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Biochemical and Molecular Actions of Nutrients

Response to Intra- and Extracellular Lipolytic Agents and Hormone-Sensitive Lipase Translocation Are Impaired in Adipocytes from Rats Adapted to a High-Protein, Carbohydrate-Free Diet¹

Departments of Physiology and Biochemistry/Immunology, School of Medicine, University of São Paulo, 14049–900 Ribeirão Preto-SP, Brazil

²To whom correspondence should be addressed. E-mail: rhmiglio{at}fmrp.usp.br.

We showed previously that rats adapted to a high-protein (70%), carbohydrate-free (HP) diet have reduced lipolytic activity. To clarify the underlying biochemical mechanisms, several metabolic processes involved in adipose tissue lipolysis were investigated. The experiments were performed in rats adapted for 15 d to an HP or a balanced diet. In agreement with previous results, microdialysis experiments showed that the concentrations of adipose tissue interstitial and arterial plasma glycerol were lower in rats adapted to the HP diet. Under nonstimulated conditions, rates of lipolysis, estimated by glycerol release to the incubation medium, were reduced in adipocytes from HP rats. Under the same conditions, there was a small, but significant (17%) reduction in the activity of hormone sensitive lipase (HSL), with no change in the content of the enzyme. Upon stimulation with isoproterenol, the percentage of the enzyme in the adipocyte cytosol translocated to the fat droplet was 20–25% in HP rats and 40–50% in rats fed the balanced diet. Adipocytes from HP diet–adapted rats had a significantly reduced response (40%) to the lipolytic action of nonspecific (norepinephrine, epinephrine, isoproterenol) and specific (CL316,243, BRL37,344, dobutamine, clenbuterol) ß-adrenergic agonists. Adipocytes from HP rats also had a reduced lipolytic response to the intracellular agents, dibutyryl cAMP (44%), forskolin (46%), and isobutylmethylxanthine (29%). The data suggest that the main mechanism responsible for the reduced basal and stimulated lipolysis in HP diet–adapted rats is an impairment in the intracellular process of lipolysis activation, with a deficient translocation of HSL to the fat droplet.

KEY WORDS: • hormone sensitive lipase (HSL) • HSL content and activity • HSL translocation • ß-adrenergic agonists • intracellular lipolytic agents

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