Increase in passive tension of spontaneously beating atria in hyperosmolar media

Spontaneously beating rabbit atria were used for study of increases in passive tension induced by hyperosmolar Tyrode bathing media. The time course and degree of passive tension change was dependent on the level of osmolality and the osmotic agent used. The effectiveness of various agents added to Tyrode solution in increasing resting tension was in the order sucrose greater than mannitol greater than or equal to NaCl greater than glucose greater than = no change. The hyperosmotically induced "contracture" was similar to contracture induced by high Ca2+ medium, but the maximum hyperosmotic effect was greater than that of high Ca2+. In contrast to active atria, inactive atria showed no increase in passive tension in hyperosmotic solutions or in Tyrode solution containing high Ca2+. After depolarization with Tyrode medium containing high K+ or zero K+, increased Ca2+ caused contracture in inactive left atria greater than that caused by hyperosmotic media (+200 mM sucrose) in active right atria or atrial pairs. Results of the study indicate that although other factors may be involved, increased Ca2+ flux inward could largely account for the passive tension response.