NHE3(-/-) mice display a profound defect in proximal tubule
bicarbonate reabsorption but are only mildly acidotic owing to reduced glomerular filtration rate and enhanced H(+) secretion in distal nephron segments. In vivo microperfusion of rat distal tubules suggests that a significant fraction of
bicarbonate reabsorption in this nephron segment is mediated by NHE2. Two approaches were used to evaluate the role of distal tubule NHE2 in compensating for the proximal defect of H(+) secretion in NHE3(-/-) mice. First, renal clearance experiments were used to assess the impact of
HOE694, an inhibitor with significant affinity for NHE2, on excretion of
bicarbonate in NHE3(-/-) and NHE2(-/-) mice. Second, in vivo
micropuncture and microperfusion were employed to measure the concentration of
bicarbonate in early distal tubule fluid and to measure distal
bicarbonate reabsorption during a constant
bicarbonate load. Our data show that
HOE694 had no effect on urinary
bicarbonate excretion in NHE3(+/+) mice, whereas
bicarbonate excretion was higher in NHE3(-/-) mice receiving
HOE694.
HOE694 induced a significant increase in
bicarbonate excretion in mice given an acute
bicarbonate load, but there was no effect during
metabolic acidosis.
Bicarbonate excretion was not affected by
HOE694 in
bicarbonate-loaded NHE2(-/-) mice. In vivo
micropuncture revealed that early distal
bicarbonate concentration was elevated in both
bicarbonate-loaded and NHE3(-/-) mice. Further, microperfusion experiments showed that HOE694-sensitive
bicarbonate reabsorption capacity was higher in acidotic and NHE3 null animals. We conclude that NHE2 contributes importantly to acidification in the distal tubule, and that it plays a major role in limiting urinary
bicarbonate losses in states in which a high
luminal bicarbonate load is presented to the distal tubule, such as in NHE3 null mice.