Heterogeneity of Tubular Transport Processes in the Nephron

This review describes the current state of knowledge regarding intraneph­ ron and intemephron heterogeneity of tubular transport function. Anato­ mists, micropuncturists, and microperfusionists have, in general, agreed upon the definitions of intranephron and intemephron heterogeneity. In­ tranephron heterogeneity is defined as the comparative properties of differ­ ent segments of an individual nephron. The nephron has been subdivided into twelve distinct segments: early and late proximal convoluted tubule, proximal straight tubule, descending and ascending thin limbs of Henle's loop, medullary and cortical thick ascending limbs of Henle's loop, distal tubule, connecting tubule, cortical collecting tubule, and medullary and papillary collecting ducts. Internephron heterogeneity is defined as the ' comparative properties of analogous structures in superficial (SF) and juxtmedullary (1M) nephrons. Anatomists offer the most precise definition of the SF and JM nephron populations: SF are nephrons whose efferent arterioles run straight to the renal surface before splitting off into capillaries; JM are nephrons whose efferent arterioles give rise to the arterial vasa recta. By this definition, in the rabbit 28% of the nephrons are SF and 9% are 1M. The remaining 63%, belonging to neither group, are called midcortical. Micropuncturists ap­ proximate this anatomical definition by defining SF nephrons as those accessible from the surface and JM as those possessing loops that extend into the tip of the papilla. Microperfusionists often do not discriminate