MECHANISM OF HALOTHANE-INDUCED INHIBITION OF ISOPROTERENOL-STIMULATED LIPOLYSIS IN ISOLATED RAT ADIPOCYTES

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  • 1 March 1988
    • journal article
    • research article
    • Vol. 33  (3) , 338-343
Abstract
The effect of halothane on isoproterenol-stimulated lipolysis was determined in isolated rat epididymal fat cells. The maximal lipolytic response (Emax) activated by isoproterenol was 350 .+-. 61 nmol of glycerol/105 cells/hr with an EC50 of 5.1 .times. 10-9 M. When the adipocytes were simultaneously bubbled with 2.5% halothane, the Emax decreased to 158 .+-. 43 nmol of glycerol/105 cells/hr and the dose response curve for isoproterenol was shifted to the right (EC50 3.5 .times. 10-8 M, p < 0.05). When lipolysis was maximally stimulated with (-)-isoproterenol (10-6 M), the inhibitory effect of halothane was found to be both dose dependent (IC50 .apprx. 2.5%, v/v) and reversible following washout. Neither the nonhydrolyzable cAMP analong, 8-(4-chlorophenylthio) adenosine 3''-,5''-cyclic monophosphate (2 .times. 10-3 M), nor forskolin (10-6 M) was able to normalize lipolysis in the presence of halothane. The activation of cAMP-dependent protein kinase (EC 2.7.1.37) activity by isoproterenol was not different in halothane-exposed cells when compared to unexposed cells. When control adipocytes were exposed to isoproterenol (10-6 M), there was a 2.5-fold increase in the activity of hormone-sensitive lipase (EC 3.1.1.3) from 0.64 .+-. 0.13 to 1.53 .+-. 0.32 pkat (pmol/sec) per mg (p < 0.005, n = 10). However, in the presence of halothane (2.5%, v/v) isoproterenol stimulation of hormone-sensitive lipase was attenuated by 50% to values of 1.06 .+-. 0.23 pkat/mg (p < 0.01, n = 10). Halotane had no direct inhibitory effect on hormone-sensitive lipase since this enzyme''s activity was unaffected when homogenates of isoproterenol-stimulated control cells were incubated with halothane. These studies suggest that halothane impairs the activation of hormone-sensitive lipase by cAMP-dependent protein kinase and in this manner inhibits .beta.-adrenergic-stimulated lipolysis.