Inhibition of lipolysis in hamster adipocytes by the cation ionophor X537A.

The present study reports the effects of the lipophylic ionophore X537A on lipolysis and accumulation of cAMP in isolated hamster epidiymal adipocytes. X537A inhibited lipolysis activated with norepinephrine, isoproterenol, dibutyryl cAMP or theophylline but failed to influence basal lipolysis. The minimum effective concentration of X537A required to inhibit lipolysis was between 1 and 3 micrograms/ml; at a concentration of 10 micrograms/ml, X537A inhibited lipolysis by approximately 50%. The antilipolytic effect of X537A does not result from decreased formation of cAMP because the accumulation of cAMP in response to isoproterenol or theophylline was significantly potentiated in the presence of the ionophore. Most of the additional cAMP that accumulated in the presence of X537A was found to be intracellelular, the distribution of cAMP between cells and incubation medium not being influenced by X537A. Neither the basal activity of cAMP dependent protein kinase nor the activity in the presence of isoproterenol or theophylline was influenced by X537A. The effects of X537A on lipolysis and on accumulation of cAMP were found to persist in the absence of extracellular calcium, but adipocytes that were preincubated in a calcium free media containing 4.0 mM EGTA failed to respond to X537A with an increase in cAMP levels. It is concluded that X537A inhibits lipolysis by uncoupling cAMP accumulation from activation of triglyceride lipase by a mechanism unrelated to activation of protein kinase.