Observations on the Production of Hydrogen Ions During Mobilization of Fatty Acids from Adipose Tissue

Lipolytic hormones of the sympathetic nervous system and pituitary gland activate a hormone-sensitive lipase in the fat cell, with resulting hydrolysis of intracellular triglyceride to glycerol and free fatty acids (FFA). The newly formed FFA are transported from the fat cell as a complex of the ionized acid, RCOO^-, with plasma albumin. The fate of the H⁺ which arise from dissociation of the mobilized FFA has not previously been investigated. In the present study, slices of epididymal or perirenal adipose tissue from rat, hamster or rabbit were incubated in albumin-free or albumin-containing Krebs- Ringer phosphate medium with and without epinephrine or ACTH. In the absence of albumin, FFA produced by lipolysis were not released into the medium but accumulated instead in the aqueous-insoluble, lipid-rich fraction of the tissue, whence they could be extracted in the form of the undissociated acids (RCOOH) by heptane without prior acidification; H⁺ likewise were not released into albumin-free medium and did not accumulate detectably in the aqueoussoluble fraction of the tissue. When albumin was present in the medium, newly formed FFA were continuously released by the hormone-stimulated tissue; this extracellular accumulation of RCOO^- complexed to albumin was associated with a progressive decline in medium pH which was equivalent at all times studied to that which would result from dissociation of the quantity of FFA found in the medium. Intravenous injection of 2 mg ACTH in the rabbit caused a 6- to 10-fold increase in plasma FFA during the following 3 hr, which was accompanied by marked hyperventilation and reductions in blood pH of 0.05 to 0.25 U, in plasma HCO₃^- concentration of 10 to 16 mEq/1, and in plasma PCO₂ of 7 to 22 mm Hg. The in vitro data demonstrate that the release of H⁺ from adipose tissue during lipolysis is coupled to release of RCOO^-, the latter process being limited by the availability of binding sites on albumin for RCOO^-. These data suggest, but do not prove, that this failure of H⁺ to be released during the lipolytic response except in association with RCOO^- results from the storage of newly formed FFA within the cells largely as RCOOH rather than RCOO^-. The in vivo observations suggest that the release of H⁺ from adipose tissue during lipolysis, even though regulated by the availability of circulating albumin's binding sites for RCOO^-, can nevertheless occur rapidly enough in the intact animal to cause acute nonketotic metabolic acidosis. (Endocrinology79: 1100, 1966