Effects of insulin on adrenoceptor binding and the rate of catecholamine-induced lipolysis in isolated human fat cells.
The mechanisms by which insulin inhibits catecholamine-induced lipolysis in fat cells are unknown. In this study the possible role of an interaction between insulin and the adrenoceptors on human fat cells was investigated. Insulin inhibited, in a dose-dependent fashion, the specific binding of hydrophobic as well as hydrophilic nonselective beta-receptor radioligands but had no effect on the binding of alpha 2-selective radioligands. The results of saturation experiments and competition-inhibition experiments under both equilibrium conditions and nonequilibrium conditions revealed that insulin reduced the total number of beta-adrenergic binding sites (maximum effect 25%) without changing the beta-adrenoceptor affinity. This insulin effect was rapid and reversible; one-third of the effect occurred within 1 min of incubation and it was completely reversed within 30 min after withdrawal of insulin. It could be mimicked by a polyclonal rabbit insulin receptor antibody but not by insulin mimickers acting distal to the initial interaction between the hormone and its specific insulin-receptor binding site. The beta-adrenoceptor binding to a plasma membrane-enriched fraction decreased at the same time as it increased to a microsomal enriched fraction after insulin treatment, indicating a redistribution of beta-adrenoceptors in the cell. In lipolysis experiments performed under conditions like those in the binding experiments, insulin inhibited the rate of lipolysis with a lag period of 3 min. Furthermore, the hormone caused a dose-dependent maximum 10-fold shift to the right of the dose-response curve for isoprenaline-induced lipolysis without changing the amplitude of the curve. This effect of insulin was specific for the beta-adrenergic receptors system, since insulin markedly decreased the amplitude of the dose-response curve for parathyroid hormone-induced lipolysis. In addition, the effect of insulin on isoprenaline-induced lipolysis could be mimicked by long-lasting fractional inactivation of the beta-adrenoceptors. The dose-response relationships for the inhibitory effects of insulin on beta-adrenoceptor binding and the lipolytic sensitivity to isoprenaline were almost identical. Half-maximum and maximum effects occurred at about 5 and 100 microunits/ml of insulin, respectively. In conclusion, the exposure of human fat cells to physiological insulin doses is followed by a rapid and dose-dependent translocation of beta-adrenoceptors from the exterior to the interior of the cell and a subsequent dose-dependent decrease in the lipolytic sensitivity to beta-adrenergic agonists, without a change in maximum lipolysis.(ABSTRACT TRUNCATED AT 400 WORDS)
Engfeldt
P
P
Department of Medicine, Huddinge Hospital, Karolinska Institute, Stockholm, Sweden.
Hellmér
J
J
Wahrenberg
H
H
Arner
P
P
D000273:Adipose Tissue / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D002395:Catecholamines / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D020120:Iodocyanopindolol; D007545:Isoproterenol / Q000378:metabolism; D008066:Lipolysis / Q000187:drug effects*; D010869:Pindolol / Q000031:analogs & derivatives / Q000378:metabolism; D011817:Rabbits; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011943:Receptors, Adrenergic, beta / Q000378:metabolism; D015016:Yohimbine / Q000378:metabolism
D002395:Catecholamines; D007328:Insulin; D011941:Receptors, Adrenergic; D011943:Receptors, Adrenergic, beta; D015016:Yohimbine; D020120:Iodocyanopindolol; D010869:Pindolol; D007545:Isoproterenol