ELECTROPHYSIOLOGY OF THE IN-SITU CONTRACTILE VACUOLE COMPLEX OF PARAMECIUM REVEALS ITS MEMBRANE DYNAMICS AND ELECTROGENIC SITE DURING OSMOREGULATORY ACTIVITY

In the freshwater protozoan Paramecium multomicronucleatum, excess cyt osolic water, acquired osmotically, is segregated and expelled to the cell exterior through the activity of the contractile vacuole complex, This process keeps the cell volume electrophysiological parameters of the measured in situ using a fine-tipped microelectrode inserted into the contractile vacuole, the exocytotic vesicle of the organelle to w hich the segregated fluid is transported before being expelled to the exterior, The input capacitance decreased markedly immediately before fluid expulsion and regained its previous value when fluid filling res umed after fluid expulsion, This change in the capacitance proved that the contractile vacuole became disconnected from its radial arms, whi ch project from the vacuole, before fluid expulsion occurred and then reconnected with the arms after fluid expulsion, A positive electrical potential was recorded from the contractile vacuole only when it was connected to the radial arms, This implies that the electrogenic mecha nism resides exclusively in the radial arms and supports the idea that the decorated spongiomes, V-type proton-pump-covered terminal tubules of the radial arms that end blindly in the cytosol, are electrogenic. The linear current-voltage relationship of the contractile vacuole me mbrane also implies that few voltage-activated ion channels are presen t in the membrane, To explain the movement of water into the contracti le vacuole complex, we favour the hypothesis that the potential genera ted across the decorated spongiome membrane can be used to drive count er-anions from the cytosol into the lumen of the complex, The anions c ould then act as an osmolite to pull cytosolic water into the lumen of the organelle.