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

A blotting procedure was developed to remove superficial fluid adhering to rat atrial tissue in a consistent manner prior to chemical analyses. The K+ and water contents of rat atria stimulated for 3 hours in Krebs-Ringer bicarbonate medium containing 6 mM potassium 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 potassium concentration and osmotic pressure. Atria stimulated for 2 hours in K+-free medium, after a one hour equilibration period in the normal medium, showed a 57.4% loss in potassium 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 epinephrine. This was followed by a phase of very slow rate and of epinephrine insensitiveness. Arrhythmias were observed in both phases. The degree of contracture observed was directly related to the rate while the force of contraction was inversely related. The intrinsic rate showed a direct relationship to Ca++ concentration both in presence and absence of potassium.