Effect of Insulin and Lipolytic Agents on Rat Adipocyte Low K Cyclic Adenosine 3′ : 5′-Monophosphate Phosphodiesterase

Data suggest that the antilipolytic effect of insulin may be mediated through activation of low Km phosphodiesterase, which may be responsible for the failure of these agents to produce sustained increases in adipocyte intracellular cyclic AMP.

Abstract

Abstract Isolated rat fat cells were incubated with insulin or lipolytic agents and measurements made of the activity of low Km cyclic adenosine 3' : 5'-monophosphate (cyclic AMP) phosphodiesterase in subsequently prepared fat-free homogenates. In the assay [3H]adenosine production from cyclic[3H]AMP was measured. Insulin consistently increased Vmax, did not alter Km (0.3 µm), and was ineffective when added to homogenates. As little as 7.5 microunits per ml of insulin increased phosphodiesterase activity by 18% and 50 microunits produced maximum stimulation of 65%. Incubation of fat cells with isoproterenol, epinephrine, or adrenocorticotropic hormone increased phosphodiesterase activity by 20 to 30 %. The effect of lipolytic hormones on phosphodiesterase activity could be reproduced when fat cells were incubated with 2.0 mm N6, O2'-dibutyryl adenosine 3' : 5'-monophosphate. The effect of insulin on phosphodiesterase activity was apparent within 2 min, was maximum at 5 min, and was sustained for at least 60 min. The response to isoproterenol was equally rapid, reaching a peak at 5 to 10 min but was not sustained. The fall in phosphodiesterase activity observed with isoproterenol was apparently related to intracellular ATP depletion. Homogenates prepared from fat cells preincubated with insulin and isoproterenol were separated into 50,000 x g particulate and supernatant fractions. The low Km phosphodiesterase activity associated with the particulate fraction was increased, while that of the supernatant was unaffected by hormonal preincubation. These data suggest that the antilipolytic effect of insulin may be mediated through activation of low Km phosphodiesterase. As well, the activation of phosphodiesterase produced by lipolytic hormones may be responsible for the failure of these agents to produce sustained increases in adipocyte intracellular cyclic AMP.