HIGH WATER PERMEABILITY OF HUMAN SPERMATOZOA IS MERCURY-RESISTANT AND NOT MEDIATED BY CHIP28

A novel integral membrane protein with an apparent molecular mass of 28 kDa (CHIP28) was first isolated from human erythrocytes and is now recognized as a water channel protein. The expression of this protein has been found in several other cell types that all require high water permeability for their functions. Recent studies have shown that the water permeability (L(p)) of human spermatozoa is among the highest reported for mammalian cells. Together with the low activation energy of human spermatozoa for L(p), this suggests that CHIP28 water channel may be present in the plasma membrane of human spermatozoa. However, our current studies do not support this hypothesis. Results from Western blot analysis on human sperm plasma membrane proteins, performed through use of an antibody against human erythrocyte CHIP28 protein, indicated that human spermatozoa do not express CHIP28 protein on their cell surface (n = 10). Consistent with the Western blot finding, mercuric chloride (HgCl2), a known water channel blocker, failed to reduce the osmotic water permeability of human spermatozoa. The calculated L(p) values were 1.30 +/- 0.29 mu m/min/atm (n = 16; mean +/- SEM) for the control group and 1.31 +/- 0.29 (n = 9; mean +/- SEM), 1.04 +/- 0.27 (n = 11; mean +/- SEM), and 1.34 +/- 0.19 (n = 6; mean +/- SEM), respectively, for the 10 mu M, 30 mu M, and 50 mu M HgCl2-treated groups. These L(p) values are not different (p > 0.05). In contrast, the same concentration of HgCl2 significantly blocked the osmotic water transport across the membrane of human erythrocytes. These data strongly suggest that the high water permeability of human sperm plasma membranes is not due to CHIP but may be mediated by other water channel proteins that are mercury-resistant.