Calcium regulates independently ciliary beat and cell contraction in Paramecium cells

Intracellular Ca2+ concentration is a well-known signal regulator for various physiological activities. In many cases, Ca2+ simultaneously regulates individual functions in single cells. How can Ca2+ regulate these functions independently? In Paramecium cells, the contractile cytoskeletal network and cilia are located close to each other near the cell surface. Cell body contraction, ciliary reversal, and rises in ciliary beat frequency are regulated by intracellular Ca2+ concentration. However, they are not always triggered simultaneously. We injected caged calcium into Paramecium caudatum cells and continuously applied weak ultraviolet light to the cells to slowly increase intracellular Ca2+ concentration. The cell bodies began to contract just after the start of ultraviolet light application, and the degree of contraction increased gradually thereafter. On the other hand, cilia began to reverse 1.4 s after the start of ultraviolet application and reversed completely within 100 ms. Ciliary beat frequency in the reverse direction was significantly higher than in the normal direction. These results indicate that cell body contraction is regulated by Ca2+ in a dose-dependent manner in living P. caudatum. On the other hand, ciliary reversal and rise in ciliary beat frequency are triggered by Ca2+ in an all-or-none manner.