The effect of calcium on heart rate, A-V conduction, and contractility was studied in the dog's heart-lung preparation. Normal plasma calcium concentration was 2.71 ± 0.20 mm (N = 25). Heart rate was increased by calcium in the range from 1.6 to 14.0 mm and fell at still higher levels. A-V conduction time was shortest at calcium concentration slightly above normal, and decreased at higher as well as lower concentrations. At levels above 17 mm conduction block, fibrillation or cardiac arrest, or both, occurred. Contractile force was markedly reduced at low calcium. Increase of calcium lowered ventricular filling pressure, increased the rate of tension development and of relaxation, and shortened duration of contraction. The effects of calcium were not altered by pretreatment with reserpine (2 x 0.5 mg/kg, s.c., given 48 and 24 hr prior to experiment) and were not influenced by atropine (2–5 mg). The effect of calcium on contractility was fully developed 100 sec after administration; the effect on sinoatrial rhythm was established only after 4.4 ± 0.54 min.
