Water Transport across ADH-Sensitive Epithelia

Abstract

Several epithelia are virtually water-impermeable under at least certain physiological or experimental conditions. In the mammalian urinary pathway examples include all segments of the renal tubule beyond the bend of the loop of Henle and all segments of the urinary tract from the renal pelvis to the urethra. In amphibia, epithelial preparations that exhibit a very low osmotic water permeability (Pos) under control conditions include the anuran epidermis and urinary bladder (e.g., of frog and toad). Under the action of a variety of peptide hormones collectively named antidiuretic hormones (ADHs), some of these epithelia increase their water permeability dramatically. In the presence of a favorable transepithelial difference in osmotic pressure, this change in permeability results in water reabsorption and is one of the major mechanisms of water conservation. Epithelia sensitive to ADH include the cortical and medullary collecting ducts of the mammalian kidney, the frog and toad urinary bladders and epidermis, and others. In mammals, both the thin and the thick ascending loop of Henle and also the epithelia of the urinary tract distal to the papillary duct do not respond to ADH with an increase in osmotic water permeability. However, salt transport by the thick ascending loop of Henle is sensitive to ADH.