Renal ultrastructure and excretion of salt and water by three terrestrial lizards

Abstract

Determinations of sodium, chloride, inulin, and osmolal concentrations were made on blood and urine collected from dehydrated, waterloaded, and salt-loaded geckos, horned toads, and Galapagos lizards. Electron micrographs were made of proximal and distal tubular cells of these animals. Kidneys of horned toads and Galapagos lizards produced only isoosmotic urine (osmolal urine-to-plasma ratio = 0. 9-1. 0) and re-absorbed 55% of the glomerular filtrate, while geckos produced dilute urine (osmolal U/P [urine/plasma] ratio = 0. 6-0. 8) and reabsorbed 75% of the filtrate. Electron micrographs proved that basal plasma membranes of proximal and distal tubular cells of horned toads and Galapagos lizards are not infolded and that the mitochondrial population is small in these cells. In geckos, the cells resembled mammalian kidney cells in having infolded basal membranes and a large population of elongated mitochondria. Infolded plasma membranes associated with mitochondria may therefore be responsible for the ability of relatively water-impermeable distal tubules to creat osmotic gradients and for the capacity of water-permeable proximal tubules for transferring large amounts of salt and water.