CALCIUM MOVEMENTS IN RESTING AND STIMULATED ISOLATED RABBIT ATRIA

¹ Department of Pharmacology, State University of New York, Upstate Medical Center, Syracuse, New York

Radioactive calcium uptake, radioactive calcium outflow and total tissue calcium concentration were studied in isolated left rabbit atria under varying conditions of stimulation and varying external calcium, sodium and potassium concentrations. Three different tissue-calcium compartments were demonstrated in both resting and stimulated atria: 1) an unexchangeable tissue-calcium compartment. 2) a rapidly exchangeable tissue-calcium compartment. Its rate of exchange was increased by stimulation of the atrium at various rates. The effects of rate of stimulation on calcium exchange were seen after 1-min but not after 5-min exposure to radioactive calcium. The rate constant for the outflow from the second compartment was not altered by stimulation. The half-times of calcium movements into and out of this compartment are sufficiently rapid to account for the speed with which changes in the external calcium concentration alter contractility. It was proposed that the concentration of calcium in this compartment is a determining factor of contractile force. 3) a tissue compartment into which uptake was increased and outflow was decreased by stimulation, resulting in a net gain of tissue calcium. This calcium compartment is probably not directly related to changes in contractile force since its rate constant is too slow to account for the rapid changes in contractility produced by the addition or removal of calcium in the external medium.