The heads of demembranated spermatozoa of the sea urchin Tripneustes gratilla, reactivated at different concentrations of ATP, were held by suction in the tip of a micropipette and vibrated laterally with respect to the head axis. This imposed vibration resulted in a stable rhythmic beating of the reactivated flagella that was synchronized to the frequency of the micropipette. The reactivated flagella, which in the absence of imposed vibration had an average beat frequency of 39 Hz at 2 mmol l-1 ATP, showed stable beating synchronized to the pipette vibration over a range of 20-70 Hz. Vibration frequencies above 70 Hz caused irregular, asymmetrical beating, while those below 20 Hz induced instability of the beat plane. At ATP concentrations of 10-100 mumol l-1, the range of vibration frequency capable of maintaining stable beating was diminished; an increase in ATP concentration above 2 mmol l-1 had no effect on the range of stable beating. In flagella reactivated at ATP concentrations above 100 mumol l-1, the apparent time-averaged sliding velocity of axonemal microtubules decreased when the imposed frequency was below the undriven flagellar beat frequency, but at higher imposed frequencies it remained constant, with the higher frequency being accompanied by a decrease in bend angle. This maximal sliding velocity at 2 mmol l-1 ATP was close to the sliding velocity in the distal region of live spermatozoa, possibly indicating that it represents an inherent limit in the velocity of active sliding.(ABSTRACT TRUNCATED AT 250 WORDS)

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