Development of the renal response to blood volume expansion in normal and fast-growing rats

Male rats from reduced (fast-growing) litters between 14 and 50 days of age were studied. Standard renal clearnce techniques were employed. After a 60-min control period, the animals were infused (2.3% body wt) with heparinized donor blood obtained from lillermates of the same age. Renal function was followed for an additional 60 min. The efficiency of the diuretic response, the percent infused volume excreted above control levels, and the sodium efficiency, the percent infused Na excreted, were calculated. Results indicate that both efficiencies develop in a discontinuous pattern and that they are comparable. This pattern of development, as well as the magnitude of the mature response, is comparable to that previously reported for rats from intact (normally growing) litters. The onset and attainment of the mature response is, however, shifted in time, such that reduced-litter animals achieve the mature response 10-15 days earlier than intact-litter rats. Results exclude chronological age or body weight alone as principal determinant of the mature response and suggest that some function of growth rate is responsible for the maturation of this regulatory function.