Endocrine regulation of avian renal function

The avian kidney contains reptilian‐type (RT) nephrons that do not function in concert and mammalian‐type (MT) nephrons that do function in concert to permit production of concentrated urine. Significant data on endocrine regulation of renal function are available only for antidiuretic hormone (arginine vasotocin, AVT) and parathyroid hormone (PTH). Plasma AVT increases and number of filtering RT nephrons decrease when plasma osmolality increases. Small doses of AVT reduce the number of filtering RT nephrons, and all RT nephrons filter following acute neurohypophysectomy. In addition to changing tubule permeability to water, AVT affects the concentrating mechanism by reducing the number of filtering RT nephrons and flow through collecting ducts. Both net tubular reabsorption and net tubular secretion of PO₄ occur. Parathyroidectomy (PTX) stimulates PO₄ reabsorption, and PTH replacement decreases reabsorption and stimulates secretion, apparently in proximal tubules of RT nephrons. PTX stimulates and PTH replacement inhibits Na‐dependent PO₄ transport by renal brush border membrane vesicles. PTH also stimulates Ca²⁺ reabsorption but apparently not along the proximal tubule of RT nephrons.