MECHANISM OF THE INHIBITORY EFFECTS OF CARDIAC GLYCOSIDES ON LIPOLYSIS

¹ Department of Pharmacology, Wyeth Laboratories, Inc., Radnor, Pennsylvania

The inhibitory effects of cardiac glycosides (CG) on drug-stimulated glycerol release were compared with those of sulfhydryl reagents and metabolic inhibitors in order to study the mechanism of CG action. The biphasic stimulation of lipolysis by norepinephrine was depressed by digitoxin (5 x 10^-4 M) in both minced tissue and isolated fat cell preparations. The norepinephrine-induced (5 x 10^-4 M) glycerol release also was reduced significantly by p-chloromercuriphenyl sulfonic acid (pCMBS), N-ethylmaleimide, iodoacetamide (IAA), 2,4-dinitrophenol (DNP) and Dicumarol. The addition of reduced glutathione completely reversed the pCMBS and N-ethylmaleimide inhibitions, partially reversed that due to IAA but was unable to antagonize those caused by DNP, Dicumarol or digitoxin. On the other hand, cysteine and 2,3-dimercaptopropanol significantly reduced only the N-ethylmaleimide inhibition, had no significant effect on pCMBS and IAA and potentiated the antilipolytic effects of DNP, Dicumarol and digitoxin. When norepinephrine (at 2.5 x 10^-5 M), theophylline or dibutyryl cyclic adenosine monophosphate were employed, all the aforementioned drugs again inhibited glycerol release. The digitoxin (10^-4 M) inhibition was completely antagonized by reduced glutathione while that of DNP and Dicumarol was partially reversed. Unlike before, cysteine completely reversed the pCMBS inhibition and significantly reduced that of IAA. The remainder of the effects were comparable to those described above. In addition to the resemblance of the effects of CG (at 10^-4 M) to —SH inhibitors, higher concentrations of CG (5 x 10^-4 M) behaved similarly to those of metabolic inhibitors (10^-3 M). These findings suggest that CG may exert their antilipolytic effects by alteration of membrane structure and/or by interference with oxidative metabolism.