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

Background n How to discriminate the own effects of acidosis from the effects of the underlying conditions => acidosis? n Is the cell the cause or the victim of acidosis? Is acidosis deleterious or protective? n 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

Where do H+ come from?

Metabolic acidosis n Cardiovascular –Impairment of cardiac contractility –Arteriolar dilatation, venoconstriction, and centralization of blood volume –Increased pulmonary vascular resistance –Reduction in cardiac output, arterial blood pressure, and hepatic and renal blood flow –Sensitization to reentrant arrhythmias and reduction in threshold of ventricular fibrillation –Attenuation of cardiovascular responsiveness to catecholamines n Respiratory –Hyperventilation –Decreased strength of respiratory muscles and promotion of muscle fatigue n Metabolic –Increased metabolic demands –Insulin resistance –Inhibition of anaerobic glycolysis –Reduction in ATP synthesis –Hyperkalemia –Increased protein degradation n Cerebral –Inhibition of metabolism and cell-volume regulation –Obtundation and coma

Metabolic versus respiratory acidosis

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

Intracellular buffering capacity n Intrinsic buffering capacity (proteins and phosphates buffers) + buffering capacity of HCO3-/CO2 system n intracellular pCO2 = extracellular pCO2 = interstitial pCO2  venous pCO2 n intracellular concentration of HCO3-  12 mmol/l n 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

Ion exchange transporters n 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

Ions exchange transporters n 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+

Ion exchange transporters n 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

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

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

Metabolism, activation, growth and cell proliferation n 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

Metabolism, activation, growth and cell proliferation n 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+)

Metabolism, activation, growth and cell proliferation n 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

Metabolism, activation, growth and cell proliferation n 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

Intracellular messengers: Ca++ & AMPc n 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 n Consequences of increased Ca++ cytosolic concentration? –Metabolic responses? –Ca++dependent contractility? –=> but may be blocked by acidosis n In contrast: acidosis could block the intracellular influx of Ca++ by voltage- dependent calcium channels

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

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

Regulation of cell volume n Changes in osmolarity => changes in cell volume by membrane ion exchangers n 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 n Metabolic acidosis => activation of Na+/H+ exchanger, cellular ballooning –activation of RVD n Competition between mechanisms of regulation of cell volume and of pHi

Other cellular properties n Membrane conductance of ion channels n membrane potentials n cytoskeleton n cellular coupling

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

Effects of acidosis on cell function n 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

Effects of acidosis on cell function n A. Response of a normal cell to an acidotic extracellular pHe –Changes of pCO2 => immediate effects on pHi > 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++

Effects of acidosis on cell function n 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 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++

Effects of acidosis on cell function n 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

Effects of acidosis on cell function n 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++

Effects of acidosis on cell function n 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

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

Particularities of poisoning n Impaired oxygen utilisation: cyanide, carbon monoxide… increased lactate production n Exogenous source of acids: methanol, salicylates