ROSInduced ROS

The close relation between O2'- and NO' synthesis rises the possibility of an ROS-induced ROS formation as occurs in other cell types. As examples, NO2 generation increases in cultured glial cells treated with xanthine+xanthine oxidase, an effect abolished by SOD [72] and, conversely, H2O2 stimulates NOS in porcine aortic endothelial cells via a complex signaling pathway [57].

Several observations confirm the hypothesis of an ROS-induced ROS system operating during sperm capacitation [46] . The first indication is that SOD blocks capacitation due to l-Arg (NOS substrate) and NO' itself (from DA-NONOate) and that the NOS inhibitor l-NMMA prevents that due to O2'- (from xanthine + xanthine oxidase) (Fig. 4.4a) [46]. Second is that endogenous NO' synthesis stops in the presence of SOD (Figs. 4.1c and 4.4b) and conversely that O2'- formation is arrested by NOS inhibitors (Fig. 4.4c) [46]. Additionally, spermatozoa treated with DA-NONOate initiate the synthesis O.'- (Fig. 4.4c) and, conversely, spermatozoa treated with O.'- (from xanthine+xanthine oxidase) produce more NO' , an effect blocked in the presence of SOD (Fig. 4.4b) [46]; it should be stressed here that the amounts of ROS (O2'- and NO') then generated are of the same magnitude as those measured in spermatozoa incubated with FCSu or l-Arg [46]. Moreover, H2O2 promotes synthesis of NO' but not of O2'- suggesting that there is no self-induction of the oxidase [46]. Finally, ONOO- has no effect on either O.'- or NO' generation pointing out that this ROS is a final product rather than initiating agent [46].

Pooling of all these data points out to spermatozoa as the first known cells with a reciprocal (two sided) ROS-induced ROS system, O2'- inducing NO' production and vice versa.

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