Calcium channels play key roles in the heartbeat, muscle contraction, communication between nerves and hormone release. Each of these processes have in common the fact that a complex intracellular chemical process is triggered by an electrical signal. The Ca channel is the device used to translate electrical signals into chemical events; it does so by allowing flux of Ca in to the cell in response to cell membrane depolarization. Ca channels are a target for drugs in cardiovascular therapy and are of general concern to the health of the nervous system. The proposed research addresses three issues: 1) the means by which the channel chooses to pass calcium over other ions; 2) the differences between skeletal and cardiac muscle Ca channels; 3) the rated by which Ca channels are incorporated into and degraded from the cell membrane. The first two issues are addressed with the electrophysiological techniques, single cell and single channel recording. The third uses a new neurotoxin, described in appendix E, to label Ca channels and measure the turnover rates. The studies on selectivity are intended to answer questions remaining from a successful line of research. Are there multiple Ca binding sites arranged in sequence within the Ca channel pore? Are the sites identical? Is the high affinity intrapore site involved in Ca-induced inactivation of Ca current? What is novel about the pore of the "T-type" Ca channel that makes its selectivity different than the classic Ca channel? The skeletal muscle Ca channel is the main target for purification, reconstitution and cloning attempts but the channel is undescribed at the single channel level in intact cells. This project is meant to fill this gap in the literature. Clearly, the number of Ca channels in the cell membrane is important for cell function and the equilibrium between incorporation and degradation determines this number. Appendix E demonstrates that a neurotoxin inhibits synaptic function by direct and persistent binding to neuronal Ca channels. This new Ca channel label should allow study of turnover rates that underlie and determine Ca channel density. Is there modulation of these by Ca channel blockade, neuronal activity, hormones or cell growth?