of 203Hg-labelled Mercurascan (MSC) in the heart was used. The increase of myocardial uptake of MSC in ISO-treated rats over control values was very rapid and might be directly proportional to the number of damaged myocardial cells actually present in the heart. MSC-uptake method of myocardial damage evaluation was more sensitive and precise than other methods tested (macroscopic evaluation according to Rona, increase of heart weight). Different modifications of MSC-uptake test may be selected in relation to experimental conditions. MSC test is especially useful for evaluation of early myocardial lesions induced by small doses of ISO (0.01 mg/kg).