EFFECTS OF TEMPERATURE AND OUABAIN ON THE RATE OF ACTION OF CALCIUM ON ATRIAL CONTRACTIONS

¹ Department of Pharmacology, University of Mississippi School of Medicine, Jackson, Mississippi

When beating left atria were transferred from normal Ringer's solution to one containing low calcium (2.4 to 0.24 mM CaCl2), isometric contractile tension declined to a new steady state level exponentially in time in one phase only. Increases of heart rate and temperature and pretreatment with 5 x 10-7 M ouabain accelerated the rate of decline without changing the exponential nature of the process. Temperature coefficients (Q₁₀) ranged from 1.5 to 2.9 for control atria, being highest at the lower ranges of temperature and heart rate, Q₁₀ values for ouabain varied from 1.3 to 1.4 and were not greatly influenced by the heart rates and temperatures examined. These findings are consistent with the hypothesis that the time course of tension change due to an altered external calcium concentration reflects a quantity of calcium taken up reversibly by the heart muscle as tension changes. The rate-limiting step or steps governing this movement of cellular calcium appeared to be more of a physical, rather than a chemical, nature. The rates of these processes can be influenced by heart rate, temperature and ouabain.