Phenformin causes
lactic acidosis in clinical situations due to inhibition of mitochondrial respiratory chain complex I. It is reportedly taken up by hepatocytes and exhibits mitochondrial toxicity in the liver. In this study, uptake of
phenformin and [(14)C]
tetraethylammonium (
TEA) and complex I inhibition by
phenformin were examined in isolated liver and heart mitochondria. Uptake of
phenformin into isolated rat liver mitochondria was higher than that into heart mitochondria. It was inhibited by several cat ionic compounds, which suggests the involvement of multispecific transport system(s). Similar characteristics were also observed for uptake of
TEA; however, uptake of
phenformin into mitochondria of organic
cation/
carnitine transporter 1 (OCTN1) knockout mice was lower than that in wild-type mice, whereas uptake of
TEA was comparable between the two strains, suggesting the involvement of distinct transport mechanisms for these two
cations in mitochondria. Inhibition by
phenformin of oxygen consumption via complex I respiration in isolated rat liver mitochondria was greater than that in heart mitochondria, whereas inhibitory effect of
phenformin on complex I respiration was similar in inside-out structured submitochondrial particles prepared from rat livers and hearts.
Lactic acidosis provoked by iv infusion of
phenformin was weaker in octn1(-/-) mice than that in wild-type mice. These observations suggest that uptake of
phenformin into liver mitochondria is at least partly mediated by OCTN1 and functionally relevant to its inhibition potential of complex I respiration. This study was, thus, the first to demonstrate OCTN1-mediated mitochondrial transport and toxicity of
biguanide in vivo in rodents.