Reptilian glomerular and tubular functions and their control

Controls for glomerular filtration (GFR) and tubular transport of solutes and water are interrelated. GFR changes with hydration, apparently as a result of changes in the number of filtering nephrons under the control of antidiuretic hormone. The resulting alterations in volume flow rate through the collecting ducts may be as important as changes in epithelial permeability in determining urine osmolarity. Tubular transport of sodium and potassium may be controlled in part by antidiuretic hormone and aldosterone but considerable variation occurs among species, and intrinsic regulation of distal tubular sodium absorption in response to the delivered load of sodium may also occur. Net tubular secretion of phosphate, under control of parathyroid hormone, occurs in some species, but no hormonal control of tubular calcium transport has been demonstrated. Net tubular urate secretion that is influenced by potassium occurs in uricotelic reptiles. Complexing of inorganic cations, especially sodium and potassium, with urate precipitates in tubular urine permits excretion of cations without their contributing to urine osmolarity. This process also may keep the distal tubule sodium concentration low enough to permit maximum dilution but may require absorption of filtered water without sodium. Such an absorptive process may exist in reptilian proximal tubules.