| Abstract: | Membrane transport of long chain fatty acids in the isolated adipocyte can be stimulated 5-10-fold by epinephrine (Abumrad, N. A., Perry, P. R., and Whitesell, R. R. (1985) J. Biol. Chem. 260, 9969-9971). This study shows that isoproterenol and norepinephrine are more potent than epinephrine in activating the transport process. The stimulatory effect on transport is mediated by beta-receptor interaction and cAMP. This was shown by the following. alpha-Receptor agonists and antagonists were ineffective; methylisobutylxanthine at low concentration (3 microM) potentiated the effect of a suboptimal dose (0.01 microgram/ml) of epinephrine and was stimulatory at high concentration (100 microM) in the absence of epinephrine; and cAMP analogs were very effective activators. Involvement of the cAMP-dependent protein kinase was indicated by two lines of evidence. 1) Combinations of cAMP analogs which are specific for sites 1 and 2 of the protein kinase, respectively, had synergistic effects on fatty acid transport. Combinations of analogs specific for the same site were only additive in their effects. This is similar to the pattern of protein kinase activation in vitro and to that of lipolysis activation in the intact adipocyte (Beebe, S. J., Holloway, R., Rannels, S. R., and Corbin, J. D. (1984) J. Biol. Chem. 259, 3539-3547). 2) Treatment of cells with various metabolic poisons abolished the stimulatory effect of norepinephrine. The response of fatty acid transport to catecholamines showed multiple parallels with that documented for lipolysis except that it was much more rapid. This suggested that the transport process was a regulatory step in fatty acid mobilization. This interpretation is supported by the observation that basal Vmax for transport is much too slow to accommodate the rate of fatty acid release which is observed following stimulation of intact cells with adrenergic hormones. |
