Tentative Role of the Na+/H+ Exchanger Type 3 in Central Chemosensitivity of Respiration

Blockade of Na+/H+ exchanger type 3 causes intracellular acidification and hyperexcitability via inhibition of pH-sensitive K+ channels in chemosensitive respiratory neurons of the dorsal vagal nucleus in rats

Extracellular pH (pHe) and intracellular pH (pHi) are important factors for the excitability of chemosensitive central respiratory neurons that play an important role in respiration and obstructive sleep apnea. It has been proposed that inhibition of central Na(+)/H(+) exchanger 3 (NHE-3), a key pHi regulator in the brainstem, decreases the pHi, leading to membrane depolarization for the maintenance of respiration. However, how intracellular pH affects the neuronal excitability of respiratory neurons remains largely unknown. In this study, we showed that NHE-3 mRNA is widely distributed in respiration-related neurons of the rat brainstem, including the dorsal vagal nucleus (DVN). Whole-cell patch clamp recordings from DVN neurons in brain slices revealed that the standing outward current (Iso) through pH-sensitive K(+) channels was inhibited in the presence of the specific NHE-3 inhibitor AVE0657 that decreased the pHi. Exposure of DVN neurons to an acidified pHe and AVE0657 (5 μmol/L) resulted in a stronger effect on firing rate and Iso than acidified pHe alone. Taken together, our results showed that intracellular acidification by blocking NHE-3 results in inhibition of a pH-sensitive K(+) current, leading to synergistic excitation of chemosensitive DVN neurons for the regulation of respiration.

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.

Role of Brainstem Sodium/Proton Exchanger 3 for Breathing Control during Chronic Acid–Base Imbalance

RATIONALE

The sodium/proton exchanger (NHE) 3 is expressed in brainstem areas with prevalence for central chemosensitivity. Selective NHE3 inhibitors can evoke CO(2) mimetic responses both in vitro and in vivo, demonstrating the functional significance of this pH-regulating protein. Moreover, levels of NHE3 expression are inversely correlated to interindividual differences of baseline ventilation in conscious rabbits.

OBJECTIVES

We explored the influence of chronic acid-base disturbances on mRNA levels of brainstem NHE3 in relation to breathing control.

METHODS

Alveolar ventilation (Va), blood gases, systemic base excess (BE), and metabolic Vco(2) were determined in rabbits shortly after exposure to either CO(2)-enriched air for 3 days (n = 5) or to ammonium chloride with drinking water for 2 days (n = 6). Untreated animals served as controls (n = 24). NHE3 mRNA within the obex region was quantified by real-time reverse transcription-polymerase chain reaction.

MEASUREMENTS AND MAIN RESULTS

After chronic hypercapnia, we found a compensatory rise of BE (mean +/- SEM) to 5.3 +/- 0.5 mmol x L(-1) with slightly elevated Pa(CO(2)). Brainstem NHE3 mRNA as well as Va were not significantly different from control levels. In the NH(4)Cl group, arterial pH was approximately 0.09 units lower than control, and BE decreased to -6.5 +/- 1.6 mmol x L(-1) with slightly decreased Pa(CO(2)), but considerably reduced Va (by approximately 25%; P < 0.05) and Vco(2). Concomitantly, brainstem NHE3 mRNA had increased from control level of 1.45 +/- 0.19 to 3.64 +/- 0.37 fg cDNA/mug RNA; P < 0.01.

CONCLUSIONS

Expression of brainstem NHE3 is up-regulated by chronic metabolic acidosis but not by prolonged hypercapnia. It is proposed that elevated brainstem NHE3 expression contributes to limit maladaptive hyperventilation during metabolic acidosis.

Enhanced c-Fos expression in the rostral ventral respiratory complex and rostral parapyramidal region by inhibition of the Na+/H+ exchanger type 3

Previous studies have shown that selective inhibition of Na+/H+ exchanger type 3 (NHE3) induces intracellular acidification and activates CO2/H+-sensitive medullary neurons, mimicking the responses evoked by hypercapnic stimuli. In addition, NHE3 blockers administration decreases the duration of apnoea induced by laryngeal stimulation, presumably by means of central chemoreceptor activation. To test the hypothesis that the central chemoreceptor network may be affected by NHE3 inhibition, brainstem c-Fos immunoreactive cell counting was performed after systemic administration of the NHE3 blocker AVE1599 (Aventis Pharma Deutschland GmbH) (2 mg/kg). The rostro-caudal quantitative c-Fos analysis showed a significant increase in the number of c-Fos positive cells in the rostral part of the ventral respiratory complex (VRC) as well as in the rostral part of the parapyramidal (Ppy) region. The VRC activated region (-4.2 to -3.2mm interaural) included the pre-Bötzinger complex, the rostral ventral respiratory group and the rostral ventrolateral medulla, all of them involved in cardiorespiratory control. The activated Ppy region corresponded with the rostral chemosensitive area, which elicits the strongest ventilatory response upon ventral medullary surface stimulation with H+/CO2. Most cells activated in Ppy after NHE3 inhibition were serotonergic. Hence, systemic application of NHE3 blockers may induce central chemoreceptors activation and an increase in the respiratory network activity in a similar way to known physiological stimuli such as hypercapnia. On the other hand, selective NHE3 blockers could be excellent tools for treatment of pathological states where central chemoreceptor function is diminished or absent, such as central hypoventilation syndrome or sudden infant death syndrome.

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.