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 [1]. In view of the myriad of pH-sensitive functions, pH must be regulated within a narrow range [2]. Accordingly, disturbances in acid-base homeostasis may lead to partially life-threatening disorders [3]. 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) [6]. Chronic kidney disease invariably leads to acidosis, which contributes to disease progression [7]. Renal HCO3 and H+ excretion are a function of diverse transport processes [8]. 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 [9]. These cells express various H+ or HCO3 transporter proteins such as pendrin [1,10], H+ ATPase B1 [11], and the Cl-/HCO3 exchanger AE1 (Slc4a1) [12], which also classify the subtype of intercalated cells [9]. 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 [14] 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 [16], as well as reduced abundance of NKCC2 [17] and Napi2a [18] in these mice. The authors also described that AC6 is not exclusively mediating lithium-induced diabetes insipidus [19]. 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 [14] 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 [14]. As H+ ATPase is stimulated by cAMP [20], 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 [14]. Poulsen and colleagues [14] 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 [14] 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 [18] leads to recruitment of alkaline phosphate from bone, the phosphaturia could add to alkalosis. The decreased NKCC2 expression in AC6-deficient mice [17] 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 [21].

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 [22]. Especially, thyroid function may affect renal acid-base regulation, and hypothyroidism may account for reduced HCO3 reabsorption [23]. 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 [24], and cAMP was shown to regulate NHE3 activity without modifying the NHE3 membrane abundance [25]. Surprisingly, although NHE3 is phosphorylated on S552 by PKA [26], 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 [27]. An important role of AC6 in the proximal tubule was already suggested previously and the AC6-deficient mice display phosphaturia [18]. There may even be a cross-talk of these effects, as increased HCO3 concentrations may impair renal phosphate reabsorption [28].

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 [14] further remind us that much is still to be learned on the complex ramifications of acid-base regulation.

Funding

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.

Competing interests

The authors declare that there are no competing interests associated with the manuscript.

Abbreviation

     
  • AC6

    adenylyl cyclase 6

  •  
  • NKCC2

    Na-K-2Cl cotransporter

  •  
  • NHE3

    sodium-hydrogen exchanger 3

  •  
  • PKA

    protein kinase A

References

References
1
Wagner
C.A.
,
Mohebbi
N.
,
Capasso
G.
and
Geibel
J.P.
(
2011
)
The anion exchanger pendrin (SLC26A4) and renal acid-base homeostasis
.
Cell. Physiol. Biochem.
28
,
497
504
[PubMed]
2
Hamm
L.L.
,
Nakhoul
N.
and
Hering-Smith
K.S.
(
2015
)
Acid-base homeostasis
.
Clin. J. Am. Soc. Nephrol.
10
,
2232
2242
[PubMed]
3
Seifter
J.L.
and
Chang
H.Y.
(
2017
)
Disorders of acid-base balance: new perspectives
.
Kidney Dis. (Basel)
2
,
170
186
[PubMed]
4
Berend
K.
,
de Vries
A.P.
and
Gans
R.O.
(
2014
)
Physiological approach to assessment of acid-base disturbances
.
N. Engl. J. Med.
371
,
1434
1445
[PubMed]
5
Kettritz
R.
,
Ritter
T.
,
Rudolph
B.
and
Luft
F.C.
(
2017
)
The case | acid-base diagnoses in the 21st century
.
Kidney Int.
92
,
1293
1294
[PubMed]
6
Brown
D.
and
Wagner
C.A.
(
2012
)
Molecular mechanisms of acid-base sensing by the kidney
.
J. Am. Soc. Nephrol.
23
,
774
780
[PubMed]
7
de Brito-Ashurst
I.
,
Varagunam
M.
,
Raftery
M.J.
and
Yaqoob
M.M.
(
2009
)
Bicarbonate supplementation slows progression of CKD and improves nutritional status
.
J. Am. Soc. Nephrol.
20
,
2075
2084
[PubMed]
8
Kurtz
I.
(
2014
)
Molecular mechanisms and regulation of urinary acidification
.
Compr. Physiol.
4
,
1737
1774
[PubMed]
9
Roy
A.
,
Al-bataineh
M.M.
and
Pastor-Soler
N.M.
(
2015
)
Collecting duct intercalated cell function and regulation
.
Clin. J. Am. Soc. Nephrol.
10
,
305
324
[PubMed]
10
Alesutan
I.
,
Daryadel
A.
,
Mohebbi
N.
,
Pelzl
L.
,
Leibrock
C.
,
Voelkl
J.
et al
(
2011
)
Impact of bicarbonate, ammonium chloride, and acetazolamide on hepatic and renal SLC26A4 expression
.
Cell. Physiol. Biochem.
28
,
553
558
[PubMed]
11
Wagner
C.A.
,
Devuyst
O.
,
Bourgeois
S.
and
Mohebbi
N.
(
2009
)
Regulated acid-base transport in the collecting duct
.
Pflugers Arch.
458
,
137
156
[PubMed]
12
Romero
M.F.
,
Chen
A.P.
,
Parker
M.D.
and
Boron
W.F.
(
2013
)
The SLC4 family of bicarbonate (HCO(3)(-)) transporters
.
Mol. Aspects Med.
34
,
159
182
[PubMed]
13
Wagner
C.A.
(
2014
)
Effect of mineralocorticoids on acid-base balance
.
Nephron Physiol.
128
,
26
34
[PubMed]
14
Poulsen
S.B.
,
Marin De Evsikova
C.
,
Murali
S.K.
,
Praetorius
J.
,
Chern
Y.
,
Fenton
R.A.
et al
(
2018
)
Adenylyl cyclase 6 is required for maintaining acid-base homeostasis
.
Clin. Sci. (Lond.)
,
132
,
1779
1796
[PubMed]
15
Shen
T.
,
Suzuki
Y.
,
Poyard
M.
,
Miyamoto
N.
,
Defer
N.
and
Hanoune
J.
(
1997
)
Expression of adenylyl cyclase mRNAs in the adult, in developing, and in the Brattleboro rat kidney
.
Am. J. Physiol.
273
,
C323
C330
[PubMed]
16
Rieg
T.
,
Tang
T.
,
Murray
F.
,
Schroth
J.
,
Insel
P.A.
,
Fenton
R.A.
et al
(
2010
)
Adenylate cyclase 6 determines cAMP formation and aquaporin-2 phosphorylation and trafficking in inner medulla
.
J. Am. Soc. Nephrol.
21
,
2059
2068
[PubMed]
17
Rieg
T.
,
Tang
T.
,
Uchida
S.
,
Hammond
H.K.
,
Fenton
R.A.
and
Vallon
V.
(
2013
)
Adenylyl cyclase 6 enhances NKCC2 expression and mediates vasopressin-induced phosphorylation of NKCC2 and NCC
.
Am. J. Pathol.
182
,
96
106
[PubMed]
18
Fenton
R.A.
,
Murray
F.
,
Dominguez Rieg
J.A.
,
Tang
T.
,
Levi
M.
and
Rieg
T.
(
2014
)
Renal phosphate wasting in the absence of adenylyl cyclase 6
.
J. Am. Soc. Nephrol.
25
,
2822
2834
[PubMed]
19
Poulsen
S.B.
,
Kristensen
T.B.
,
Brooks
H.L.
,
Kohan
D.E.
,
Rieg
T.
and
Fenton
R.A.
(
2017
)
Role of adenylyl cyclase 6 in the development of lithium-induced nephrogenic diabetes insipidus
.
JCI Insight
2
,
e91042
[PubMed]
20
Paunescu
T.G.
,
Ljubojevic
M.
,
Russo
L.M.
,
Winter
C.
,
McLaughlin
M.M.
,
Wagner
C.A.
et al
(
2010
)
cAMP stimulates apical V-ATPase accumulation, microvillar elongation, and proton extrusion in kidney collecting duct A-intercalated cells
.
Am. J. Physiol. Renal Physiol.
298
,
F643
F654
[PubMed]
21
Hodges
G.J.
,
Gros
R.
,
Hegele
R.A.
,
Van Uum
S.
,
Shoemaker
J.K.
and
Feldman
R.D.
(
2010
)
Increased blood pressure and hyperdynamic cardiovascular responses in carriers of a common hyperfunctional variant of adenylyl cyclase 6
.
J. Pharmacol. Exp. Ther.
335
,
451
457
[PubMed]
22
Bouchard
M.
,
Souabni
A.
and
Busslinger
M.
(
2004
)
Tissue-specific expression of cre recombinase from the Pax8 locus
.
Genesis
38
,
105
109
[PubMed]
23
Marcos Morales
M.
,
Purchio Brucoli
H.C.
,
Malnic
G.
and
Gil Lopes
A.
(
1996
)
Role of thyroid hormones in renal tubule acidification
.
Mol. Cell. Biochem.
154
,
17
21
[PubMed]
24
Liu
F.Y.
and
Cogan
M.G.
(
1989
)
Angiotensin II stimulates early proximal bicarbonate absorption in the rat by decreasing cyclic adenosine monophosphate
.
J. Clin. Invest.
84
,
83
91
[PubMed]
25
Moe
O.W.
,
Amemiya
M.
and
Yamaji
Y.
(
1995
)
Activation of protein kinase A acutely inhibits and phosphorylates Na/H exchanger NHE-3
.
J. Clin. Invest.
96
,
2187
2194
[PubMed]
26
Zhao
H.
,
Wiederkehr
M.R.
,
Fan
L.
,
Collazo
R.L.
,
Crowder
L.A.
and
Moe
O.W.
(
1999
)
Acute inhibition of Na/H exchanger NHE-3 by cAMP. Role of protein kinase a and NHE-3 phosphoserines 552 and 605
.
J. Biol. Chem.
274
,
3978
3987
[PubMed]
27
Kocinsky
H.S.
,
Dynia
D.W.
,
Wang
T.
and
Aronson
P.S.
(
2007
)
NHE3 phosphorylation at serines 552 and 605 does not directly affect NHE3 activity
.
Am. J. Physiol. Renal Physiol.
293
,
F212
F218
[PubMed]
28
Steele
T.H.
,
Challoner-Hue
L.
,
Gottstein
J.H.
,
Stromberg
B.A.
and
Underwood
J.L.
(
1981
)
Acid-base maneuvers and phosphate transport in the isolated rat kidney
.
Pflugers Arch.
392
,
178
182
[PubMed]
29
Leibrock
C.B.
,
Alesutan
I.
,
Voelkl
J.
,
Michael
D.
,
Castor
T.
,
Kohlhofer
U.
et al
(
2016
)
Acetazolamide sensitive tissue calcification and aging of klotho-hypomorphic mice
.
J. Mol. Med.
94
,
95
106
[PubMed]