* Author to whom correspondence should be addressed. E-mail: sitaram@unipune.ernet.in J. theor. Biol. (2000) 206, 419}428 doi:10.1006/jtbi.2000.2134, available online at http://www.idealibrary.com on What does a Common Channel for Electrolytes and Non-electrolytes in the Sperm Mean? V. SITARAMAM* AND ZUBEN E. SAUNA Department of Biotechnology, ; niversity of Pune, Pune 411 007, India (Received on 12 April 2000, Accepted on 5 July 2000) Cell volume is central to osmoregulation in intact cells. Bovine spermatozoa, as also other mammalian spermatozoa, exhibit a very rapid regulatory volume decrease (RVD) when exposed to hypotonic saline media. This response, fastest known in animal cells, is mediated by a putative potassium channel which the pharmacological properties of a conventional channel and yet admits both electrolytes and non-electrolytes. The evolutionary basis and functional role of this conserved quinine-sensitive channel in mammalian sperm could o!er hitherto unexplored facets of the link(s) between ecology and reproduction.  2000 Academic Press Introduction The role of stretch-sensitive K channels and their conservation through evolution has been emphasized (Jan & Jan, 1994). The sperm cells from various mammalian species have a stretch- sensitive, putative, potassium channel that admits both electrolytes and non-electrolytes (Kulkarni et al., 1997). The importance of non- electrolyte permeation across sperm, however, was not clear. We brie#y explore here the advant- ages of such a common channel for electrolytes and non-electrolytes based on the premise that such a channel must have been conserved in evolution and could o!er valuable molecular means for tailoring the reproduction in organ- isms to ecology during evolution. Availability of the solvent water is the major di!erence in the aquatic and terrestrial habitats. Osmotic balance, of the organism and the gametes, is a necessary condition for sustenance of the species. Manag- ing solutes within tolerable limits would be the preoccupation when the availability of water be- comes limiting. This idea is fully supported in the evolution of the K channels wherein its modulation is suggested to be related in a range of organisms between E. coli (msc¸) to the mammalian recep- tor cells of the inner ear (Jan & Jan, 1994) and erythrocytes (Sambasivarao et al., 1986). Mechano-sensitivity was suggested to antedate even voltage and ligand sensitivities for the K channel (Jan & Jan, 1994). Volume regulation, wherein the role of K is well established (Ho!- man, 1987), was envisaged to be among the earliest functions of this ubiquitous channel (Deutsch & Chen, 1993). If primitive and yet conserved aspects of this channel are to be ex- plored, the abundantly available, and the primor- dial, male gamete appears to be the logical choice. The variable of importance would be the cell volume as the focus of regulation. 0022}5193/00/190419#10 $35.00/0  2000 Academic Press