An explanation for the positive inotropic effect of hypertonic solutions

Hypertonic solutions of mannitol and sucrose have positive inotropic effects on both cardiac and skeletal muscle in vivo and in vitro. By increasing the tonicity of dog plasma of 50 mOsm in vitro with sucrose or mannitol Ca++ activity. The negative results with calcium proteinate and simple salt solutions suggest that the increased activity increases of 20--50 percent can be measured with a calcium-specific electrode. Increased tonicity had no effect on calcium proteinate complexes. By increasing the tonicity of solutions of small molecular weight complexes, a 5--50 percent increase in Ca++ activity results, depending on the specific complex used. Increasing the tonicity of solutions of inorganic salts such as CaSO4 or CaCl2 did not increase Ca++ activity seen with small molecular weight complexes is caused by dissociation of the Ca++, not by an interaction of the sucrose or mannitol solutions with the electrode. The absolute change inducible in a 10(-3) M solution of a small molecular weight complex is in the range of 0.5 mEq/l, a physiologically significant range. Thus it is suggested that part of the positive inotropic effect of increase osmolarity is a result of increased Ca++ activity from small molecular weight complexes in plasma and interstitial fluid.