Centrally injected atropine reduces hyperglycemia caused by 2-DG or immobilization stress in awake rats.

The physiological significance of central cholinergic neurons was investigated by verifying the effect of previous intracerebroventricular administration of atropine on the hyperglycemia induced by 2-deoxyglucose (2-DG) or by immobilization stress in unrestrained, nonanesthetized rats. Intravenous 2-DG induced a marked increase in plasma glucose that was not affected by atropine injected intracerebroventricularly 30 min before. However, the hyperglycemia induced by intracerebroventricular 2-DG was significantly reduced by previous intracerebroventricular injection of atropine. Immobilization induced a rapid increase of plasma glucose levels that was reduced by about 50% by intracerebroventricular injection of atropine. The increase in plasma lactate induced by intravenous 2-DG, or immobilization, was not significantly affected by previous intracerebroventricular injection of atropine. The data suggest that central cholinergic neurons participate in the complex neural events responsible for the hyperglycemic response to neurocytoglucopenia and to stressful situations.

Brown adipose tissue triacylglycerol synthesis in rats adapted to a high-protein, carbohydrate-free diet.

Adaptation of rats to a high-protein, carbohydrate-free (HP) diet induced a marked reduction of brown adipose tissue (BAT) fatty acid (FA) synthesis from both 3H2O and [14C]glucose in vivo, with pronounced decreases in the activities of four enzymes associated with lipogenesis: glucose-6-phosphate dehydrogenase, malic enzyme, citrate lyase, and acetyl-CoA carboxylase. In both HP-adapted and control rats, in vivo incorporation of 3H2O and [14C]glucose into BAT glyceride-glycerol was much higher than into FA. It could be estimated that most of the glycerol synthetized was used to esterify preformed FA. Glycerol synthesis from nonglucose sources (glyceroneogenesis) was increased in BAT from HP rats, as evidenced by an increased capacity of tissue fragments to incorporate [1-14C]pyruvate into glycerol and by a fourfold increase in the activity of phosphoenolpyruvate carboxykinase activity, a key glyceroneogenic enzyme. The data suggest that high rates of glyceroneogenesis and of esterification of preformed FA in BAT from HP-adapted rats are essential for preservation of tissue lipid stores, necessary for heat generation when BAT is recruited in nonshivering thermogenesis.

Glucose contribution to in vivo synthesis of glyceride-glycerol and fatty acids in rats adapted to a high-protein, carbohydrate-free diet.

Triacylglycerol (TAG) synthesis from all carbon sources and from glucose carbon was evaluated in rats fed a high-protein, carbohydrate-free (HP) diet or control diet by determining simultaneously in the same animal the rate of incorporation of 3H2O and of 14C-glucose into the two TAG moieties in the carcass, liver, and retroperitoneal and epididymal adipose tissue. Incorporation rates of 3H2O into TAG-fatty acids (FAs) in the two adipose tissues and in liver were reduced in HP rats to about 20% and 50%, respectively, of the rates in control rats. In the two experimental groups, glucose was a poor precursor for FA synthesis, contributing only 22.8% of whole-body (carcass plus liver) total FA synthesis in control rats and even less (14%) in HP rats. In contrast to the reduction in FA synthesis, incorporation of 3H2O into TAG-glycerol in HP rats did not differ significantly or was even higher (in epididymal tissue) versus the control level. In all tissues of both HP and control rats, the rate of 14C-glucose incorporation into TAG-glycerol was much higher than the rate of incorporation into FA. Glyceroneogenesis, estimated by subtracting TAG-glycerol synthesis from glucose from the rate obtained with 3H2O, was significantly increased in adipose tissue from HP rats, with almost all of the glycerol formed by this route being used to esterify preformed FAs. It is suggested that the increased adipose tissue glyceroneogenesis is important for esterification of diet-derived FA and preservation of body fat stores in rats adapted to the HP diet.

Sympathetic activity in brown adipose tissue from rats adapted to a high protein, carbohydrate-free diet.

Previous studies have shown that adaptation of rats to a high protein, carbohydrate-free (HP) diet results in a marked reduction in brown adipose tissue (BAT) thermogenic capacity. The present experiments were carried out to assess BAT sympathetic activity in HP diet-adapted rats. It was found that interscapular brown adipose tissue (IBAT) norepinephrine (NE) content, fractional turnover rate and calculated rate of turnover were markedly reduced in HP-fed rats. Replacement of the HP diet by a control, balanced diet for 24 h did not affect BAT NE content significantly, but restored fractional turnover rate and turnover rate values to those of control animals. Exposure to cold (4 degrees) for a short period (8 h) induced a marked increase in IBAT NE fractional turnover rate and calculated turnover rate in both HP and control rats. The cold-induced rise of turnover rate over values at ambient temperature (25 degrees C) was higher in HP rats than in controls. Rectal temperature after 8 h of cold exposure did not differ in HP and control rats. The data suggest that the decreased thermogenic capacity of rats adapted to a high protein, carbohydrate-free diet is due to a reduced sympathetic outflow to BAT, which can be rapidly reactivated by cold stimulation.

Intra-ventromedial hypothalamic injection of cholinergic agents induces rapid hyperglycemia, hyperlactatemia and gluconeogenesis activation in fed, conscious rats.

Microinjections of carbachol (50 nmol) or neostigmine (25 nmol) in 0.5 ml saline into the ventromedial nucleus of the hypothalamus (VMH) (but not into the lateral hypothalamic area) of fed, conscious rats produced marked increases in plasma glucose and lactate, which were suppressed or markedly reduced by previous adrenodemedullation. The rate of incorporation of 14C from infused bicarbonate (0.60 microliter, 0.20 microCi/min), an index of gluconeogenic activity, increased significantly after VMH administration of neostigmine. The data suggest that cholinergic synapses in the VMH participate of a central glucoregulatory system that increases hepatic glucose production mainly through a stimulation of adrenal medulla epinephrine secretion.

Thermogenic capacity of brown adipose tissue is reduced in rats fed a high protein, carbohydrate-free diet.

The functional state of interscapular brown adipose tissue (IBAT) was examined in rats fed for 20-30 d a high protein, carbohydrate-free diet [70% (wt/wt) protein, 8% fat] or a balanced diet (66% carbohydrate, 17% protein, 8% fat). In rats fed the high protein diet, body weight did not differ from that of control rats, but relative IBAT weight (grams per 100 g body wt) and lipid concentration (per gram of tissue) were 37% and 14% lower, respectively. In vivo rates of lipogenesis in IBAT, epididymal and retroperitoneal adipose tissue of rats fed the high protein diet were 20, 30 and 40%, respectively, of control values. Mitochondrial protein and cytochrome oxidase activity per total IBAT were significantly lower in rats fed the high protein diet than in controls; GDP binding was lower even when expressed per total tissue or per milligram of mitochondrial protein. The increase of IBAT temperature following norepinephrine infusion was significantly smaller than in controls. It is suggested that the decrease in IBAT capacity in the rats fed the high protein diet was due, at least in part, to a sustained reduction of sympathetic activity.