Читать книгу Disorders of Fluid and Electrolyte Metabolism - Группа авторов - Страница 28

The DCT is the segment between the macula densa and CD. This is a morphologically heterogeneous segment [12] that is also water impermeable. The CDs are formed in the cortex by the confluence of several distal tubules. They descend through the cortex and outer medulla individually, but successively fuse together on entering the inner medulla. In humans, a terminal inner medullary CD draws from as many as 7,800 nephrons. The CDs possess vasopressin-sensitive adenylate cyclase in all species studied; they are virtually impermeable to water in the absence of the hormone. The vasopressin-sensitive AQP2 in the apical membrane mediates water reabsorption in this segment of the nephron in concert with vasopressin-independent aquaporins 3 and 4 in the basolateral membrane [13, 14]. The CD in its cortical and medullary segments is also impermeable to urea, but in response to the vasopressin-sensitive UTA1, the inner medullary CD is rendered urea permeable [15].