[Li+ as a Na+ analog in ion transport process in vertebrates]

Special features of Li+ transport in several organs ensuring ion homeostasis of vertebrates (in kidneys of fishes and mammals, in gills of teleost and rectal gland of elasmobranchs) are observed in comparison to Na+ transport in the same organs. Transport processes in extrarenal excretory organs--in gills of marine teleosts and in rectal gland of marine elasmobranchs--are distinguished by low lithium-sodium selectivity, i.e. in these processes Li+ behaves very much like Na+. On the contrary, renal excretion of these ions is accompanied by significant (more than ten times) increase of [Li]/[Na] ratio in urine in comparison with that of blood plasma. At the same time, there is no difference between Li+ and Na+ in process of glomerular filtration and their transfer across proximal tubule wall; discrimination takes place in distal parts of a nephron where Li+ is not involved in the process of Na+ reabsorption. Data on lithium-sodium selectivity of the kidneys and rectal gland were obtained by a direct measurement of Li+ and Na+ concentrations in body fluids. Estimations of lithium sodium-selectivity of gills of freshwater and marine teleosts were performed by means of the balance equation of Li+ and Na+ net fluxes in processes of exchange between the animal and its environment.