The ciliary responses of Paramecium caudatum, extracted in Triton X-100 andglycerol, to the external application of ATP and various divalent cations wereexamined. Ciliary beating could not be reactivated, but changes in the pointing direction of the cilia (the reorientation response) could be reactivated. The free Ca2+ concentration determined the final orientation of the cilia, which was towards the front when the Ca2+ concentration was above 10−6 moll−1, and towards the rear when below 10−7 moll−1. The reorientation response was inhibited by vanadate. These results indicate that the mechanism for the reorientation response is separable into two components. One is the movement of cilia to change their pointing direction, which, like normal ciliary beating, is energized by Mg-ATP2-. The other is the determination of the final pointing direction of the cilia, which is Ca2+ -dependent. Divalent cations can be classified into two groups according to their mode of action on the Ca2+ -dependent component. Sr2+ is an agonist and Ba2+, Zn2+ and Co2+ are antagonists to Ca2+ for the component.

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