Brainstem NHE-3 Expression and Control of Breathing

pH regulating transporters in neurons from various chemosensitive brainstem regions in neonatal rats

We studied the membrane transporters that mediate intracellular pH (pH(i)) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH(i) recovery from an NH(4)Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO(2)) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na(+)/H(+) exchange (NHE). Recovery in RTN neurons was blocked approximately 50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO(3)-dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked approximately 65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na(+) and increased by external HCO(3)(-). On the basis of these findings, pH(i) recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO(3)-dependent transporter in LC neurons. Thus, pH(i) recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Sodium/Proton Exchanger 3 in the Medulla Oblongata and Set Point of Breathing Control

RATIONALE

In vivo inhibition of the sodium/proton exchanger 3 (NHE3) in chemosensitive neurons of the ventrolateral brainstem augments central respiratory drive in anesthetized rabbits.

OBJECTIVES

To further explore the possible role of this exchanger for the control of breathing, we examined the individual relationship between brainstem NHE3 abundance and ventilation in rabbits during wakefulness.

METHODS

In 32 adult male rabbits on standard nutritional alkali load, alveolar ventilation, metabolic CO2 production, and blood gases were determined, together with arterial and urinary acid-base status and renal base control functions. Expression of NHE3 in brainstem tissue from the obex region was determined by quantitative real-time reverse-transcription polymerase chain reaction analysis.

MEASUREMENTS AND MAIN RESULTS

Regarding the distribution above and below the median, we classified high and low brainstem NHE3 animals, expressing a mean (+/- SEM) NHE3 mRNA of 2.08 +/- 0.28 and 0.72 +/- 0.06 fg cDNA/mg RNA, respectively. Alveolar ventilation of high brainstem NHE3 animals was lower than that of low brainstem NHE3 animals (715 +/- 36 vs. 919 +/- 41 ml . minute(-1); p < 0.01), a finding also reflected by a marked difference in Pa(CO2) (5.24 +/- 0.16 vs. 4.44 +/- 0.15 kPa; p < 0.01). Among possible secondary factors, CO2 production, systemic base excess, and fractional renal base reabsorption were not found to be different.

CONCLUSIONS

We conclude that the level of brainstem NHE3 expression-most likely via intracellular pH modulation-contributes to the individual control of breathing and Pa(CO2) in conscious rabbits by adjusting the set point and the loop gain of the system.