This study was prompted by three problems: 1) the role of vasopressin in the countercurrent mechanism for concentrating urine; 2) the effect of dehydration on this mechanism; and 3) the behavior of this mechanism in the absence of vasopressin when the body fluid volumes are contracted rather than expanded.Although it is generally accepted that countercurrent systems in the renal medulla effect the concentration of urine, several questions about the operation of these systems remain unanswered (1). For example, it is not known whether vasopression enhances the sequestration of nonurea solutes in the medullary interstitium, either by increasing the reabsorption of sodium from the loops of Henle, by slowing medullary blood flow, or both. Levitin, Goodman, Pigeon, and Epstein demonstrated a progressive and significant increase in sequestered sodium within the renal papilla and medulla of dogs as vasopressin was superimposed on a water diuresis by intravenous infusion (2). Since they also found the enrichment of papillary and medullary sodium by infused 22Na to be more rapid during water diuresis than during antidiuresis, they suggested that vasopressin may promote medullary sequestration of sodium by slowing medullary blood flow, not by increasing reabsorption of sodium from the ascending loops of Henle. In contrast, Jaenike has pre-

A B S T R A C T Postnatal renal development was studied in dogs between 2 and 77 days. Single, superficial nephrons were evaluated by micropuncture, concurrently with measurements of total renal function and morphometric analyses in the same animals.Glomerular filtration rate for the entire kidney increased linearly from 0.13 ml/min per g kidney weight at 2 days to 0.91 at 77 days. Extraction of p-aminohippurate increased from about 20 to 80%, and renal plasma flow per g kidney weight, measured as CPAH/EPAH, increased threefold during the same period. Filtration fraction increased to the mature value during the first half of the postnatal period studied.The clearance of urea per unit of renal mass increased with age, whereas the fraction of filtered urea reabsorbed declined during the early part of the postnatal period. The pattern of fractional urea reabsorption may be due mainly to increased medullary recycling of urea and to a rise in the reabsorption of water from the medullary collecting duct.Urine osmolality was higher than plasma from birth onward and rose with age. Osmolal equality of collecting duct fluid and medullary interstitium reflected mature vasopressin (ADH)-induced water permeability. The rise in urinary concentration was predominantly due to increasing medullary sequestration of urea.Glomerular filtration rate of the superficial nephron A preliminary report on part of this work appeared in abstract form: Horster, M. 1970. Postnatal fluid reabsorption in single nephrons of the dog kidney. J. Clin. Invest.

Despite the seemingly ubiquitous admonition to “drink at least eight 8-oz glasses of water a day” (with an accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking. This review sought to find the origin of this advice (called “8 × 8” for short) and to examine the scientific evidence, if any, that might support it. The search included not only electronic modes but also a cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids. No scientific studies were found in support of 8 × 8. Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentaryexistence, precisely the population and conditions that the “at least” in 8 × 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 × 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates. Since it is difficult or impossible to prove a negative—in this instance, the absence of scientific literature supporting the 8 × 8 recommendation—the author invites communications from readers who are aware of pertinent publications.

Familial hypothalamic diabetes insipidus ( DI) has arisen as an apparently spontaneous mutation from a strain of Long-Evans hooded rats being bred for unrelated researches not involving radioactivity. The DI rats decrease water intake and urine flow, and increase urine osmolality in response to injected vasopressin. They concentrate their urines only minimally or not at all in response to dehydration, hypertonic saline, nicotine, or stress, and their serum osmolalities and sodium concentrations are significantly higher than those of normal animals. They show marked diminution of neurosecretory material in the neurohypophysis and supraoptic nucleus. The data suggest that the deficiency causing DI in these rats is a lack or dearth of synthesis of vasopressin or its carrier protein, or both.

No abstract