The calcium‐induced curvature reversal of rat sperm is potentiated by cAMP and inhibited by anti‐calmodulin

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Abstract
Rat sperm, demembranated with 0.1% Triton X‐100, were used to explore the reversal in flagellar curvature induced by calcium ion. As reported earlier (Lindemann and Goltz, Cell Motil. Cytoskeleton, 10:420–431, 1988), the radius of curvature of the flagellar midpiece of rat sperm is controlled by the free Ca2+ concentration. A reversal of the direction of curvature (judged by the asymmetric sperm head) takes place at ≈ 2.5 + 10−6 M free Ca2+.In our current study, the time course of the curvature change, after elevating free Ca2+ to 3.5 ± 10−4 M, was utilized to assess the effects of the cAMP‐kinase A pathway on the calcium response. In addition, calmodulin's involvement in this response was explored using anti‐calmodulin and Cd2+. The activity state of the sperm models (which could be directly influenced through cAMP) was found to control the rate of curvature change in response to increased free Ca2+. In the most extreme case, fully quiescent sperm did not respond to Ca2+ at all, and cAMP‐primed sperm models completed the response to Ca2+ in two minutes or less.Anti‐calmodulin demonstrated strong inhibitory effects on the curvature reversal. Cadmium ion was also extremely potent at blocking the response to Ca2+, completely eliminating the curvature reversal at 2 × 10−10 M free Cd2+.Based on these findings, it appears that the Ca2+‐activated curvature reversal of rat sperm is potentiated by cAMP‐dependent kinase and may be mediated through calmodulin.