Regulation of renal acid-base balance by Epac isoforms in proximal tubule and collecting duct

Proper regulation of acid-base homeostasis has utmost significance for well-being and survival of the living organisms. Up to 50% of emergency room patients have notable disturbances in pH and associated morbific manifestations, which impedes effective treatment. Kidneys play a central role in regulation of acid-base balance by reabsorbing HCO3− via NHE-3 in the proximal tubule (PT) and secreting H+ via V-ATPase pump by the intercalated cells of collecting duct (CD). Exchange proteins directly activated by cAMP (Epac) are most prominently expressed in the proximal tubule and collecting ducts, the sites critical for acid-base transport in the renal tubule. However, the significance and physiological relevance of Epac isoforms (Epac1 and Epac2) for controlling HCO3− reabsorption and H+ secretion and by extension, the systemic pH regulation is not known. Here, we used transgenic mouse models lacking both or individual Epac1 and Epac2 isoforms to examine the systemic adaptation to acid-base insults and to inquire on the perturbed molecular signaling mechanisms and end-effectors in freshly isolated split-opened PTs and CDs using biochemical and fluorescent imaging tools. Deletion of Epac1&2 caused a dramatic decrease in NHE-3 protein levels in an additive manner. Moreover, we detected a marked redistribution of NHE-3 to the base of brush border in proximal tubule cells rendering its inactivation. Consistently, NHE-3 activity was more than 70% decreased upon Epac1&2 deletion, as was assessed by a rate of recovery after intracellular acidification in freshly isolated split-opened PTs. Furthermore, V-ATPase activity was similarly decreased in A-type of intercalated cells in freshly isolated CDs despite compensatory accumulation on the apical side, as monitored with confocal microscopy in cortical and medullary kidney sections of the knockout. In unstressed conditions, Epac1&2 deletion did not affect arterial pH and HCO3− levels, while urinary pH was significantly reduced. Dietary acid load (280 mM NH4Cl in drinking water for 3 days) unmasked severe hyperchloremic metabolic acidosis with pH 7.17±0.01 and HCO3− 14.5±0.6 mM and impaired urinary acidification. Deletion of Epac2 alone recapitulated the phenotype of double knockout, whereas Epac1−/− mice did not demonstrate notable deficiencies in response to dietary acid load. Overall, we concluded that Epac signaling plays a major role in urinary acidification with the dominant contribution of Epac2 isoform by regulating acid-base transport in both proximal tubule (via NHE-3) and the collecting duct (via V-ATPase). Epac1&2 and Epac2 deletion of impairs renal adaptation to the acid load resulting in severe metabolic acidosis. This research was supported by NIH-NIDDK DK117865, DK119170, AHA EIA35260097 (to O. Pochynyuk) and AHA-23POST1020372 (to K. Pyrshev). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.