Mechanisms of reduced lipolysis in adipose tissue of rats adapted to a high-protein, carbohydrate-free diet

Author(s)

Maristela Polachini Martins Afférri

• Document type: Doctoral Thesis
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Medicina de Ribeirão Preto (PCARP/BC)
• Defense date: 2002-08-16

Examining board members

Renato Helios Migliorini; Leida Maria Botion; Nilton de Almeida Brito; Isis do Carmo Kettelhut; Fabio Bessa Lima

• Advisor: Renato Helios Migliorini

Abstract

Previous studies from this laboratory have shown that rats adapted to a high-protein, carbohydrate-free (HP) diet have reduced rates of lipolysis, evidenced by low levels of plasma free fatty acids (FFA) and by reduced rates of glycerol release by incubated fragments of adipose tissue or isolated adipocytes. To obtain more information about the mechanism of this reduction the effects of the HP diet on central processes involved in lipolysis control and on the lipolytic activity of adipocytes were investigated in this work. Rats were adapted for 15 days to a HP (w/w, 70% protein, 8% fat, no carbohydrate) or to a control (16% protein, 66% carbohydrate and 8% fat) before the experiments, which were performed in the fed state. The results obtained can be summarized as follows: - Adaptation to the HP diet did not affect significantly the hyperglycemia or the increase in plasma glycerol and FFA induced by central neuroglucopenia provoked by intracerebroventricular infusion of 2-deoxyglucose (10mg:30µL, for 20 min). - The concentration of norepinephrine or the neuronal activity of neurotransmitters (dopamine, serotonine) in VMH and LH did not differ significantly in HP-fed and control rats. - ln agreement with previous studies in vivo and with fragments of adipose tissue or adipocytes in vitro, the lipolytic activity of adipose tissue perfused in situ, was reduced in HP-diet adapted rats, as shown by decreased concentration of glycerol in the dialysate. - Confirming previous observations, basal (non-stimulated) rates of lipolysis, estimated by the release of glycerol to the incubation medium, was reduced in adipocytes from HP diet-fed rats. The lipolytic response to non-selective (norepinephrine, epinephrine, isoproterenol and phenilephrine) and selective (dobutamine, clembuterol, CL 316,243 and BRL 37,344) agonists was reduced to about 40% in adipocytes from HP rats. However, the composition of adrenoceptors, as judged by the relative potency of the different agonists, was similar to that in adipocytes from control rats. - The inhibition by a specific antagonist β3 SR 59,230 of the increase in lipolysis induced by norephinephrine or CL 316,243 was more marked in adipocytes from HP-rats when calculated as % of rates in the antagonist absence, but did not differ significantly in the two experimental groups in absolute terms. - The lipolytic response to lipolytic agents that act intracellularly (dibutyryl cAMP- DBcAMP, forskolina - FSK, ou isobutylmethylxanthine - IBMX) was reduced in HP-rats adipocytes. Lipolysis stimulation by DBcAMP and FSK was reduced to about 45% of controls in HP rats adipocytes, a reduction similar to that obtained in the experiments with agonists. The reduction in HP-adipocytes of lipolysis stimulation by IBMX was smaller (29%). - Addition of adenosine deaminase (ADA) to the incubation medium resulted in quantitatively similar increases in the lipolytic activity of adipocytes from HP and control rats. Addition of ADA plus IBMX induced an increase in lipolysis similar to that induced by DBcAMP alone in adipocytes from HP-rats, but not in control adipocytes, in which the increase induced by the two drugs together was smaller than that produced by DBcAMP. - Adaptation to the diet did not affect the adipocyte content of hormone sensitive lipase protein, but induced a 17% reduction in the activity of the enzyme. Taken together, the above data allow the following inferences: - The reduced lipolytic activity in rats adapted to the HP diet does not seem to be due to an inability of the central nervous system to respond to lipolysisstimulating signals or to changes in the concentration of central neurotransmitters involved in sympathetic activation. - The main reason for the reduced basal (non-stimulated) lipolysis in adipocytes from HP-rats seems to be an impairment in the intracellular processes of lipolysis activation with a smaller proportion of the hormone sensitive lipase in the phosphorilated (active) form. ln view of recent studies in the literature demonstrating that the same lipolytic agents used in the present work increase adipocyte lipolysis with no change in HSL content or activity, the reduced response of HP-adipocytes to lipolytic agents probably resulted from an impairment of intracellular processes of lipolysis activation, which do not depend on HSL phosphorilation. The possibility can not be excluded that the HP diet did not affect the function or activity of adipocyte membrane adrenoceptors. The cause of the changes in the lipolytic activity of adipocytes induced by the HP diet remains to be determined. The hypothesis is advanced that they result from a sustained decrease in the adipose tissue sympathetic activity. (AU)