Follicular fluid proteins stimulate nitric oxide (NO) synthesis in human sperm: A possible role for NO in acrosomal reaction

Nitric oxide (NO) is a free radical involved in the regulation of several f unctions of the male genitourinary system. It is produced by neurons and th e endothelium and epithelia of reproductive system; it mediates penile erec tion and regulates sperm motility, viability, and metabolism. Here we show that human spermatozoa exhibit a detectable NO synthase (NOS) activity, mea sured both as ability of the intact sperm and cell lysate to convert L-[H-3 ]arginine into L-[H-3]citrulline and as 24 h accumulation of extracellular nitrite in intact sperm suspensions. NOS activity (identified as an endothe lial isoform) was inhibited by L-canavanine and N-G-monomelhyl-L-arginine, and nitrite accumulation was inhibited by the NO scavenger hemoglobin; both enzyme activity and nitrite production were increased by a 24 h incubation of spermatozoa with protein-enriched extracts of human follicular fluid (P FF); a significant increase of citrulline synthesis was observed only after a 24 h incubation with 40% PFF, a time period during which acrosomal react ivity was significantly increased. PFF-induced acrosomal reaction was inhib ited by L-canavanine and hemoglobin, and the NO donors sodium nitroprusside (SNP), S-nitroso-N-acetyl-penicillamine (SNAP), and DETA NONOate were able to increase the percentage of reacted spermatozoa. Our results suggest tha t NO synthesized by human sperm may play a role in follicular fluid-induced acrosomal reaction. (C) 1999 Wiley-Liss, Inc.