The Relationship between Intravascular Volume Expansion and Natriuresis in Developing Puppies

Abstract

SummaryThe role played by the relative degree of expansion of the intravascular and extravascular compartments in limiting the natriuretic response of fluid-loaded developing animals was determined in I-, 2-, 3-and 6-wk-old puppies. Volume expansion was induced by infusing either isotonic saline, 10% body weight or isoncotic albumin in saline 5% body weight, and measurements of giomerular filtration rate, sodium excretion, fractional excretion of sodium, and plasma volume were made. Each expansion procedure resulted in an increase in the absolute excretion of sodium at all ages ( P < 0.001). The greatest natriuretic effect was observed in the 3-wk-old puppies, the average of the two solutions being 19, 30, 70, and 28 pEq/min/kg in the 1, 2, 3, and dwk-old animals, respectively. The difference in natriuresis among the age groups was due predominantly to differences in the magnitude of the increase fractional excretion of sodium. At all ages, a greater absolute excretion of sodium was encountered during volume expansion with saline than observed with albumin ( P < 605). The intravascular volume increased by a similar % at all ages (P > 0.1), and saline and albumin yielded equivalent degrees of intravascular expansion (approximately 50%). The results demonstrate that age-related changes in natriuretic response to volume expansion cannot be attributed to differences in either the degree of expansion or the distribution of the load. In addition, the observations indicate that the mechanism underlying the difference between the response to isotonic saline and isoncotic albumin in saline is already operative at birth, and that it is independent of nephron heterogeneity since the proportion of superficial nephrons must have changed during the period of nephrogenesis. SpeculationThe collecting duct, which is sensitive to both isotonic saline and isoncotic albumin, is likely to be responsible for the parallel development of the renal response to these two methods of intravascular volume expansion.It has been amply demonstrated that the excretory response to saline expansion is less in neonatal and developing animals than it is in mature subjects (1,7,12,13,16). Early investigators have suggested that the low glomerular filtration rate (GFR), which prevails during this period of development, limits the natriuresis elicited by volume expansion. This concept has been recently challenged. Our work (7) and that performed by others (12,13) have demonstrated that an avid reabsorption by the renal tubule rather than a low GFR is the main cause of the limited natriuresis observed in infancy. The mechanism responsible for the persistence of a high fractional reabsorption in developing animals, even under conditions of volume expansion, has not as yet been established. We reasoned that "immaturity" of the transport mechanism could not provide an adequate explanation for this phenomenon because reabsorption of sodium is a process that requires the expenditure of energy, and as such it might be expected to be less efficient in the dev...

Role of renal sympathetic nerves in response of the ovine fetus to volume expansion

“…during early development or later in life. Unlike the short-term effects of volume expansion after birth (1,21), changes in fetal systemic hemodynamics and renal function are sustained during fetal hypervolemia. It is known from previous studies in fetal sheep that dextran increases fetoplacental blood volume to a greater extent than isotonic saline and has a higher retention time in the fetal vascular compartment (3).…”

Species comparison of anatomical and functional renal development

“…The fractional reabsorption of water in dogs is constant during postnatal maturation and similar in adults (Kleinman, 1982). Sodium excretion was studied in 1‐, 2‐, 3‐, and 6‐week‐old puppies by expanding intravascular volume with either isotonic saline or isoncotic albumin in saline (Aladjem et al, 1982). Glomerular filtration rate, sodium excretion, fractional excretion of sodium, and plasma volume measurements were made.…”

“…The highest level of sodium excretion was observed in 3‐week‐old puppies. The authors state that the mechanism underlying the difference between the response to isotonic saline and isoncotic albumin in saline is already operative at birth in dogs (Aladjem et al, 1982). A graph depicting the timing of urine concentrating ability in dogs is presented in Figure 5.…”

“… Cumulative sodium excretion in developing animals sustaining volume expansion with either saline (open bars) or isonocotic albumin (hatched bars). Reproduced from Aladjem et al (1982) with permission of the publisher. …”