Contribution of the cAMP-Dependent Signal Pathway to Circadian Synchrony of Motility and Resting Membrane Potential in Paramecium

It is known that the ciliated protozoan Paramecium multimicronucleatum has synchronized circadian rhythms of motility, resting membrane potential and cyclic adenosine 3′,5′-monophosphate (cAMP) and cyclic guanosine 3′,5′-monophosphate (cGMP) concentrations. The present study shows that (1) extracellularly added 4 mM tetraethylammonium (TEA)+ (a K+ channel blocker) almost completely abolishes the diurnal oscillation of intracellular cAMP concentrations; (2) even 32 mM TEA+ fails to abolish the circadian motility rhythm; but (3) the motility rhythm is highly damped when 4 mM TEA+ and 100 μM CdCl2 (a Ca2+ channel blocker) are added simultaneously. A cAMP analogue (N6-monobutyryl-cAMP) added extracellularly accelerates swimming velocity. Both a K+ channel blocker (e.g. TEA+) and an inhibitor (trifluoperazine) of adenylate cyclase (AC) suppress cAMP formation, supporting the hypothesis that AC in Paramecium has dual functions, as a K+ channel and as an enzyme for cAMP formation. It is hypothesized that the circadian synchrony is due to circadian fluctuations of AC causing separate circadian changes both in ciliary motion and membrane potential through a cAMP-dependent signal pathway that forms a sophisticated network of second messengers to govern the synchrony together with Ca2+- and cGMP-dependent pathways in a manner antiphasic and/or complementary to one another.