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The regulation of CaQ2 concentration in relation to myofibrils plays a central role in the production of contraction and relaxation in cardiac muscle (Langer, 1968). The calcium pump of the sarcoplasmic reticulum is a major factor producing relaxation by removing Ca2+ from the region of the myofibrils, and may secondarily produce changes in Ca2+ release on activation leading to alterations in myocardial contraction. Cyclic AMP (adenosine 3': 5'-cyclic monophosphate) has been shown to stimulate the calcium pump (Epstein et al., 1970; Shinebourne & White, 1970), and variations in the production of cyclic AMP by adenylate cyclase may form the basis of a regulating mechanism. We have measured Ca2+ uptake (Gertz et al., 1967) and adenylate cyclase activity (Krishna et al., 1968) in the same preparations of fresh sarcoplasmic reticulum from canine myocardium. Ca2+ uptake was measured in the presence of oxalate and imidazole in duplicate at 1 min intervals, and the mean uptake over 5min was used in the calculation. Under these con-

The relation between calcium ion transport and adenylate cyclase activity in myocardial sarcoplasmic-reticulum preparations