Calcium-osmolality interaction in the inotropic response of isolated rat atria to increased sodium concentration

The effects of osmolality (300 and 450 mOsm/l) and external calcium concentration ([Ca2+]o: 0.87, 1.13, 1.47 and 1.92 mM) on the inotropic response of isolated rat left atria to an increase of 97.8 mM in extracellular sodium concentration ([Na+]o) were studied. The evoked tensions developed during 30 min after the exposure to increased [Na+]o increased with increasing [Ca2+]o. In iso-osmotic solutions, tension was lower than in hyperosmotic solutions when [Ca2+]o was 0.87 and 1.13 mM, but not for higher [Ca2+]o, revealing a Ca2(+)-osmolality interaction in the determination of the inotropic response.