Regional differences in the response of cardiac cells to triiodothyronine administration across the left ventricle free wall of rat heart.

We have studied the effect of T3 administration (50 micrograms/Kg/day) on the phenotype expression of several glucose-metabolizing enzymes (hexokinase, HK, glucose-6-phosphate dehydrogenase, G6P-DH, aldolase, ALD, phosphofructokinase, PFK, lactate dehydrogenase, LDH) in the different myocardial layers of the left ventricle wall. In the control rats, most of these enzyme activities are uniformly distributed across the left ventricle wall, G6P-DH being the only exception. In the rats given T3 for 14 days, the mean levels of PFK, HK and ALD activities increased significantly. With regard to the transmural distribution patterns, that of PFK was unchanged, unlike those of HK and ALD which exhibited their maximum increase in activity in the midmyocardium or in the mid- and subepicardial myocardium. With LDH, a significant increase in activity was found in the subepicardial layers which escaped detection on the whole homogenate. It is concluded that the administration of thyroid hormone has different effects on enzyme phenotype expression of cardiomyocytes in different regions of the cardiac wall.