INSULIN ACTION ON ADIPOCYTES - EVIDENCE THAT THE ANTI-LIPOLYTIC AND LIPOGENIC EFFECTS OF INSULIN ARE MEDIATED BY THE SAME RECEPTOR

The dose-response relationships of insulin stimulation of lipogenesis and inhibition of lipolysis were studied simultaneously by using rat adipocytes to determine whether these different effects of insulin are mediated through the same or different sets of receptors. The sensitivity (defined as the concentration of insulin required to produce a half-maximal effect) of the stimulated lipogenic response to insulin was not significantly different from the sensitivity of the anti-lipolytic response to insulin. The addition of different adrenaline and glucose concentrations did not alter the half-maximal concentration of insulin required to inhibit lipolysis. The specificities of the lipogenic and anti-lipolytic responses were studied by using insulin analogues. The sensitivities of the lipogenic and anti-lipolytic responses were the same for five chemically modified insulins and hagfish insulin, which have potencies compared with bovine insulin of between 3 and 90%. Starving rats for 48 h significantly increased the sensitivities of both the anti-lipolytic and lipogenic responses to insulin, but the changes in the sensitivities of the two effects were not significantly different. After re-feeding for 24 h the sensitivities of both lipogenesis and anti-lipolysis returned to that of fed rats. We conclude that insulin stimulates lipogenesis and inhibits lipolysis over the same concentration range. These observations provide powerful evidence that the different effects of insulin are mediated through the same set of receptors.