Regulation of glucose metabolism by free fatty acid availability in septic and nonseptic rats

Abstract

An inhibitory effect of fatty acid oxidation on glucose uptake and oxidation has been demonstrated in heart and skeletal muscle under certain experimental conditions. This reciprocal relationship has been termed the glucose-fatty acid cycle. The purpose of the present study was to determine under in vivo conditions whether this interaction was operational in various nonmuscle tissues, and whether infection altered the activity of this cycle. Oral administration of alpha-methylpalmoxirate (MPA; 75 mg/kg), a known inhibitor of long-chain fatty acid oxidation, suppressed hepatic glucose production by 54% and increased whole body glucose disappearance by 15% in nonseptic rats. In contrast, MPA produced a larger reduction of glucose output in septic rats, but did not enhance glucose disposal. In vivo glucose uptake (Rg) by individual tissues was determined using the tracer 2-deoxyglucose technique. In nonseptic animals, MPA increased Rg in "working" muscles (heart and diaphragm; 12-fold and two-fold respectively), but not in "resting" skeletal muscles. MPA increased the Rg of heart and diaphragm to the same level in septic animals. Inhibition of fatty acid oxidation in nonseptic rats also enhanced Rg in liver (174%), spleen (158%), lung (153%), ileum (52%), skin (28%), kidney (115%), and epididymal fat (135%). In septic rats, MPA only increased Rg in the ileum (23%) and kidney (50%). This increased glucose uptake was independent of increases in plasma glucose and insulin concentrations. The infusion of heparin and intralipid, which increased circulating levels of fatty acids, failed to produce consistent changes in either the whole body glucose turnover or glucose uptake by individual tissues. We conclude that under basal in vivo conditions the inhibition of fatty acid oxidation suppresses glucose production and increases peripheral glucose disposal in nonseptic animals. These data support the presence of the glucose-fatty acid cycle in nonmuscle tissues and emphasizes its importance in whole body glucose homeostasis in situations where fatty acid oxidation is impaired. Infection increases glucose uptake by nonmuscle tissues which, for the most part, cannot be further enhanced by the inhibition of lipid oxidation.