Effects of the Human Immunodeficiency Virus-Protease Inhibitor, Ritonavir, on Basal and Catecholamine-Stimulated Lipolysis

Several of the aspartic acid protease inhibitors used to treat HIV infection increase basal lipolysis in adipocytes, but the cellular mechanisms leading to this augmentation are not well understood. We therefore studied the effects of chronic exposure to the HIV protease inhibitor, ritonavir, on the lipolytic cascade in 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with ritonavir for 14 d (during and after differentiation) enhanced basal, isoproterenol (Iso)-stimulated, and cAMP analog-stimulated lipolysis. Enhancement of lipolysis was observed after Iso at concentrations between 0.1 and 10 μm. Despite a significant decrease in cyclic nucleotide phosphodiesterase (PDE)3B activity and protein levels, there were no changes in Iso-stimulated intracellular cAMP, protein kinase A (PKA) expression, or PKA activity. Ritonavir-augmented lipolysis was also observed under conditions that reversed the effect on PDE3B activity via preincubation with 1 μm (-)-N6-(2-phenylisopropyl)adenosine. In ritonavir-treated cells, protein expression of the lipid droplet-protective protein, perilipin, was significantly decreased, whereas there was no change in hormone-sensitive lipase. Activation of ERK1/2 by Iso did not play a role in the augmentation. We conclude that ritonavir decreases PDE3B and perilipin protein expression and affects both basal and catecholamine-stimulated lipolysis in 3T3-L1 adipocytes primarily through actions at sites downstream of PKA.

Myocardial adenosine A1-receptor sensitivity during juvenile and adult stages of maturation

In the heart, endogenous adenosine attenuates the β-adrenergic-elicited increase in contractile performance via activation of adenosine A1 receptors. It has been recently reported that this function of adenosine becomes more pronounced with myocardial maturation. The purpose of the present study was to determine whether mature hearts possess a greater sensitivity than immature hearts to this antiadrenergic effect of adenosine. Isolated perfused hearts or atria from immature (ca. 23 days) and mature (ca. 80 days) rats were stimulated with isoproterenol (Iso), a β-adrenergic agonist, at 10−8 M and concomitantly exposed to increasing concentrations of 2-chloroN6-cyclopentyladenosine (CCPA), a highly selective and potent adenosine A1-receptor agonist, from 10−12 to 10−6 M. CCPA at 10−10–10−6M dose dependently reduced the Iso-elicited contractile response more in immature than in mature hearts or atria. At 10−6 M, CCPA reduced the Iso-elicited contractile response by 103% in immature hearts and by 55% in mature hearts. These effects of CCPA were attenuated by the adenosine A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine at 10−7 M. In additional experiments, CCPA exhibited similar effectiveness in reducing the spontaneous heart rate of immature and mature hearts, an effect also mediated by activation of adenosine A1 receptors. Similar to CCPA, the adenosine A1-receptor agonist R- N 6-(2-phenylisopropyl)adenosine reduced the Iso-elicited contractile response more in immature than in mature hearts, albeit with less effectiveness than CCPA. In agreement with these results, CCPA reduced Iso-elicited adenylyl cyclase activity more in immature than in mature hearts. Overall, in contrast with our original hypothesis, these results indicate that immature hearts display greater sensitivity than mature hearts to the antiadrenergic effect of adenosine A1-receptor activation.

Adenylyl cyclase inhibitory pathway is differentially modified in rat white and brown fat by high-energy diets

Incubation of white adipose tissue (WAT) adipocytes from rats fed a high-energy diet (Exp group) with antilipolytic Gi-coupled adenylyl cyclase inhibitory agonists, nicotinic acid (Nic) and N8-(L-2-phenylisopropyl)adenosine (PIA), resulted in lower cellular adenosine 3',5'-cyclic monophosphate (cAMP) levels than in stimulated adipocytes from rats fed a nutritionally balanced diet (Con group). In contrast to WAT, incubation of brown adipose tissue (BAT) adipocytes with Nic yielded higher cAMP levels in the Exp vs. Con rats. In both WAT and BAT adipocytes, pertussis toxin treatment abolished the differences in Nic- and PIA-inhibited cAMP formation between Exp and Con animals. Immunoblotting of adipocyte membranes indicated a lower content of Gi alpha but not Gs alpha in BAT membranes of Exp vs. Con animals after 6 and 10 wk of feeding. No such differences were found in the Gs alpha or Gi alpha contents of WAT membranes. Thus the inhibitory pathway of adenylyl cyclase is proposed to be sensitized in WAT and desensitized in BAT of rats fed high-energy diets. These modifications in sensitivity are in line with reduced cAMP and lipolysis in WAT and increased cAMP and thermogenesis in BAT during obesity.

Enhancement of glucose uptake in stunned myocardium: role of glucose transporter

This study quantifies the myocardial glucose uptake and clarifies the pathway of augmented glucose uptake in myocardium reperfused after a brief period of ischemia (stunned myocardium). The glucose uptake rate was determined from the time course of the sugar phosphate (SP) resonance in rat myocardium (d[SP]/dt) with 31P nuclear magnetic resonance after the substitution of glucose with its analog 2-deoxyglucose. The d[SP]/dt in stunned myocardium [1.03 +/- 0.05 (SE) micromol x g wet wt(-1) x min(-1); n = 8] increased significantly compared with nonischemic control myocardium (0.18 +/- 0.03 micromol x g wet wt(-1) x min(-1); n = 8; P < 0.0001), reaching the maximal stimulatory uptake rate during exposure to insulin (1.05 +/- 0.04 micromol x g wet wt(-1) x min(-1); n = 8). Twenty minutes after reperfusion, the d[SP]/dt was still augmented (0.41 +/- 0.05 micromol x g wet wt(-1) x min(-1); n = 5; P < 0.05 vs. control myocardium). To elucidate further the mechanism of augmented glucose uptake, N6-(L-2-phenylisopropyl)-adenosine (PIA; 100 micromol/l), a potent blocker of the glucose transporter, was administered to stunned hearts and, as a control, to insulin-stimulated hearts. PIA significantly and comparably inhibited the increase in d[SP]/dt in stunned myocardium (0.36 +/- 0.07 micromol x g wet wt(-1) x min(-1); n = 4; P < 0.0001 vs. without PIA) and in insulin-stimulated myocardium (0.38 +/- 0.02 micromol x g wet wt(-1) x min(-1); n = 4; P < 0.0001 vs. without PIA). These results indicate that the augmented glucose uptake in stunned myocardium is maintained by the glucose transporter, the amount of which is almost equal to that which can be maximally recruited by insulin.