Agradecimentos: This work was supported by FAPESP/Brazil (2000/08836-3 and 2003/04688-5) and CNPq (350088/2003-9 and 300632/2005-3). The excellent technical support by Ms. Elizângela S. Oliveira and Ana C. Fantin is acknowledged. We are indebted to Mr. Marcos A. Rocha and to the Division of Clinical Pathology of the Hospital das Clínicas da UNICAMP for the osmolality measurement of the solutions
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Abstract: Hypertonic NaCl solutions have been used for small-volume resuscitation from hypovolemic shock. We sought to identify osmolality- and Na+-dependent components of the effects of the hyperosmotic NaCl solution (85 mOsm/kg increment) on contraction and cytosolic Ca2+ concentration ([Ca2+]i) in isolated rat ventricular myocytes. The biphasic change in contraction and Ca2+ transient amplitude (decrease followed by recovery) was accompanied by qualitatively similar changes in sarcoplasmic reticulum (SR) Ca2+ content and fractional release and was mimicked by isosmotic, equimolar increase in extracellular [Na+] ([Na+]o). Raising osmolality with sucrose, however, augmented systolic [Ca2+]i monotonically without change in SR parameters and markedly decreased contraction amplitude and diastolic cell length. Functional SR inhibition with thapsigargin abolished hyperosmolality effects on [Ca2+]i. After 15-min perfusion, both hyperosmotic solutions slowed mechanical relaxation during twitches and [Ca2+]i decline during caffeine-evoked transients, raised diastolic and systolic [Ca2+]i, and depressed systolic contractile activity. These effects were greater with sucrose solution, and were not observed after isosmotic [Na+]o increase. We conclude that under the present experimental conditions, transmembrane Na+ redistribution apparently plays an important role in determining changes in SR Ca2+ mobilization, which markedly affect contractile response to hyperosmotic NaCl solutions and attenuate the osmotically induced depression of contractile activity Ver menos