Regulation of ciliary motility in Paramecium by cAMP and cGMP

Paramecium tetraurelia is a useful model system for the study of signal transduction mechanisms that couple changes in membrane potential to changes in ciliary motility, and cAMP and cGMP have been implicated as second messengers in this system. Changes in ciliary beat are correlated with changes in the membrane potential, V m , with hyperpolarizations resulting in increased beat frequency and increased forward swimming speed. The intercellular levels of cAMP and cGMP vary with V m , and increasing internal cAMP by microinjection or the use of membrane permeant analogs results in increased swimming speed. In addition, cAMP and cGMP modify the swimming behavior of permeabilized Paramecium , and the sensitivity of cilia to cAMP and cGMP depends upon the location of the cilia on the cell surface. However, increasing internal cAMP also results in hyperpolarization and increasing internal cAMP does not override the effect of V m . These results have lead to two conflicting models to describe the role of cAMP: (1) stimulus → hyperpolarization of V m → increased [cAMP] in → increased beat frequency, and (2) stimulus → increased [cAMP] in → hyperpolarization of V m → increased beat frequency. This review discusses the data that support these models and possible experimental approaches to resolve the paradox presented by these models.