Effects of euvolemic and hypervolemic hyperbicarbonemia on segmental nephron HCO3 reabsorption in the newborn dog

Studies were undertaken to determine the effect of elevated plasma bicarbonate concentration (PHCO3) with and without volume expansion on segmental nephron HCO3 reabsorption in newborn and adult dogs using the technique of distal blockade. Reabsorption of bicarbonate per mL glomerular filtrate (RHCO3/GFR) in the proximal nephron was more suppressed in euvolemic newborns than euvolemic adults as PHCO3 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 HCO3 delivery and the fraction of the distal HCO3 load reabsorbed must have been greater in euvolemic newborns than adults when PHCO3 was elevated. Total nephron RHCO3/GFR was less suppressed by NaHCO3 volume expansion in the newborn than it was in the adult. However, proximal nephron RHCO3/GFR was similarly suppressed by NaHCO3 volume expansion in newborns and adults. Thus, the NaHCO3-expanded newborn must have reabsorbed a greater proportion of the increased distal HCO3 load than did the NaHCO3-expanded adult. Proximal nephron HCO3 reabsorption is a balance between reabsorption effected by active proton secretion and passive HCO3 back leak. Euvolemic increase in peritubular HCO3 concentration has been shown to suppress proximal tubular proton secretion; volume expansion increases proximal tubule HCO3 permeability and back leak.(ABSTRACT TRUNCATED AT 250 WORDS)