title: Metformin overdose, but not lactic acidosis per se, inhibits oxygen consumption in pigs.
abstract: BACKGROUND
  Hepatic mitochondrial dysfunction may play a critical role in the pathogenesis of metformin-induced lactic acidosis. However, patients with severe metformin intoxication may have a 30 to 60% decrease in their global oxygen consumption, as for generalized inhibition of mitochondrial respiration. We developed a pig model of severe metformin intoxication to validate this clinical finding and assess mitochondrial function in liver and other tissues.
  
  
  METHODS
  Twenty healthy pigs were sedated and mechanically ventilated. Ten were infused with a large dose of metformin (4 to 8 g) and five were not (sham controls). Five others were infused with lactic acid to clarify whether lactic acidosis per se diminishes global oxygen use. Arterial pH, lactatemia, global oxygen consumption (VO2) (metabolic module) and delivery (DO2) (cardiac output by thermodilution) were monitored for nine hours. Oxygen extraction was computed as VO2/DO2. Activities of the main components of the mitochondrial respiratory chain (complex I, II and III, and IV) were measured with spectrophotometry (and expressed relative to citrate synthase activity) in heart, kidney, liver, skeletal muscle and platelets taken at the end of the study.
  
  
  RESULTS
  Pigs infused with metformin (6 ± 2 g; final serum drug level 77 ± 45 mg/L) progressively developed lactic acidosis (final arterial pH 6.93 ± 0.24 and lactate 18 ± 7 mmol/L, P < 0.001 for both). Their VO2 declined over time (from 115 ± 34 to 71 ± 30 ml/min, P < 0.001) despite grossly preserved DO2 (from 269 ± 68 to 239 ± 51 ml/min, P = 0.58). Oxygen extraction accordingly fell from 43 ± 10 to 30 ± 10% (P = 0.008). None of these changes occurred in either sham controls or pigs infused with lactic acid (final arterial pH 6.86 ± 0.16 and lactate 22 ± 3 mmol/L). Metformin intoxication was associated with inhibition of complex I in the liver (P < 0.001), heart (P < 0.001), kidney (P = 0.003), skeletal muscle (P = 0.012) and platelets (P = 0.053). The activity of complex II and III diminished in the liver (P < 0.001), heart (P < 0.001) and kidney (P < 0.005) while that of complex IV declined in the heart (P < 0.001).
  
  
  CONCLUSIONS
  Metformin intoxication induces lactic acidosis, inhibits global oxygen consumption and causes mitochondrial dysfunction in liver and other tissues. Lactic acidosis per se does not decrease whole-body respiration.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Protti
authors.forename: Francesco
authors.initials: F
authors.lastname: Fortunato
authors.forename: Massimo
authors.initials: M
authors.lastname: Monti
authors.forename: Sarah
authors.initials: S
authors.lastname: Vecchio
authors.forename: Stefano
authors.initials: S
authors.lastname: Gatti
authors.forename: Giacomo P
authors.initials: GP
authors.lastname: Comi
authors.forename: Rachele
authors.initials: R
authors.lastname: De Giuseppe
authors.forename: Luciano
authors.initials: L
authors.lastname: Gattinoni
mesh_terms: D000140:Acidosis, Lactic / Q000378:metabolism*; D000818:Animals; D062787:Drug Overdose / Q000378:metabolism*; D007004:Hypoglycemic Agents / Q000008:administration & dosage / Q000633:toxicity*; D007262:Infusions, Intravenous; D008687:Metformin / Q000008:administration & dosage / Q000633:toxicity*; D018384:Oxidative Stress / Q000187:drug effects / Q000502:physiology; D010101:Oxygen Consumption / Q000187:drug effects* / Q000502:physiology*; D013552:Swine
chemical_list: D007004:Hypoglycemic Agents; D008687:Metformin