Page Wages

Advisor: Dr. Joe Erlichman

SLU Festival of Science 2001 Poster Presentation

Developmental Changes in the Intracellular pH Regulation in the Medullary Neuronof the Rat and the Importance of Sodium/Hydrogen Exchange on pH Regulation and Ventilation

Abstract

CO₂ chemoreceptors in the brainstem are thought to play a role in the regulation of the rate of ventilation. The retrotrapezoid nucleus (RTN), a chemosensitive site was compared to two non-chemosensitive sites, the hypoglossal nucleus (HYP) and the medial vestibular nucleus (MeV) in the medulla. The chemoreceptive areas differ from neighboring non-chemoreceptive areas because they do not regulate the intracellular pH (pH_i). Using an ammonia (NH₄Cl) prepulse, which acidifies only pH_i, it was found that both the chemosensitive and non-chemosensitive areas regulated their pH, recovering quickly from the acid load. An increase in CO₂ concentration decreases the pH of both the pH_i of chemoreceptive cells and the external pH (pH_o). The RTN, in rats younger than 8 days, was the only area tested that did not recover from the acidosis, both the HYP and MEV recovered normally. Hypercapnia, therefore, resulted in an increase in ventilation due to nerve firings from the chemoreceptive areas, which did not recover from the acidification. However, the pH_irecovery abilities of the neurons in the HYP were found to decrease progressively with age. In rats older than 8 days, hypercapnia resulted in sustained acidification in both the HYP and the MeV, similar to the effects seen in the RTN. However, the HYP and MeV neurons were unable to recover from the NH₄Cl acidification of only the pH_i. Comparing these three areas in the medulla, the mechanism of the pH regulation in the chemoreceptors was labeled, using amiloride and DIDS, as the Na⁺/H⁺ transporter. The pH_i recovering from acidification in the RTN neurons was found to be dependent on the extracellular Na⁺ concentration, which was blocked by 1 mM amiloride. DIDS blocks the CO₂/HCO₃^- exchanger and 0.05mM concentration of the drug had no effect. This indicates the bicarbonate ion does not play a vital role in the regulation of pH_i and pH_o in the chemosensitive regions of the brainstem. The pH_i recovery from intracellular acidification in the RTN, HYP, and MeV neurons is sensitive to changes in pH_o in younger rats (less than 8 days) and not older rats (older than 8 days), indicating a developmental change in the chemoreceptive regions. Therefore, rats younger than 8 days have not yet developed the differential pH regulation between the chemoreceptive regions and the non-chemoreceptive regions. Transporters on the membrane must play a role in the regulation of cellular pH, and these transporters were only found on the membranes of the chemosensitive neurons.

In the whole animal, amiloride and DIDS prepulses were done and the ventilations were measured using a whole-body plethysmograph on Sprague-Dawley rats. The amiloride effects caused an increase in ventilation, due to the blocking of the Na⁺/H⁺ transporter. DIDS had no significant effect on the ventilation of the rats, verifying the in vitro studies.