Characterization of the frequency-force relationship in the dog atrium in situ.

Isometric developed tension and rate of development of tension in the in situ atrium of the anesthetized dog have been shown to increase consistently with increasing heart rate over a wide range. This study thus definitely demonstrates that both the ascending and descending staircase effects occur in the dog atrium in situ. At high cardiac rates the velocity of force development may increase without a change in the extent of force development. These changes were independent of variations in aortic pressure, intactness of betaadrenergic receptors, level of cardiac catecholamine stores, cardiac innervation, or changes in coronary flow. The character of the frequency-force relationship was found to be markedly influenced by isolation of the heart with its concomitant depression of contractility. ADDITIONAL