Normal cardiac function depends on the precise control of myoplasmic calcium concentration. The voltage-regulated calcium channel is an important part of this control mechanism. Cardiac disease such as hypertrophic cardiomyopathy, which is characterized by excessive intracellular Ca++ accumulation, may reflect a primary disorder in either calcium channel function, intracellular processing mechanisms, or calcium channel synthesis or degradation rates. These abnormalities may be expressed as changes in the number and distribution of channels within the cell membrane, or as a specific channel malfunction. To test this hypothesis, the functional properties of calcium channels in both normal and cardiomyopathic cells will be compared. Synthesis and degradation rates of the calcium channel and some aspects of its intracellular processing will be examined in both normal and cardiomyopathic cardiac cells. Cells will also be exposed to drugs known to influence calcium channel function (Beta-adrenergic agonists, Ca++ channel blockers) and the effects of these drugs on channel synthesis and degradation rates, and the number and distribution of channels on the cell will be determined. All parameters will be measured as a function of time in culture. Normal and cardiomyopathic (Syrian hamster BIO 14.6) heart cells will be cultured for study. Calcium channels will be identified and quantitated using the radioligand, [3H]nitrendipine. The affinity and number of binding sites will be determined by a filtration binding assay. The distribution of calcium channels ([3H]nitrendipine binding sites) on the cell surface will be determined by light microscopic autoradiography using tritium sensitive film. The functional properties of the channels will be characterized by measurement of the rate of 45Ca++ flux into the cells during K+ induced depolarization and by single channel recording using the patch clamp technique. The rates of synthesis and degradation of calcium channels will be determined by the density shift method which was developed by Devreotes et al. for measurement of acetylcholine receptor turnover rates. The role of glycosylation in intracellular processing will be examined using tunicamycin, an inhibitor of to protein glycosylation. The effects of cardiovascular drugs and several other manipulations on the number and turnover of calcium channels will be studied in order to determine if up- or down-regulation occurs with these treatments.
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