A redox-regulated tyrosine phosphorylation cascade in rat spermatozoa

Rat spermatozoa from both the caput and cauda epididymidis were shown to ge nerate superoxide anion (O-2(-)) both spontaneously and following stimulati on with NAD(P)H. Caput spermatozoa gave a significantly greater O-2(-) resp onse to NADPH stimulation than caudal cells, whereas in both cell types the responses to exogenous NADPH and NADH were approximately equivalent. Analy sis of H2O2 production revealed that this oxidant was generated only by cau dal epididymal cells and only in these cells did the stimulation of reactiv e oxygen species (ROS) production with NADPH lead to an increase in tyrosin e phosphorylation. Stimulation of ROS production with NADPH increased intra cellular cyclic adenosine monophosphate (cAMP) levels in both caput and cau dal epididymal cells, but only in caudal cells did cAMP stimulate tyrosine phosphorylation, in keeping with the NADPH results. On the basis of these f indings we propose that tyrosine phosphorylation in rat spermatozoa is driv en by ROS acting via 2 different but complementary mechanisms; O-2(-) stimu lates tyrosine kinase activity indirectly through the elevation of intracel lular cAMP while H2O2 acts directly on the kinase/phosphatase system. stimu lating the former and inhibiting the latter. Zinc was examined as a potenti al regulator of this signal transduction cascade and was shown to suppress tyrosine phosphorylation in caput cells but to promote this activity in cau dal spermatozoa, possibly through an inhibitory effect on tyrosine phosphat ase activity. These results reveal the maturation of a redox-regulated, cAM P-mediated, signal transduction cascade during epididymal transit in the ra t that is sensitive to zinc acid plays a key role in the control of tyrosin e phosphorylation events associated with capacitation.