Systemic acid-base balance is tightly controlled within a narrow range of pH. Disturbances in systemic acid-base homeostasis are associated with diverse detrimental effects. The kidney is a key regulator of acid-base balance, capable of excreting HCO3− or H+, and chronic kidney disease invariably leads to acidosis. However, the regulatory pathways underlying the fine-tuned acid-base sensing and regulatory mechanisms are still incompletely understood. In the article published recently in Clinical Science (vol 132 (16) 1779-1796), Poulson and colleagues investigated the role of adenylyl cyclase 6 (AC6) in acid-base homeostasis. They uncovered a complex role of AC6, specifically affecting acid-base balance during HCO3− load, which causes pronounced alkalosis in AC6-deficient mice. However, the phenotype of AC6-deficient mice appears much more complex, involving systemic effects associated with increased energy expenditure. These observations remind us that there is much to be learned about the intricate signaling pathways involved in renal control of acid-base balance and the complex ramifications of acid-base regulation.
Systemic acid-base balance results from the complex interplay of diverse organs including lung, kidney, liver, gastrointestinal tract, and bone . In view of the myriad of pH-sensitive functions, pH must be regulated within a narrow range . Accordingly, disturbances in acid-base homeostasis may lead to partially life-threatening disorders . Several organs contribute to the fine-tuning of pH regulation [2,4,5]. A key regulator of acid-base balance is the kidney, which can either counteract alkalosis by excreting HCO3− or counteract acidosis by excretion of H+ (and generation of HCO3−) . Chronic kidney disease invariably leads to acidosis, which contributes to disease progression . Renal HCO3− and H+ excretion are a function of diverse transport processes . The bulk of filtered HCO3− is reabsorbed in the proximal tubule. Fine-tuning of urinary pH is accomplished by intercalating cells and principal cells of the collecting duct . These cells express various H+ or HCO3− transporter proteins such as pendrin [1,10], H+ ATPase B1 , and the Cl−-/HCO3− exchanger AE1 (Slc4a1) , which also classify the subtype of intercalated cells . These transport proteins are tightly regulated to maintain acid-base balance and are responsive to changes in pH and hormonal signals [9,11,13]. However, the intracellular pathways regulating the activity of these transport molecules are still incompletely understood.
In their current work, Poulsen and colleagues  investigated a possible signaling pathway of renal pH regulation. They report intriguing observations uncovering a role of adenylyl cyclase 6 (AC6)-dependent cAMP production in the regulation of acid-base balance. At least on the mRNA level in rat and mouse kidneys, AC6 is the most abundant adenylyl cyclase isoform [15,16]. The authors previously observed reduced medullary cAMP formation, impaired membrane trafficking of aquaporin-2 , as well as reduced abundance of NKCC2  and Napi2a  in these mice. The authors also described that AC6 is not exclusively mediating lithium-induced diabetes insipidus . The current findings add new aspects about the role of AC6 in metabolic regulation. Indeed, AC6 deficiency in mice leads to mild alkalosis with elevated HCO3− plasma concentrations together with enhanced urinary acidity  paralleled by significantly higher renal abundance of the H+-ATPase B1 subunit. AC6 deficiency further blunts the response of urinary pH to HCO3− treatment. AC6 is expressed in intercalated cells, but subcellular distribution of H+-ATPase B1 subunit, pendrin, and anion exchangers 1 and 2 were not appreciably affected by AC6 deficiency under normal diet. The decline of H+-ATPase B1 subunit abundance and number of type A intercalated cells under HCO3− treatment was, however, blunted in AC6-deficient mice . As H+ ATPase is stimulated by cAMP , a reduction in cAMP in AC6-deficient mice cannot account for this effect.
Specific knockout of renal tubule and collecting duct AC6 (AC6loxloxPax8Cre mice) did not appreciably affect urinary pH and plasma HCO3− concentration under baseline conditions but resulted in an enhanced increase in blood HCO3− following HCO3− treatment . Poulsen and colleagues  conclude that AC6 is not required for renal acid elimination but becomes important for regulation of acid-base balance under HCO3− treatment.
The observations of Poulson and colleagues  shed light on the complexity of acid-base regulation and on the multiple renal and extrarenal functions of AC6, which could in turn affect acid-base balance. AC6-deficiency leads to increased energy expenditure suggestive of increased CO2 production, which could, at least in theory, enhance distal tubular H+ secretion. To the extent that the phosphaturia of AC6-deficient mice  leads to recruitment of alkaline phosphate from bone, the phosphaturia could add to alkalosis. The decreased NKCC2 expression in AC6-deficient mice  is expected to result in renal salt and water loss with volume depletion alkalosis. The complex role of AC6 is further underscored by the observation that a hyperfunctional variant of AC6 is associated with an increased vasodilator and heart-rate response in humans .
Mechanisms outside renal intercalated cells may be operative even in AC6loxloxPax8Cre mice, as the Pax8-CRE may also be expressed in the thyroid, hindbrain, adrenal gland, and inner ear . Especially, thyroid function may affect renal acid-base regulation, and hypothyroidism may account for reduced HCO3− reabsorption . Whether these disturbances exist in AC6-deficient mice remains to be determined.
In the proximal tubule, decreasing cAMP levels are associated with increased HCO3− reabsorption , and cAMP was shown to regulate NHE3 activity without modifying the NHE3 membrane abundance . Surprisingly, although NHE3 is phosphorylated on S552 by PKA , no differences were observed in AC6-deficient mice, indicating a possible compensation by other AC isoforms or mechanisms other than AC. The phosphorylation at S552 may, however, be dispensable for NHE3 activity . An important role of AC6 in the proximal tubule was already suggested previously and the AC6-deficient mice display phosphaturia . There may even be a cross-talk of these effects, as increased HCO3− concentrations may impair renal phosphate reabsorption .
At this moment, many questions about the mechanisms of AC6 in renal regulation of acid-base balance remain but highlight the complexity of the various contributing aspects of acid-base homeostasis. Nonetheless, in view of the tremendously important role of acid-base balance, especially in patients with chronic kidney disease [2,7,29], these observations extend the horizons and warrant further investigations of adenylyl cyclases in metabolism and renal transport function. The observations of Poulson and colleagues  further remind us that much is still to be learned on the complex ramifications of acid-base regulation.
This work was supported by the European Union Seventh Framework Programme (experiments in the authors’ laboratories) [grant number FP7/2007-2013]; the Systems Biology to Identify Molecular Targets for Vascular Disease Treatment [grant number SysVasc, HEALTH-2013 603288]; the Deutsche Forschungsgemeinschaft [grant number VO2259/2-1]; the Else Kröner-Fresenius-Stiftung [grant number 2017_A32]; and the Sonnenfeld Foundation.
The authors declare that there are no competing interests associated with the manuscript.