The Functional Role of Sodium/Hydrogen Exchangers in the Rat Suprachiasmatic Nucleus

• Main Authors: Hsin Yi Lin, 林欣毅
• Other Authors: R. C. Huang
• Format: Others
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/78852639119662768196

碩士 === 長庚大學 === 生物醫學研究所 === 99 === Our lab previously demonstrated extracellular acidification in the rat suprachiasmatic nucleus (SCN), which is in part mediated via carbonic anhydrase. As the sodium/hydrogen exchanger (NHE) acts to regulate intracellular proton concentrations by using the sodium concentration gradient to extrude one H+ in exchange for one Na+ flowing into the cell, the NHE may also mediate extracellular acidifications in the SCN. Here I used the ion-selective electrode recording technique to study the functional roles of NHE in the SCN. RT-PCR analysis indicates expression of mRNAs for multiple plasma membrane NHE isoforms, NHE1, NHE3, NHE4, and NHE5, in the SCN. Consistently, dose dependence of amiloride-induced alkalinization suggests the presence of at least two NHE isoforms in action. To determine if astrocytes contribute to extracellular acidifications in the SCN, we compared the basal extracellular pH and pH responses to 100 μM amiloride, zero K+, and 1 mM NaCN in control slices and slices treated with the glia toxin fluoroacetate. The results indicate no difference between control and fluoroacetate-treated SCN slices, suggesting a negligible contribution of astrocytes in extracellular acidifications. To investigate if the proton produced by the pumping activity of Na/K pump is extruded by the NHE to cause extracellular acidification, we determine the effect of 100 μM amiloride on zero K+-induced extracellular alkalinization. The result indicates a decrease in the zero K+-induced extracellular alkalinization by amiloride, suggesting one (or two) of the NHE extrudes the protons produced by the Na/K pump. To determine if internal Na+ regulates the NHE activity, we investigated the effects on extracellular pH of TTX and veratridine to block and open the voltage-dependent Na+ channels, respectively. The result indicates a lack of effect of TTX on extracellular pH, suggesting that in physiological settings, the Na+ ions flow through TTX-sensitive Na+ channels do not alter the NHE activity. In contrast, veratridine produced extracellular alkalinization at a concentration of 10 μM but caused extracellular acidification at a concentration of 50 μM, suggesting that the artificially increased Na+ flow through TTX-sensitive Na+ channels may alter the NHE activity. It remains to be determined if the increased Na+ loads via 10 μM veratridine reduces the NHE activity to produce extracellular alkalinization. Nonetheless, the extracellular acidification produced by 50 μM veratridine may be associated with membrane depolarization, as 20 mM K+ also produced extracellular acidification. In conclusion, the SCN expresses multiple isoforms of NHE (NHE1, NHE3, NHE4, and NHE5), at least two of which mediating extracellular acidifications by extruding protons. Furthermore, the protons produced by Na/K pumping activity may be extruded by one (or two) of the NHEs. Understanding how different isoforms of NHE interact locally with channels/transporters and how they contribute to proton extrusion and extracellular acidification will be a focus in the future.