The Effects of Calcium on the Switch-Point of the Beat in Reactivated Bull Sperm.

K. A. Lesich, T. G. dePinho, B. J. Dionne, C. B. Lindemann; Biological Sciences, Oakland University, Rochester, MI

Triton X-100-extracted and ATP reactivated bull sperm attain the greatest curvature of the flagellum at a position between 5 and 10 μm from the flagellar base. This bend is conventionally designated as the principal (P) bend. In the current study we examine the effect of the addition of 1 mM Ca2+ on the switch-point of the beat. We found that 1 mM Ca2+ selectively and significantly reduced the curvature of the P-bend, with little effect on bending in the opposite (R bend) direction. An earlier report showed that the P bend greatly increased in curvature when ADP was included in the reactivation (Lesich et al. 2008, Biophys. J. 95:472). Ca2+ significantly reduced the switch-point curvature in the P bend direction when 1 mM ADP was present (p=<0.001), enough to completely eliminate the effect of ADP as reported earlier. In 18 of 20 cases, the addition of Ca2+ reduced the P bend curvature such that the R bend was then of greater curvature. by the traditional definition of P and R bends this would require redefining the R bend as the new P bend. While the P and R bend direction could be changed by the action of 1 mM Ca2+ on 0.1 mM ATP + 1 mM ADP reactivated cells, we noticed another feature of the beat that remained true to the original P bend direction. The shear angle that the proximal flagellum (first 5 μm) makes to the midline of the head axis has a permanent asymmetry that favors the beat direction that is dominant in the absence of Ca2+ presence of ADP. This suggests that the flagellar connecting piece has an inherent anatomical asymmetry which allows the flagellum to hinge at the head more strongly in one direction independent of the Ca2+ effect. (Supported by N.S.F. Grants MCB 0516181 & MCB 0918294)