Effects of a sympathetic activation by a lower body negative pressure on glucose and lipid metabolism

Residual sympathetic tone is associated with reduced insulin sensitivity in patients with autonomic failure

PURPOSE

Parkinson disease, an α-synucleinopathy, is associated with reduced insulin sensitivity, impaired glucose tolerance, and diabetes mellitus. Importantly, these metabolic alterations have been shown to contribute to disease progression. The purpose of this study was to determine if reduced insulin sensitivity is also present in other α-synucleinopathies associated with autonomic failure.

METHODS

We studied 19 patients with multiple system atrophy and 26 patients with pure autonomic failure. For comparison, we studied 8 healthy controls matched for body mass index. Insulin sensitivity and beta cell function were calculated using fasting glucose and insulin levels according to the homeostatic model assessment 2. A multiple linear regression model was performed to determine factors that predict insulin sensitivity in autonomic failure.

RESULTS

There was a significant difference in insulin sensitivity among groups (P = 0.048). This difference was due to lower insulin sensitivity in multiple system atrophy patients: 64% [interquartile range (IQR), 43 to 117] compared to healthy controls 139% (IQR, 83 to 212), P = 0.032. The main factor that contributed to the reduced insulin sensitivity was the presence of supine hypertension and residual sympathetic tone.

CONCLUSIONS

Multiple system atrophy patients have reduced insulin sensitivity that is associated with residual sympathetic activation and supine hypertension. These patients may therefore be at high risk for development of impaired glucose tolerance and diabetes mellitus.

Relationship of muscle sympathetic nerve activity to insulin sensitivity

PURPOSE

An association between insulin resistance and activation of the sympathetic nervous system has been reported in previous studies. However, potential interactions between insulin sensitivity and sympathetic neural mechanisms in healthy people remain poorly understood. We conducted a study to determine the relationship between sympathetic activity and insulin resistance in young, healthy humans.

METHODS

Thirty-seven healthy adults (18-35 years, BMI <28 kg m(-2)) were studied. Resting muscle sympathetic nerve activity (MSNA) was measured with microneurography and insulin sensitivity of glucose and free fatty acid metabolism was measured during a hyperinsulinemic-euglycemic clamp with two levels of insulin.

RESULTS

During lower doses of insulin, we found a small association between lower insulin sensitivity and higher MSNA (P < 0.05) but age was a cofactor in this relationship. Overall, we found no difference in insulin sensitivity between groups of low and high MSNA, but when women were analyzed separately, insulin sensitivity was lower in the high MSNA group compared with the low MSNA group of women.

CONCLUSIONS

These data suggest that MSNA and insulin sensitivity are only weakly associated with young healthy individuals and that age and sex may be important modifiers of this relationship.

Dairy calcium supplementation in overweight or obese persons: its effect on markers of fat metabolism

BACKGROUND

Dairy calcium supplementation has been proposed to increase fat oxidation and to inhibit lipogenesis.

OBJECTIVE

We aimed to investigate the effects of calcium supplementation on markers of fat metabolism.

DESIGN

In a placebo-controlled, crossover experiment, 10 overweight or obese subjects who were low calcium consumers received 800 mg dairy Ca/d for 5 wk. After 4 wk, adipose tissue was taken for biopsy for analysis of gene expression. Respiratory exchange, glycerol turnover, and subcutaneous adipose tissue microdialysis were performed for 7 h after consumption of 400 mg Ca or placebo, and the ingestion of either randomized slow-release caffeine (SRC; 300 mg) or lactose (500 mg). One week later, the test was repeated with the SRC or lactose crossover.

RESULTS

Calcium supplementation increased urinary calcium excretion by 16% (P = 0.017) but did not alter plasma parathyroid hormone or osteocalcin concentrations. Resting energy expenditure (59.9 +/- 3.0 or 59.6 +/- 3.3 kcal/h), fat oxidation (58.4 +/- 2.5 or 53.8 +/- 2.2 mg/min), plasma free fatty acid concentrations (0.63 +/- 0.02 or 0.62 +/- 0.03 mmol/L), and glycerol turnover (3.63 +/- 0.41 or 3.70 +/- 0.38 micromol . kg(-1) . min(-1)) were similar with or without calcium, respectively. SRC significantly increased free fatty acid concentrations, resting fat oxidation, and resting energy expenditure. During microdialysis, epinephrine increased dialysate glycerol concentrations by 250% without and 254% with calcium. Expression of 7 key metabolic genes in subcutaneous adipose tissue was not affected by calcium supplementation.

CONCLUSION

Dairy calcium supplementation in overweight subjects with habitually low calcium intakes failed to alter fat metabolism and energy expenditure under resting conditions and during acute stimulation by caffeine or epinephrine.

Short-term administration of isotretinoin elevates plasma triglyceride concentrations without affecting insulin sensitivity in healthy humans

The mechanism underlying hypertriglyceridemia-associated insulin resistance in humans remains poorly understood. It has been proposed that hypertriglyceridemia only produces insulin resistance when associated with an increased lipid delivery to muscle. Accordingly, hypertriglyceridemia secondary to a decreased clearance of triglyceride-rich particles should not cause insulin resistance. To verify this hypothesis, we assessed whole body and adipose tissue insulin sensitivity in 15 healthy male volunteers before and after a 5-day administration of isotretinoin (1 mg/kg/d), a vitamin A derivative that decreases the clearance of triglyceride-rich particles. Whole body insulin-mediated glucose disposal (6,6 (2)H(2)glucose), glucose oxidation (indirect calorimetry), lipolysis ((2)H(5) glycerol), and subcutaneous adipose lipolysis (microdialysis) were evaluated during a 3-step hyperinsulinemic euglycemic clamp. Isotretinoin increased plasma triglyceride from 0.97 +/- 0.15 to 1.30 +/- 0.22 mmol/L (P <.02), but did not change whole body insulin-mediated glucose disposal and lipolysis. These observations are consistent with an isotretinoin-induced inhibition of very-low-density lipoprotein (VLDL)-triglyceride clearance. The suppression of endogenous glucose production and the reduction in subcutaneous adipose glycerol concentrations by insulin remained equally unaffected after isotretinoin administration. We conclude that the impaired clearance of triglyceride-rich particles secondary to a 5-day isotretinoin administration does not impair insulin-mediated antilipolysis or glucose disposal. The data support the concept that hypertriglyceridemia-associated insulin resistance develops primarily when triglyceride production is increased.

Fish oil prevents the adrenal activation elicited by mental stress in healthy men

OBJECTIVES

A diet rich in n-3 fatty acids (fish oils) is associated with reduced risks of cardiovascular and metabolic diseases, but the mechanisms remain incompletely understood. Sympathoadrenal activation is postulated to be involved in the pathogenesis of these diseases, and may be inhibited by n-3 fatty acids. We therefore evaluated the effects of a diet supplemented with n-3 fatty acids on the stimulation of the sympathetic nervous system and of stress hormones elicited by a mental stress.

METHODS

Seven human volunteers were studied on two occasions, before and after 3 weeks of supplementation with 7.2 g/day fish oil. On each occasion, the concentrations of plasma cortisol, and catecholamines, energy expenditure (indirect calorimetry), and adipose tissue lipolysis (plasma non esterified fatty acid concentrations) were monitored in basal conditions followed by a 30 min mental stress (mental arithmetics and Stroop's test) and a 30 min recovery period.

RESULTS

In control conditions, mental stress significantly increased heart rate, mean blood pressure, and energy expenditure. It increased plasma epinephrine from 60.9 +/- 6.2 to 89.3 +/- 16.1 pg/ml (p<0.05), plasma cortisol from 291 +/- 32 to 372 +/- 37 micromol/l (p<0.05) and plasma non esterified fatty acids from 409 +/- 113 to 544 +/- 89 micromol/l (p<0.05). After 3 weeks of a diet supplemented with n-3 fatty acids, the stimulation by mental stress of plasma epinephrine, cortisol, energy expenditure, and plasma non esterified fatty acids concentrations, were all significantly blunted.

CONCLUSION

Supplementation with n-3 fatty acids inhibits the adrenal activation elicited by a mental stress, presumably through effects exerted at the level of the central nervous system.

Effect of metformin on insulin sensitivity and insulin secretion in female obese patients with normal glucose tolerance

OBJECTIVES

Metformin is recognized as the treatment of chronic obese, insulin-resistant type 2 diabetic patients. Whether it improves insulin sensitivity in obese patients with normal glucose tolerance remains unknown.

METHODS

Eight obese female patients with normal glucose tolerance were studied during a double blinded, randomized cross-over study including a 2-week administration of metformin and a 2-week administration of placebo. Insulin secretion and insulin sensitivity were assessed after metformin and placebo by means of a 3-hour hyperglycemic clamp.

RESULTS

The plasma insulin and C-peptide concentrations during the hyperglycemic clamp were identical after placebo or metformin (both first and second phases). Insulin-mediated glucose disposal, stimulation of glucose oxidation and suppression of endogenous glucose production were identical after metformin and placebo.

CONCLUSIONS

Metformin does not improve insulin sensitivity nor insulin secretion in obese female patients with normal glucose tolerance.

Measurement of the whole body clearance of infused glycerol as a test of liver function after major hepatectomy

Major liver resection can be used in the treatment of liver cancer. The functional capacity of liver parenchyma needs to be evaluated preoperatively because it conditions the outcome. We assessed whether the whole body clearance of glycerol, a substrate essentially metabolized in liver cells, may be suitable as a simple test of liver function. Seven patients after major hepatectomy, six patients after colectomy and 12 healthy subjects were studied. Patients were investigated on the first day after surgery. All participants were studied during a 150-min basal period followed by a 120-min infusion of 16 mumol kg-1 min-1 13C-labelled glycerol. Whole body glycerol clearance was calculated from the change in plasma glycerol concentration. Whole body glucose production was measured with 6,6 2H2 glucose infused as a tracer in the basal state and during glycerol infusion. In addition, 13C glucose synthesis was monitored to quantitate gluconeogenesis from glycerol. Patients after liver resection had higher plasma glycerol concentrations and lower whole body glycerol clearance than healthy subjects and patients after colectomy. They also had higher plasma glucagon concentrations. Their fasting glucose production was mildly elevated in the fasting state and did not change after glycerol infusion, indicating a normal hepatic autoregulation of glucose production. These results indicate that whole body glycerol clearance can be simply determined from plasma glycerol concentrations during exogenous glycerol infusion. It is significantly reduced in patients after major hepatectomy, suggesting that it constitutes a sensitive test of hepatic function. Its use as a preoperative testing procedure remains to be evaluated.

Effects of dexamethasone on the metabolic responses to mental stress in humans

The haemodynamic effects of the sympathetic nervous system (SNS) activations elicited by hypoglycaemia, acute alcohol administration, or insulin can be prevented by a pretreatment with dexamethasone in humans. This suggests a possible role of central corticotropin releasing hormone (GRIT) release. Mental stress activates the SNS, and decreases systemic vascular resistances though a beta-adrenergic-mediated vasodilation thought to involve vascular nitric oxide release. It also increases insulin-mediated glucose disposal, an effect presumably related to vasodilation. In order to evaluate whether activation of SNS by mental stress is glucocorticoid-sensitive, we monitored the haemodynamic and metabolic effects of mental stress during hyperinsulinaemia in healthy humans with and without a 2-day treatment with 8 mg day(-1) dexamethasone. Mental stress decreased systemic vascular resistances by 21.9% and increased insulin-mediated glucose disposal by 2 8.4% without dexamethasone pretreatment. After 2 days of dexamethasone treatment, whole body insulin-mediated glucose disposal was decreased by 40.8%. The haemodynainic effects of mental stress were however, not affected. Mental stress acutely increased insulin-mediated glucose disposal by 28.0%. This indicates that mental stress elicits a stimulation of SNS through dexamethasone-insensitive pathway, distinct of those activated by insulin, alcohol, or hyperglycaemia.

Intraneural stimulation elicits an increase in subcutaneous interstitial glycerol levels in humans

1. The effect of intraneural electrical stimulation of the lateral femoral cutaneous nerve on lipolysis in the innervation territory of the stimulated nerve fascicle was studied in seven healthy women. Lipolysis was evaluated by microdialytic measurement of the interstitial glycerol concentration in subcutaneous adipose tissue. 2. Ten minutes of unilateral intraneural stimulation elicited a 22 +/- 8 % (mean +/- s.e.m.) increase in glycerol levels in the stimulated region (P < 0.05), whereas no change was registered in the corresponding area of the contralateral unstimulated leg. 3. Significantly higher glycerol levels in the stimulated vs. contralateral unstimulated region (47 +/- 13 %, P < 0.05) were already observed at baseline (30 min resting period preceding the 10 min stimulation), in all probability as a consequence of the nerve searching procedure and trial stimulations. After the 10 min stimulation, the overall glycerol increase was 72 +/- 17 % compared with the contralateral leg, illustrating the degree of lipolysis induced by the whole experimental procedure. 4. The sympathetic discharge in the lateral femoral nerve (6 recordings) showed typical characteristics of skin sympathetic activity, and the firing pattern was strikingly similar to simultaneously recorded sympathetic discharge in cutaneous nerve fascicles innervating regions without prominent subcutaneous fat stores (2 double nerve recordings). Thus, no component of cutaneous sympathetic outflow specific for the nerve innervating prominent subcutaneous fat stores could be identified. 5. Our findings suggest that sympathetic nerve fibres travelling in cutaneous nerve fascicles exert a regulatory influence on subcutaneous fat tissue in humans. The combination of intraneural recording/stimulation and subcutaneous microdialysis provides a model for evaluating neural control of human fat metabolism.