Spatial distribution of functional OH- carriers along a characean internodal cell: determined by the effect of cytochalasin B on H14CO3- assimilation.

Abstract

The present study was aimed at testing an hypothesis relating to the OH− efflux pattern developed on internodal cells ofChara corallina. It was suggested that OH− efflux carriers were restricted to a limited number of sites along the internodal cell. Hydroxyl ions were considered to be delivered to these sites via the streaming cytoplasm. To test this hypothesis, cytochalasin B was used to inhibit cyclosis. Under these conditions OH− efflux and H14CO 3 − assimilation should have been inhibited. Our results indicated that cyclosis was inhibited within 60 min in the presence of 30 μg/ml cytochalasin B. When14CO2 (pH 5.0) fixation experiments were conducted under these conditions, no inhibition of photosynthesis was observed. Cytochalasin B was shown, therefore, to inhibit cyclosis without affecting thein vivo reactions of photosynthesis. A parallel study on the effect of increasing cytochalasin B concentrations on cyclosis and H14CO 3 − assimilation revealed that there was a limiting velocity of cyclosis below which H14CO 3 − assimilation was inhibited. These results were interpreted in terms of a rate-limiting step associated with the supply of OH− to the operational efflux sites. At cytochalasin B concentrations below 15–20 μg/ml, no effect was observed on H14CO 3 − assimilation. This suggested that cytochalasin B does not interfere with the membrane-bound HCO 3 − and OH− transport systems of this species. The OH− efflux pattern underwent significant modification following cyclosis inhibition. A change from discrete band efflux sites to a network of numerous small, localized, disc-shaped efflux sites was observed. The discovery of this modified OH− efflux system provided an explanation for the observed limited sensitivity of H14CO 3 − influx to cessation of cyclosis. These results invalidated part of our hypothesis, since they revealed that the OH− carriers are uniformly distributed over the plasmalemma surface. A modified hypothesis to account for the spatial distribution of the OH− efflux sites is presented.