Influence of osmolarity of perfusate on contractility of mammalian myocardium

The effect of changes in the osmolarity of the perfusing fluid on strength and time course of myocardial contraction was studied in cat papillary muscles and atrial strips. Osmolar concentration was varied by adding up to 750 mm sucrose or mannitol to a slightly hypoosmolar solution, leaving molar concentrations of the original solutes unchanged. Optimum osmolarity for tension development by papillary muscles was close to that of serum (330 mosmol/ liter), but for atrial muscles was 150 mosmol/liter higher. Osmolarity changes apparently influence contractility by altering the intracellular water concentration. Increases of osmolarity up to the optimum augment tension development by increasing the degree of activation of the contractile elements. This may be due to increased concentration of calcium in a superficial area of the fiber. Increases of osmolar concentration above the optimum lower developed tension, probably by reducing the intrinsic speed of the muscle. Tension development in response to external stimuli is abolished by raising osmolarity to three times that of serum (1 osmol/liter). Spontaneous contractions cease in solutions of twice normal osmolar concentration.