Osmoregulation in Paramecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole.

In vivo K(+), Na(+), Ca(2+) and Cl(-) activities in the cytosol and the contractile vacuole fluid of Paramecium multimicronucleatum were determined in cells adapted to a number of external osmolarities and ionic conditions by using ion-selective microelectrodes. It was found that: (1) under standardized saline conditions K(+) and Cl(-) were the major osmolytes in both the cytosol and the contractile vacuole fluid; and (2) the osmolarity of the contractile vacuole fluid, determined from K(+) and Cl(-) activities only, was always more than 1.5 times higher than that of the cytosol. These findings indicate that excess cytosolic water crosses the contractile vacuole complex membrane osmotically. Substitution of choline or Ca(2+) for K(+) in the external solution or the external application of furosemide caused concomitant decreases in the cytosolic K(+) and Cl(-) activities that were accompanied by a decrease in the water segregation activity of the contractile vacuole complex. This implies that the cytosolic K(+) and Cl(-) are actively coimported across the plasma membrane. Thus, the osmotic gradients across both the plasma membrane and the membrane of the contractile vacuole complex ensure a controlled cascade of water flow through the cell that can provide for osmoregulation as well as the possible extrusion of metabolic waste by the contractile vacuole complex.