We have shown that calmidazolium (R24571) causes a transient increase in the cytosolic free Ca2+ concentration ([Ca2+]i) in Dictyostelium discoideum [Schlatterer and Schaloske (1996) Biochem. J. 313, 661–667]. Here we have used R24571 to artificially increase [Ca2+]i during light-scattering oscillations and have found that, depending on the time of addition during the oscillatory cycle, R24571 suppressed cAMP synthesis and delayed the next spike for several minutes. Addition of Ca2+ to the medium, which also elevates [Ca2+]i, induced phase delays and resulted in a similar phase response curve as R24571. The magnitude of the phase delay was correlated with the point during the oscillatory cycle at which Ca2+ was added, indicating that an artificial increase in [Ca2+]i also resets the phase of the intrinsic oscillator.

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