Metabolic acidosis

1. Metabolic acidosis P Hantson Department of Intensive Care, Cliniques St-Luc, Université catholique de Louvain, Brussels, Belgium

2. Background • How to discriminate the own effects of acidosis from the effects of the underlying conditions => acidosis? • Is the cell the cause or the victim of acidosis? Is acidosis deleterious or protective? • Different mechanisms leading to acidosis: • mineral acidosis: normal cells in an acidotic extracellular pH • acidosis is the cause of cellular dysfunction • organic acidosis: cellular failure with organic acids overproduction • acidosis is the consequence of cellular dysfunction

3. Where do H+ come from?

4. Where do H+ come from?

5. Where do H+ come from?

6. Regulation of intracellular pH (pHi) • Values of pHi: experimental conditions, types of cells, level of metabolic activation • Usually: 6,8-7,2 • Strict regulation of pHi • at least two systems • intracellular buffering capacity • several systems of ion exchange transporter

7. Intracellular buffering capacity • Intrinsic buffering capacity (proteins and phosphates buffers) + buffering capacity of HCO3-/CO2 system • intracellular pCO2 = extracellular pCO2 = interstitial pCO2  venous pCO2 • intracellular concentration of HCO3-  12 mmol/l • intracellular acid load: 99.99% of the protons kept by the buffering systems => decrease of intracellular HCO3-, changes in the electrical load of the proteins

8. Ion exchange transporters • A. Na+/H+ exchanger • energy: gradient Na+ e - i, ejection of H+ • activation: • alcalanisation of the intracellular compartment • entry of Na+, and of water • selectively inhibited by amiloride • activated by a decrease of pHi, hypertonic shock, some anabolic hormones (insulin, cortisol, growth hormone) • sensitivity of Na+/H+ exchanger different from cell to cell

9. Ions exchange transporters • B. Transport of HCO3- • also activated by changes in pHi • Cl-/HCO3- exchanger activated: acidification of the intracellular compartment • HCO3- out, Cl- in • Cl-/HCO3-Na+dependent exchanger activated: alcalinisation of the intracellular compartment • HCO3- in, Cl- out • electrogenic Na+ - HCO3- co-exchanger • entry of HCO3- and Na+

10. Ion exchange transporters • C. Other systems • production of organic acids • cetogenesis and glycolysis are stimulated in presence of alcalosis • normalisation of pHi • regulation of pHi level of cellular activation

11. Metabolic acidosis • Does acidemia itself cause clinical effects? • Or are these effects caused by the variables producing acidosis? • ischemia • anoxia • Are the clinical consequences associated with acidosis related to the intra-cellular acid-base status or that of the extracellular fluid? • Comparison of the effects associated with respiratory vs metabolic acidosis • diffusibility of CO2 compared to strong ions

12.  metabolic activity changes in cytoskeleton  cell volume  intracellular membrane transporters  contractility pHi cell coupling changes membrane conductance changes of intracellular messengers  activation,  growth and  cell proliferation Interactions between pHi and cellular functions

13. Metabolism, activation, growth and cell proliferation • A. Metabolism • activation of cell metabolism => increased production of organic acids => decrease in pHi • decreased pHi => decreased cellular metabolic activity • changes in enzymes activity: phosphofructokinase, phosphorylase • also relationship between acidosis and energy demand

14. Metabolism, activation, growth and cell proliferation • A.Metabolism • hibernating mammals: decrease of pHi induced by a rise of pCO2 => decreased oxygen consumption • decrease of pHi induced by extracellular acidosis => inhibition of neoglucogenesis, decrease of hepatic urea, increase of the cytoplasmic ATP/ADP ratio • but the activation of Na+/H+ exchanger could in a first step increase E demand (activation of Na+/K+ ATPase pump secondary the cytoplasmic load of Na+)

15. Metabolism, activation, growth and cell proliferation • A. Metabolism • In conclusion, • Biphasic effect of extracellular acidosis on energy metabolism • 1. Increase of energy demand <= activation of the mechanisms of regulation of pHi • 2. With prolonged and severe acidosis, decrease of energy demand <= decrease of pHi

16. Metabolism, activation, growth and cell proliferation • B. Activation, growth and proliferation • increase of pHi by the activation of the Na+/H+ exchanger after exposure to anabolic hormones • role of pHi on cell proliferation in humans controversial • on the whole • alcalosis: anabolic responsiveness, metabolic activation, cell growth, proliferation • acidosis: reduced metabolic activity

17. Intracellular messengers: Ca++ & AMPc • Intracellular acidosis => increase of cytosolic Ca++ • 1. Removal of Ca++ from protein binding sites • 2. Activation of a Na+/Ca++ exchanger secondary to increase of Na+ intracellular influx due to the decrease of pHi • Consequences of increased Ca++ cytosolic concentration? • Metabolic responses? • Ca++dependent contractility? • => but may be blocked by acidosis • In contrast: acidosis could block the intracellular influx of Ca++ by voltage-dependent calcium channels

18. Intracellular messengers: Ca++ & AMPc • In summary • acidosis could increase intracellular Ca++ concentration • acidosis may decrease cellular responsiveness to Ca++ influx • acidosis may decrease intracellular Ca++ influx

19. Intracellular messengers: Ca++ & AMPc • Acidosis: variable effect on AMPc according to the type of cells • AMPc may be  or , but is usually reduced following intracellular acidosis

20. Regulation of cell volume • Changes in osmolarity => changes in cell volume by membrane ion exchangers • Hypertonic shock => passive decrease of cell volume • restoration of initial volume by RVI • activation of Na+/H+ exchanger with alcalinisation of intracellular compartment, entry of Na+ and water • Metabolic acidosis => activation of Na+/H+ exchanger, cellular ballooning • activation of RVD • Competition between mechanisms of regulation of cell volume and of pHi

21. Other cellular properties • Membrane conductance of ion channels • membrane potentials • cytoskeleton • cellular coupling

22. Effects of acidosis on cell function • Cellular response to metabolic acidosis  effect of lowering extracellular pH on cell function • Decrease of plasma pH during metabolic acidosis • impaired elimination of an extracellular acid load • overproduction of intracellular acid due to energy failure • Effects of extracellular acidotic pH on a normal cell >< effects of extracellular and intracellular acidotic pH on a cell under hypoxic conditions

23. Effects of acidosis on cell function • A. Response of a normal cell to an acidotic extracellular pHe • 1. Cell in normoxia exposed to acid pHe with constant pCO2 level: progressive  of pHi // degree of extracellular acidosis • 2. Cell exposed to a decrease of pHe with decreased pCO2 level: first sudden  of pHi, then progressive  of pHi // degree of extracellular acidosis • 3. Cell exposed to a decrease of pHe with increased pCO2 level, first sudden  of pHi, then progressive  of pHi

24. Effects of acidosis on cell function • A. Response of a normal cell to an acidotic extracellular pHe • Changes of pCO2 => immediate effects on pHi >< changes of HCO3- => progressive effects • Value of pHi at equilibrium: inhibition of the intracellular transfer mechanisms of HCO3- / activation of the Na+/H+ exchanger • With decreased pHi: swelling, catabolism,  sensitivity to Ca++

25. Effects of acidosis on cell function • B. Effects of acidosis on cells during hypoxia • 1. Mechanisms leading to cell death • 1.1 Energy failure • Anoxia => reduction of mitochondrial ATP production • Inhibition of the Na+/K+ ATPase pump • 1.2. Activation of cytolytic enzymes • Reduction of membrane phospholipides, phospholipase A2 activated by intracellular influx of Ca++ • 1.3. Ischemia - reperfusion • Hypoxic-ischemic stress: production of free radicals • Changes of mitochondrial membrane permeability • Activation of Na+/H+ exchanger: influx of Na+, activation of Na+/Ca++ , with influx of Ca++

26. Effects of acidosis on cell function • B. Effects of acidosis on cells during hypoxia • 2. Is acidosis deleterious or protective during hypoxia? • Classically said detrimental: acidosis inhibits phosphofructokinase, activates Na+/H+ exchanger, stimulates free radical production • Protective? Several experimental models • hepatocytes intoxicated by cyanide • anoxic cells and acidotic environment

27. Effects of acidosis on cell function • Effects of acidosis on cells during hypoxia • 2. Is acidosis deleterious or protective during hypoxia? • Decrease of pHi is responsible for the protective effect of external acidosis • « Sparing » effect of intracellular acidosis on metabolism • Prevention of the activation of phospholipase A2 in the presence of an influx of Ca++

28. Effects of acidosis on cell function • B. Effects of acidosis on cells during hypoxia • 2. Is acidosis deleterious or protective during hypoxia? • Also with ischemia-reperfusion models • the « pH paradox »: re-oxygenation or re-perfusion in an acidotic environment give better results • …but deleterious effects when pH too low (< 6.5) • …but protective effects only shown at cellular level

29. Conclusions • Results on cell function may vary according to the origin of acidosis • Hypoxic cell: adaptative energetic metabolism, protective effect of acidosis • « Normal » cell exposed to external acidosis: increased energy demand to maintain homeostasis • What is important for the cell? • Energy vs pH homeostasis?