The calcium-sensitive bioluminescent protein aequorin will be used to monitor the changes in intracellular calcium ion concentration involved in excitation-contraction coupling in vertebrate striated muscle. Physiological and pharmacological studies will be continued on isolatad frog skeletal muscle fibers and preparations of mammalian cardiac and smooth muscle that have been microinjected with aequorin. Image intensification will be used to localize sites of calcium release and uptake in aequorin-injected cells. Aequorin signals and transmembrane currents will be correlated in voltage-clamped dog Purkinje fibers. Estimates of resting (Ca ions) in frog skeletal muscle obtained with aequorin will be compared with estimates made inthe same fibers with Ca ions-sensitive microelectrodes. The influence of loading conditions on calcium transients will be studied in skeletal and cardiac muscle.