In previous in vitro studies the level of oxidized nicotinamide adenine dinucleotide (NAD+) in renal cortex changed parallel to changes in gluconeogenesis and NAD+ inhibited phosphate transport by renal cortical brushborder membrane (BBM) vesicles. To determine whether or not changes in renal gluconeogenesis in vivo were accompanied by altered renal handling of phosphate, possibly related to NAD+ action on BBM phosphate transport in vivo, renal gluconeogenesis was stimulated in rats by metabolic acidosis. Chronic acidosis in rats previously adapted to low phosphorus diet was associated with increased UPiV (controls, 68 +/- 19; acidotic, 1055 +/- 428 nmoles/mg creatinine; P less than 0.05) without changes in plasma phosphate and creatinine (Cr) and in UCrV compared to controls. The initial rate of sodium gradient-dependent transport of phosphate by renal cortical BBM vesicles was lower in acidotic TPTX rats compared to TPTX controls (controls, 3.10 +/- 0.16; acidotic, 1.50 +/- 0.06 nmole/mg protein/0.5 min; P less than 0.001), attributed to a decrease in the apparent Vmax. In renal cortex, gluconeogenesis and the NAD+/NADH ratio were increased in acidosis. Decreased BBM transport of phosphate in proximal tubules of acidotic rats may explain the increased UPiV. This change indicates reversal of the adaptation of the BBM phosphate transport system to dietary phosphorus deprivation and may be related to increases in gluconeogenesis and the NAD+/NADH ratio in renal cortex.