Mechanism of Adrenaline-induced Lipolysis in Adipose Tissue

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Abstract
Lipolysis in isolated fat cells was increased by dibutyryl-cyclic AMP (DBcAMP) and adrenaline but not by cyclic AMP. 3H-DBcAMP and 3H-cyclic AMP were incorporated into fat cells to the same extent. Therefore, lack of lipolytic action (free fatty acid release) of cyclic AMP was not due to an inability to be incorporated into the cells. Adipose tissue was found to contain enough lipase [EC 3.1.1.3] (ediol hydrolyzing activity) to elicit lipolysis even in the absence of lipolytic agents, such as adrenaline and DBcAMP. The amount of lipase did not increase, even in the presence of these lipolytic substances. Fat cells were disrupted in a hypotonic medium and lipid micelles were collected by centrifugation. These lipid micelles contained lipase and triglyceride. Incubation of these lipid micelles with adrenaline or DBcAMP resulted in marked lipolysis, but only negligible lipolysis was observed in the absence of these agents. Adrenaline and DBcAMP did not stimulate lipolysis in homogenates of lipid micelles. On the basis of these results, the mechanism of the lipolytic action of adrenaline or DBcAMP was discussed.

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