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Quinidine-like immunoreactivity (Abbott TDx Quinidine fluorescence polarization immunoassay) was measured in control, sham-operated, abdominal aorta stenotic and hyperthyroid (thyroxine feeding 0.1 mg/rat/day rats. The mean quinidine levels (mumol quinidine/1 serum) were 0.07, 0.14, 0.13 and 0.25, respectively. The elevation in experimental hyperthyroidism was statistically (analysis of variance, Duncan's test) significant. The possibility of the existence of an endogenous antiarrhythmic, immunologically cross-reacting in quinidine immunoassay, is discussed.

The effect of various autonomic blockers on the heart rate (from the ECG recording) of intact rats and of animals with experimental hyperthyroidism induced by the administration of dried thyroid was studied. In intact rats, the heart rate fell significantly (by 22%) during the first week after the peroral administration of propranolol (0.05% in food), but trimepranol (0.02%), reserpine (0.001%) and guanethidine (0.1%) had no effect (in a suplementary experment, a small, but statistically significant decrease was also found after trimepranol). In experimental hyperthyroidism, using the same blocker doses, the heart rate fell significantly after propranolol (by 23%), trimepranol (22%), reserpine (16%) and, in a preliminary experiment, guanethidine (19%). On injecting 0.05 mg propranolol intravenously, the maximum drop in the heart rate in intact rats was 22%, while in hyperthyroidism it was hardly more than half this value (at all the intervals from 2 to 30 minutes the decrease was statistically significant). Practolol, in a dose of 0.3 mg i.v., was less effective -- in intact rats the heart rate fell by not more than 12% (at all the intervals from 2 to 30 minutes the decrease was significant), but in hyperthyroidism the drop was slower and statistically non-significant. The results do not furnish an unequivocal answer to the question of the role of adrenergic regulation of the heart rate under physiological conditions and in experimental hyperthyroidism in rats.

With reference to the indirect evidence showing that heart growth in the presence of a cardiac overload is modulated by the adrenal cortex, we studied heart reactions to overloading (ligation of the abdominal aorta or hyperthyroidism) with simultaneous administration of aminoglutethimide, an inhibitor of steroidogenesis. Aminoglutethimide (Elipten CIBA, 0.5% in the diet) markedly inhibited the growth reaction of the heart 14 days after ligation of the abdominal aorta (reducing the aortal blood flow to about 40%) or after the administration of dried thyroid (Thyreoidin SPOFA, 0.2% in the diet). The effect of aminoglutethimide is evidently not due to its simultaneous antithyroid action, because methylthiouracil (Alkiron SPOFA, 0.2% in the diet) did not limit heart growth after ligation of the abdominal aorta, despite its having a stronger goitrogenic effect than aminoglutethimide. The results are a further contribution to the series of indirect evidence of the modulating effect of the adrenal cortex on heart growth.