Immunofluorescent studies of the rat adipocyte cell surface

Antibodies against rat adipocyte plasma membranes have been shown to mimic insulin action in isolated adipocytes (Pillion & Czech, J. biol. Chem. Vol. 253, pp. 3761 - 3764, 1978). Immunofluorescent studies with antimembrane antibodies reveal capping on the adipocyte cell surface. Dose - response studies showed that the number of fat cells with obvious caps did not increase as the concentration of antimembrane antibodies was increased, whereas stimulation of glucose oxidation was proportional to the antibody concentration. At fairly high dilutions of antiserum there was no effect on rat adipocyte glucose oxidation, but caps were still visible, suggesting that there is not a direct correlation between capping and stimulation of adipocyte metabolism. At 15 degrees C, it was found that capping was not significantly impaired, while the basal rate of glucose oxidation was reduced considerably. Both insulin and antimembrane antibodies were still able to stimulate glucose oxidation at 15 degrees C, although the maximal rate of glucose oxidation which could be achieved at this temperature was considerably lower than that observed at 37 degrees C. Fat cells that were fixed with paraformaldehyde before being exposed to antimembrane antibodies showed a similar number of caps to unfixed cells, suggesting that some fat cells had a polarized distribution of membrane antigens even before exposure to antimembrane antibodies. These results demonstrate for the first time that antibodies against the rat adipocyte plasma membrane, which are known to mimic insulin action on rat fat cells, can associate with antigens arranged non-randomly on the cell surface, but it appears unlikely that capping plays a critical role in the expression of this biological activity.