Abstract

Quiescent atria and driven atria (2/sec) were exposed to ouabain (1 µg/mI) for 30-min periods under conditions of varying temperature (25°-35°C) and calcium concentration (0.625-5.0 mM). Contractile force at stimulus frequencies of 2/sec and 1/sec, the maximal contractile force obtained by paired pulse stimulation and the response to an abrupt reduction in stimulus frequency were measured before and after exposure to ouabain. The effects of temperature were studied at 2.5 mM calcium concentration, and the effects of calcium were studied at 30°C. Exposure of the quiescent atria to ouabain demonstrated that myocardial activity is not essential for ouabain action. However, ouabain effects were not clearly demonstrable in all responses studied at temperatures below 30°C or at calcium concentrations below 125 mM. Contracting atria showed a larger increase in the contractile responses upon exposure to ouabain than did quiescent atria exposed for the same time period, which confirms that activity does enhance the effect of ouabain.