Lithium secretion in kidneys of amphibians and reptiles under hydrated conditions

Renal lithium transport was studied at different hydration levels in five species of anuran amphibians (Bufo bufo, B. danatensis, B. viridis, Rana ridibunda, and R. temporaria), two species of urodeles (Triturus vulgaris and T. cristatus) and four species of reptiles (lizards Eremias multiocellata, Lacerta vivipara, Trapelus sanguinolentus, and Teratoscincus scincus). Under dehydration conditions, Li⁺ was reabsorbed in the kidneys of amphibians and reptiles, but to a lesser degree than in mammalian kidneys: the ratio of lithium clearance (C_Li) to glomerular filtration rate (GFR)—fractional lithium excretion—in dehydrated animals was in the range 0.5–0.8. The transition to the hydrated state resulted in a cessation of net renal lithium reabsorption. Under condition of high hydration, all the animals studied, except for urodeles, showed net renal secretion of Li⁺, i.e., C_Li exceeded GFR. The ratio $\text{C}{}_{\mathrm{Li}}\text{GFR}$ was 1.2–1.3 in hydrated anurans and 1.7–2.3 in hydrated lizards. In urodeles, this ratio was approximately unity. It is suggested that renal lithium secretion in hydrated amphibians and reptiles reflects fluid secretion in the proximal tubule, which is additional to the glomerular filtration mechanism of fluid delivery to nephron under water loading.