HIGH WATER PERMEABILITY OF HUMAN SPERMATOZOA IS MERCURY-RESISTANT ANDNOT MEDIATED BY CHIP28

A novel integral membrane protein with an apparent molecular mass of 2 8 kDa (CHIP28) was first isolated from human erythrocytes and is now r ecognized as a water channel protein. The expression of this protein h as been found in several other cell types that all require high water permeability for their functions. Recent studies have shown that the w ater permeability (L(p)) of human spermatozoa is among the highest rep orted for mammalian cells. Together with the low activation energy of human spermatozoa for L(p), this suggests that CHIP28 water channel ma y 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 throu gh use of an antibody against human erythrocyte CHIP28 protein, indica ted 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) v alues were 1.30 +/- 0.29 mu m/min/atm (n = 16; mean +/- SEM) for the c ontrol 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), respect ively, for the 10 mu M, 30 mu M, and 50 mu M HgCl2-treated groups. The se L(p) values are not different (p > 0.05). In contrast, the same con centration 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 n ot due to CHIP but may be mediated by other water channel proteins tha t are mercury-resistant.