Functional role of NHE4 as a pH regulator in rat and human colonic crypts

Arena EA, Longo WE, Roberts KE, Geibel P, Nateqi J, Brandstetter M, Geibel JP. Functional role of NHE4 as a pH regulator in rat and human colonic crypts. Am J Physiol Cell Physiol 302: C412-C418, 2012. First published November 2, 2011; doi:10.1152/ajpcell.00163.2011.-To regulate ionic and fluid homeostasis, the colon relies upon a series of Na+-dependent transport proteins. Recent studies have identified a sodium/hydrogen exchanger (NHE) 4 (NHE4) protein in the gastrointestinal tract but to date there has been little description of its function. Additionally, we have previously shown that aldosterone can rapidly modulate Na+-dependent proton excretion via NHE proteins. In this study we examined the role of NHE4 in rat and human colonic crypts, determined the effect of aldosterone on NHE4 specifically, and explored the intracellular pathways leading to activation. Colonic samples were dissected from Sprague-Dawley rats. Human specimens were obtained from patients undergoing elective colon resections. Crypts were isolated using ethylenediaminetetraacetic acid and intracellular pH (pH(i)) changes were monitored using 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Crypts were exposed to 7 mu M ethylisopropylamiloride or 400 mu M amiloride, doses previously shown to inhibit NHE1 and NHE3 but allow NHE4 to remain active. Functional NHE4 activity was demonstrated in both rat and human colonic crypts. NHE4 activity was increased in the presence of 1 mu M aldosterone. In the rat model, crypts were exposed to 100 mu M 3-isobutyl-1-methylxanthine/1 mu M forskolin and demonstrated a decrease in NHE4 activity with increased cAMP levels. No significant change in NHE4 activity was seen by increasing osmolarity. These results demonstrate functional NHE4 activity in the rat and human colon and an increase in activity by aldosterone. This novel exchanger is capable of modulating intracellular pH over a wide pH spectrum and may play an important role in maintaining cellular pH homeostasis.