Chemosensory Transduction in Paramecium

Paramecium tetraurelia is attracted and repelled by a variety of chemical stimuli. The attractants probably signify the presence of nutrients. For attractants, there are at least three signal transduction pathways that all lead to a hyperpolarization of the cell that results in relatively fast and smooth swimming. The three pathways differ in stimuli, receptor mechanisms and second messengers. Receptors for one and possibly two of the pathways couple surface events to the activation of a plasma membrane calcium pump. One pathway is not receptor-mediated, but affects intracellular pH by diffusion of the stimulus across the membrane. A variety of techniques are described in an effort to probe the steps of each of these pathways, which remain only partially defined.