Influence of Potassium and Calcium on Behavior of Isolated Rat Atria.

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Abstract
The intrinsic rate of the isolated rat atria showed a direct relationship to calcium concentration both in presence and absence of potassium. The K+ and water contents of rat atria stimulated for 3 hours in Krebs-Ringer HCO3- medium containing 6 mM K+ were not significantly different from those of freshly dissected atria, indicating that the medium used in the present investigation is physiological with respect to its K+ concentration and osmotic pressure. Atria stimulated for 2 hours in K+ -free medium, after a 1 hour equilibration period in the normal medium, showed a 57.4% loss in K+ content. No significant change in water content, however, was observed under these conditions. Atria stimulated 200 times per minute in K+ -free medium showed idiopathic rhythmicity, contracture, and a temporary increase in contractile force. Self-stimulated atria placed in K+-free medium showed a temporary period of increased rate which was sensitive to the chronotropic action of epinphrine. This was followed by a phase of very slow rate and of epinephrine insensitive-ness. Arrhythmias were observed in both phases. The degree of contracture observed was directly related to the rate.