EFFECTS OF DIVALENT AND LANTHANIDE IONS ON MOTILITY INITIATION IN RAT CAUDAL EPIDIDYMAL SPERMATOZOA

1 Sperm motility initiation of rat caudal epididymal spermatozoa in vitro has been studied. 2 Spermatozoa flushed out from the cauda epididymis with a sodium‐free medium exhibited a transient motility which decreased progessively. At 40 min, the forward motility was completely suppressed. However, if they were resuspended in a sodium containing medium their motility was completely restored to normal within 15 min. 3 This initiation of sperm motility required the presence of extracellular calcium. Maximal stimulation was obtained at a calcium concentration of 10 −3 M. Above this concentration, further increase in calcium produced a fall in motility and at 10 −2 M, motility initiation was completely suppressed. 4 The initiation of sperm motility has been shown to depend closely on sodium. Sperm motility initiation alters in a curvilinear fashion with extracellular sodium concentration showing saturation kinetics. High calcium (above 10 −3 m) was found to depress motility initiation induced by sodium, without affecting the apparent affinity constant for sodium. 5 The effect of Ca 2+ on sperm motility initiation could be mimicked by Sr 2+ but not by Mg 2+ , La 3+ or Eu 3+ . 6 In the presence of extracellular calcium (1.27 or 2.54 mM), La 3+ and Eu 3+ exerted a dose‐dependent inhibition of sperm motility initiation. The IC 50 values for both ions were about 5 × l0 −5 M. 7 The possible mechanism of inhibition by lanthanide ions is discussed.