Effect of Aging on Norepinephrine and Phenylephrine Stimulated Lactate Production by White Adipocytes

We have recently demonstrated that adipose tissue can produce lactate independently of lipolysis in insulin-resistant rats and that lactate production depends on aj-adrenergic stimulation. In this study, we have investigated the influence of aging on norepinephrine-and-phenylephrine-stimulated lactate production and glycerol production. We showed that basal and norepinephrine stimulated lactate production were significantly increased in adipocytes isolated from old vs. young rats (0.165 ± 0.006 vs. 0.055 ± 0.008 for basal and 0.576 ± 0.026 vs. 0.277 ± 0.019 umol lactate/10⁶ cell/15 minutes for norepinephrine-stimulated lactate production, respectively, p<0.05). The sensitivity of lactate production to norepinephrine stimulation in adipocytes isolated from old rats was significantly decreased (EC₅₀=523 ± 63.7 vs. 46.7 ± 6.34 nM, respectively, p><0.05). Maximal lactate production obtained with norepinephrine and phenylephrine was not significantly different in either group (0.576 ± 0.026 vs. 0.520 ± 0.036 in old and 0.277 ± 0.019 vs. 0.275 ± 0.017 umol/10⁶ cell/15 minutes in young rats, respectively, ns). Lactate production by adipocytes isolated from old rats were significantly less sensitive to phenylephrine stimulation compared with young (EC₅₀=3.67 ± 1.16 vs. 0.07 ± 0.01 nM, respectively, p<0.05) indicating that the effects of aging on norepinephrine and phenylephrine stimulation were probably induced by a decreased number of α₁-adrenoceptors. The mechanism by which aging increases adipocyte responsiveness of lactate production has not yet been elucidated.