Physiology and biophysics of chloride and cation cotransport across cell membranes.

Many important questions remain to be answered about the mechanism that mediates coupled Na,K,Cl cotransport. We still do not know what the ATP requirement involves. Is ATP the direct energy source? Such an energy source does not seem to be necessary, inasmuch as the net free energy in the combined transmembrane chemical gradients of Na, K, and Cl is quite sufficient to maintain the observed high Cl(i). Could a protein kinase-mediated mechanism be responsible for the ATP requirement? How does reducing Cl(i) stimulate the transporter? What are the kinetic relationships for the co-ions at the outward- and inward-facing transport sites? Are they symmetrical? Can the squid axon regulate its cell volume? If so, is the Na,K,Cl transporter directly involved? Thus, the squid axon remains a fruitful preparation to study a transport mechanism similar to that found in a variety of cells. Its large size confers unique experimental advantages that should help us in our quest to understand this widely distributed transport mechanism.