ABSTRACT
The twitch force of isolated electrically paced ventricular strips of flounder, Platichthys flesus L., increased after lowering the extracellular sodium chloride concentration by 50 mmol l−1. This response was markedly reduced by replacing the sodium chloride with either Tris-HCl or sucrose, so that osmolarity was unchanged.
The 45Ca efflux decreased and the 45Ca influx increased when the extracellular sodium concentration 
was lowered. In contrast, changing only the osmolarity had no observable effect on these fluxes.
An increased resting tension appeared in strips exposed to a Na+-, Ca2+-free solution. This was transient at an unchanged osmolarity but became permanent at an osmolarity lowered by 100 mosmol l−1.
These results suggest that both a lowered Na0 and a lowered osmolarity have a positive inotropic effect, due respectively to an increased cellular uptake of Ca2+ and a redistribution of cellular Ca2+.
