Distribution of ³H-nitrendipine in the isolated perfused rat heart as revealed by electron microscopic autoradiography

Subcellular distribution of ³H-nitrendipine was investigated in the isolated perfused rat heart by means of electron microscopic autoradiography. Mechanical measurements of cardiac contractile activity (dP/dt) were performed on the hearts used for autoradiography. There was a non-random pattern of silver grain distribution over the myocardial cells. The grain density, i.e. percentage of total grains/percentage of cell area, was determined for each cell component. A grain density higher than one was noted on the cell membrane, T-tubules and capillaries, indicating a significant incorporation of ³H-nitrendipine into the structures. Concentrations of unlabeled nitrendipine containing radiolabeled nitrendipine caused increased grain density and a decreased contractile activity (dP/dt) by 6% to 30%. It is therefore possible that the pharmacological action of nitrendipine is associated with the localization of ³H-nitrendipine on or in the cell membrane, T-tubules and capillaries.