REACTIVATION OF EXTRACTED SPERM CELL MODELS 255 



restores the normal proclivity for wave initiation at the base of the 

 tail and its subsequent propagation along the flagellum. 



ATP is dephosphorylated by sperm models but, on a cell basis, at a 

 very low rate — on the order of 0.2 fig P liberated per milligram N per 

 minute by bull sperm. Substrate specificity is not limited to ATP 

 since adenosine diphosphate (ADP) and inosine triphosphate (ITP) 

 are also utilized by mammalian sperm models (Bishop and Hoffman- 

 Berling, 1959). ATP is effective on these models over a range of about 

 0.1 to 10 mM; ITP requires a higher minimum concentration. ADP 

 appears to be split, but is probably first converted to ATP and AMP 

 by an adenylatekinase which is present in briefly stored sperm models 

 but which appears to be leached out with continued extraction and 

 storage. 



The rate of flagellation of sperm models of some species varies with 

 changes in concentration of nucleotide. The response of sea urchin 

 and starfish sperm models, studied by Kinoshita (1958), showed a 

 maximal frequency at 1 mM ATP (Fig. 1). Above a supraoptimal 

 limit of about 10 mM, ATP was ineffective in inducing motility. This 

 closely corresponds to the supraoptimal concentration of ATP for iso- 

 lated muscle systems (Weber and Portzehl, 1954). Two interrelated 

 causes may be suggested for the failure of ATP in high concentration 

 to induce flagellation: substrate inhibition of the splitting enzyme and 

 the plasticizing action of unsplit nucleotide. A curvilinear relation 

 between ATP concentration and frequency of response of intact 

 sperm models is shown in Fig. 2. Originally portrayed as a direct effect 

 of splitting activity, the increase in frequency of beat was found to 

 result from the plasticizing action of ATP, since at a constant con- 

 centration of nucleotide, the same curvilinear relation occurs with 

 increasing concentrations of inorganic pyrophosphate (Hoffmann- 

 Berling, 1955a). A change in ionic strength also increases the rate of 

 flagellation, presumably by a "softening" effect (Fig. 2). In most mam- 

 malian sperm, on the other hand, no increase in frequency occurs 

 with increasing ATP concentration; the flagellation rate remains con- 

 stant over the concentration range (Bishop and Hoffmann-Berling, 

 1959). Possibly this type of response is attributable to the presence of 

 adequate plasticizing capacity of extracted mammalian sperm not 

 found in the invertebrate models. 



The rate of induced flagellation is temperature-dependent and 



