Effects of Euvolemic and Hypervolemic Hyperbicarbonemia on Segmental Nephron HCO3 Reabsorption in the Newborn Dog1

Abstract

ABSTRACT. Studies were undertaken to determine the effect of elevated plasma bicarbonate concentration (PHC03) with and without volume expansion on segmental nephron H C 0 3 reabsorption in newborn and adult dogs using the technique of distal blockade. Reabsorption of bicarbonate per mL glomerular filtrate (RHC03lGFR) in the proximal nephron was more suppressed in euvolemic newborns than euvolemic adults as PHC03 was increased from baseline to 50-70 mM. Inasmuch as total nephron reabsorption has been shown to be essentially complete under these conditions in both newborns and adults, distal nephron H C 0 3 delivery and the fraction of the distal H C 0 3 load reabsorbed must have been greater in euvolemic newborns than adults when P H C 0 3 was elevated. Total nephron RHC03/GFR was less suppressed by NaHC03 volume expansion in the newborn than it was in the adult. However, proximal nephron RHC03/GFR was similarly suppressed by NaHC03 volume expansion in newborns and adults. Thus, the NaHC03-expanded newborn must have reabsorbed a greater proportion of the increased distal HC03 load than did the NaHC03-expanded adult. Proximal nephron H C 0 3 reabsorption is a balance between reabsorption effected by active proton secretion and passive H C 0 3 back leak. Euvolemic increase in peritubular HCO3 concentration has been shown to suppress proximal tubular proton secretion; volume expansion increases proximal tubule H C 0 3 permeability and back leak. Thus, we interpret these results to indicate that: 1 ) the limited capacity of the immature proximal tubule for proton secretion is most likely responsible for the greater suppression of proximal RHC03/GFR by increased PHC03 in euvolemic newborn dogs compared with adults; 2) the increase in H C 0 3 permeability of the immature proximal nephron with volume expansion is no greater than that which occurs in the mature proximal nephron with volume expansion; and 3) the immature distal nephron displays a greater propensity for HCO3 reabsorption as PHC03 is increased than does the mature distal nephron, with or without volume expansion. (Pediatv Res 27: 604-611, 1990) Abbreviations FRHC03, fractional reabsorption of the filtered bicarbonate load GFR, glomerular filtration rate PHC03, plasma bicarbonate concentration