Aquaporins

Dear Physiologist,

Exposure of red cells to hypotonic physiological solutions leads to swelling and breakdown of cells. The swelling is due to the entry of water into the cells. Have you ever wondered what is the pathway for the entry of water? The first choice that comes to mind is the water channel Aquaporin which plays such an important role in the renal handling of water. Yes, there are aquaporin channels on the red cell and indeed blocking them with mercury salts leads to inhibition of entry of water into the human erythrocytes. But not completely! Only by 50%. And interestingly similar experiments with chicken red cells showed that water entry can not be stopped by the blockade of aquaporin.  This suggests the presence of other waterways in the red cell membrane. Osmotic stress seems to cause physiochemical reorganization of the membrane leading to the formation of gaps or voids in the membrane and these serve as conduits water transport into the cell under hypotonic conditions. Such voids are shown to develop even in artificial lipid bilayers exposed to hypotonic solutions!

Will the osmotic fragility of chicken red cells differ from human red cells? Why not experiment and find out for yourself? Chicken red cells are nucleated whereas human cells are not nucleated. Is this in any way related to the aquaporin density on the cell membrane? Mercury salts to block aquaporins are easily available. Lastly, as we move from the fish to mammals will the fragility of red cells change in a predictable manner? Do try and let me.

J. Prakasa Rao
Professor of Physiology (Rtd)