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 approximately 2.5 x 10(-6) M free Ca2+. In our current study, the time course of the curvature change, after elevating free Ca2+ to 3.5 x 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 x 10(-10) M free Cd2+. Based on these findings, it appears that the Ca(2+)-activated curvature reversal of rat sperm is potentiated by cAMP-dependent kinase and may be mediated through calmodulin.