Electrocardiographical, Biochemical and Morphological Effects of Chronic Low Level Cadmium Feeding on the Rat Heart

Cadmium is known to dissociate myocardial excitation-contraction coupling and to depress the excitability of the cardiac conduction system in vitro (1-7). Short-term feeding experiments concluding that cadmium increases conduction time through the atrioventricular node-His-Purkinje system have partially extended these in vitro observations to in vivo systems (8, 9). Although the cadmium content of the heart increases with age and exposure (10-14), the effects of cadmium have not been systematically analyzed for possible significance as a contributing factor to degenerative heart disease. This preliminary study was undertaken to investigate electrocardiographical, biochemical, and morphological changes in mammalian hearts associated with long-term, low-level cadmium feeding. Methods. Eighteen weanling female rats of the Long-Evans strain were obtained and housed as described (15). They received a special rye-based, low-cadmium diet and deionized water fortified with Mn, Co, Cu, Zn, and Mo, as described by Schroeder and Vinton (16), for a total of 24 months when all were sacrificed. For half of them, cadmium acetate was added to the water to provide a final concentration of five parts per million (ppm) cadmium; for the half which served as controls, no cadmium was added; however, they were otherwise treated identically (16). For 16 rats (8 cadmium-fed and 8 control), indirect systolic pressures were determined 12, 18, and 21 months after weaning. Eight of the rats (4 cadmium-fed and 4 control) were studied electrocardiographically and their blood pressures determined directly after which they were sacrificed so that the various parts of the heart could be assayed for cadmium and zinc. The phosphate spectrum of hearts from the eight remaining rats (4 cadmium-fed and 4 control) were analyzed by phosphorus nuclear magnetic resonance spectroscopy (31P NMR). In addition hearts from a pair of rats not otherwise studied (1 cadmium-fed and 1 control) were examined by electron microscopy.