Direct phosphorylation effects of epinephrine on the plasma membranes of intact rat fat cells.

Collagenase-treated isolated cells, prepared from epididymal fat pads of Sprague-Dawley rats exposed to 32Pi or (gamma-32P)-ATP, result in differences in label incorporation into peptides as determined by autoradiography of dried SDS polyacrylamide gels. 32Pi-labelled cells respond to epinephrine by increase in labeling of a 67,000-dalton band, presumably the activated lipase. (gamma-32P)-ATP exposed cells gave a dose-dependent increase in a 53,000-dalton band, a finding shared by cells exposed to cAMP in the absence of epinephrine. However, whereas, cAMP also significantly increased the labeling of an 18,000-dalton band, epinephrine had only a minor effect in the labeling of the 18,000-dalton component. Also, the degree of labeling of a 42,000-dalton band is diminished after epinephrine compared to unstimulated cells. By contrast, cAMP does not affect the labeling of the 42,000-dalton component. The localization of the 53,000- and the 18,000-dalton peptides as well as the enzyme(s) that catalyze their phosphorylation on the external surface of the fat cell is supported by studies using a number of macromolecular probes as well as by subcellular fractionation studies. The absence of such phosphopeptides or kinase activity in the infranatants of such cell suspensions eliminates the possibility that these phenomena are the result of leakage of cytoplasmic components. Thus, epinephrine appears to have effect and do not appear to be explicable simply by the release of cAMP to the extracellular compartment.