Insulin action on glucose transport in cardiac muscle

The mechanism of insulin action on glucose transport in rat hearts was studied. The glucose transport activity was determined after reconstitution into egg lecithin liposomes. Isolated rat hearts were perfused in the presence or absence of insulin and homogenized. The homogenate was fractionated by differential and sucrose density gradient centrifugations. Two subcellular fractions, designated as Fractions P-5 and P-6, contained glucose transport activity. Both fractions were enriched with 5'-nucleotidase (commonly known as a plasma membrane marker) and UDP-Gal:N-acetylglucosamine galactosyltransferase (known as a Golgi marker). However, only Fraction P-5 was concentrated with the insulin receptor and ouabain-sensitive p-nitrophenylphosphatase (both plasma membrane markers). The sedimentation properties of the glucose transport activity in Fraction P-6 were considerably different from those of galactosyltransferase. Insulin added to the heart before homogenization increased the glucose transport activity in Fraction P-5 approximately 1.6-fold while decreasing the activity in Fraction P-6 to approximately 62% of the control. These results are interpreted as follows. Both Fractions P-5 and P-6 are heterogeneous; nevertheless, Fraction P-5, but not Fraction P-6, may be enriched with the plasma membrane, which is assumed to be associated with glucose transport activity. Fraction P-6 may be concentrated with the Golgi apparatus; however, the latter may not be the structure (or vesicles) to which (intracellular) glucose transport activity is associated. Insulin appears to increase the glucose transport activity in rat hearts, at least in part, by inducing translocation of the glucose transport mechanism from the unidentified vesicles (in Fraction P-6) to the plasma membrane (in Fraction P-5).