Catecholamine regulation of local lactate production in vivo in skeletal muscle and adipose tissue: role of -adrenoreceptor subtypes.

CONTEXT: The regulation of lactate production in skeletal muscle (SM) and adipose tissue (AT) is not fully elucidated. OBJECTIVE: Our objective was to investigate the catecholamine-mediated regulation of lactate production and blood flow in SM and AT in healthy, normal-weight subjects by using microdialysis. METHODS: First, lactate levels in SM and AT were measured during an iv norepinephrine infusion (n = 11). Local blood flow was determined with the 133Xe-clearance technique. Second, muscle lactate was measured during hypoglycemia and endogenous epinephrine stimulation (n = 12). Third, SM was perfused with selective beta(1-3)-adrenoreceptor agonists in situ (n = 8). Local blood flow was measured with the ethanol perfusion technique. RESULTS: In response to iv norepinephrine, the fractional release of lactate (difference between tissue and arterial lactate) increased by 40% in SM (P = 0.001), whereas remaining unchanged in AT. Blood flow decreased by 40% in SM (P < 0.005) and increased by 50% in AT (P < 0.05). In response to hypoglycemia, epinephrine increased 10-fold, and the fractional release of lactate in SM doubled (P < 0.0001). The blood flow remained unchanged. The beta2-agonist, terbutaline, caused a marked concentration-dependent increase of muscle lactate and blood flow (P < 0.0001). The beta(1)-agonist, dobutamine, induced a discrete increase of muscle lactate (P < 0.0001), and the blood flow remained unchanged. The beta3-agonist, CPG 12177, did not affect muscle lactate or blood flow. CONCLUSIONS: Catecholamines stimulate lactate production in SM, but not in AT. In SM, the beta2-adrenoreceptor is the most important beta-adrenergic receptor subtype in the regulation of lactate production.

Human skeletal muscle lipolysis is more responsive to epinephrine than to norepinephrine stimulation in vivo.

CONTEXT: Triglyceride (TG) deposits in skeletal muscle (SM) are an important energy reservoir, and increased im TG content is associated with muscle insulin resistance. OBJECTIVE: The objective of the study was to investigate the effect of endogenous catecholamines on TG lipolysis in human SM in vivo. Adipose tissue (AT) was studied for comparison. DESIGN AND MAIN OUTCOME MEASURES: Glycerol levels (index of lipolysis) were measured using microdialysis in the gastrocnemius muscle and abdominal sc adipose tissue during a hyperinsulinemic, hypoglycemic clamp (n = 13) and in response to in situ perfusion of epinephrine and norepinephrine (10(-10) to 10(-5) m) (n = 12). Local tissue blood flow was monitored with the ethanol perfusion technique. SETTING: This was an experimental study. PARTICIPANTS: The study population consisted of healthy subjects. RESULTS: Plasma epinephrine increased 10-fold and plasma norepinephrine 2-fold in response to insulin-induced hypoglycemia. In parallel, the fractional glycerol release (difference between tissue and arterial glycerol) increased 2-fold in both tissues (P < 0.0001). No changes in AT and SM blood flow were registered. When the catecholamines were perfused in situ, tissue glycerol increased significantly at 10(-7) m of either epinephrine and norepinephrine (P < 0.0001) in AT. The maximum stimulation was seen at 10(-6) m norepinephrine (2-fold increase) and 10(-5) m epinephrine (3-fold increase). In SM, tissue glycerol increased at 10(-7) m epinephrine and 10(-6) m norepinephrine, respectively (P < 0.0001); the maximum increase of glycerol values (at 10(-6) m) was 2.5 times for epinephrine and 1.6 times for norepinephrine, respectively (P < 0.01). CONCLUSIONS: The lipolytic activity of SM is increased by endogenous catecholamines in vivo and appears to be more responsive to epinephrine than norepinephrine stimulation.

Marked reutilization of free fatty acids during activated lipolysis in human skeletal muscle.

Release of glycerol and free fatty acids (FFA) was investigated in human skeletal muscle strips. In the basal state, glycerol and FFA were released at almost equimolar rates (0.3 nmol/ng tissue.90 min). A nonselective beta-adrenoceptor agonist, isoprenaline, caused a concentration-dependent stimulation of glycerol release, whereas FFA release was unaffected. Basal and isoprenaline-induced glycerol release correlated positively with the age of the donors (r = 0.5, P < 0.005) but not with their body mass index (P > or = 0.4). Biochemical experiments with hormone-sensitive lipase (HSL) showed that most enzyme activity was both in the cytosol and mitochondrial fraction and that it constituted the common long and active form of the protein. Electron microscopy studies in rat skeletal muscle using labeled highly specific HSL antibodies verified the cytosolic location of HSL and, furthermore, indicated an accumulation of HSL-adjoining mitochondria. These results suggest that FFA produced in myocytes during catecholamine-induced lipolysis are retained by the muscle and, therefore by inference, reused. It is conceivable that efficient hydrolysis of acylglycerol by HSL located in the cytosol as well as near the mitochondria may facilitate mitochondrial FFA oxidation. In addition, muscle lipolysis activity increases during aging and may be independent of total body fat.

Combined hyperinsulinemia and hyperglycemia, but not hyperinsulinemia alone, suppress human skeletal muscle lipolytic activity in vivo.

Effects of circulating insulin and glucose concentrations on skeletal muscle and adipose tissue lipolytic activity were investigated in 10 type 1 diabetes patients with no endogenous insulin secretion. Microdialysis measurements of interstitial glycerol and determination of fractional glycerol release were carried out during standardized combinations of relative hypoinsulinemia/moderate hyperglycemia (11 mmol/liter), hyperinsulinemia/ normoglycemia (5 mmol/liter), and hyperinsulinemia/moderate hyperglycemia, respectively. Local tissue blood flow rates were measured with the (133)Xe clearance technique. In response to the change from hypo- to hyperinsulinemia, the fractional release of glycerol decreased from 159.6 +/- 17.8 to 85.1 +/- 13.7 micromol/liter (P < 0.0001) in adipose tissue, whereas it remained unchanged in skeletal muscle (44.6 +/- 6.4 vs. 36.0 +/- 7.4 micromol/liter; not significant). When hyperinsulinemia was combined with hyperglycemia, fractional glycerol release was further reduced in adipose tissue (64.5 +/- 12.2 micromol/liter; P < 0.05), and in this situation it was also markedly decreased in skeletal muscle (18.1 +/- 4.8 micromol/liter; P < 0.0001). Skeletal muscle blood flow was unaltered over the respective study periods. Adipose tissue blood flow decreased by 50% in response to hyperinsulinemia (P < 0.0005), but no further change was seen when hyperinsulinemia was combined with hyperglycemia. It is concluded that in patients with type 1 diabetes, insulin does not exert an antilipolytic effect in skeletal muscle during normoglycemia. However, in response to combined hyperinsulinemia and hyperglycemia, the lipolytic activity in skeletal muscle is restrained in a similar way as in adipose tissue. This may be explained by a glucose-mediated potentiation of the antilipolytic effectiveness of insulin.

Defective activation of skeletal muscle and adipose tissue lipolysis in type 1 diabetes mellitus during hypoglycemia.

The increased risk of hypoglycemia during intensified treatment of type 1 diabetes mellitus (T1DM) patients, who have a deficient glucagon secretory response, is largely attributed to the development of suppressed adrenomedullary responses. A consequence of this impairment of catecholamine secretion might be reduced lipolysis in major target tissues (muscle and adipose) and, in turn, increased glucose metabolism. To test this hypothesis, we used microdialysis to monitor glycerol (index of lipolysis) in the extracellular fluid of skeletal muscle and adipose tissue and assessed whole-body glucose use by measuring [6,6-(2)H(2)]glucose enrichment in plasma in seven intensively treated T1DM patients and eight nondiabetic subjects who received a 3-h insulin infusion (0.8 mU/kg.min) on two occasions: during mild-moderate hypoglycemia or euglycemia. In the hypoglycemic study, the rise in plasma epinephrine was approximately 50% less in the T1DM patients despite a greater fall in plasma glucose (to 3.0 vs. 3.5 mM in controls; P < 0.05). Moreover, the rate of glucose flux and the plasma-extracellular fluid glucose gradient in muscle was increased during hypoglycemia in T1DM subjects compared with controls. Glycerol levels in muscle, adipose, and plasma fell similarly in both groups in the first hour. Thereafter, tissue glycerol remained suppressed in the T1DM patients but rebounded significantly (P < 0.01) in the control subjects. The glycerol response in muscle and adipose tissue was significantly correlated with plasma epinephrine concentration (r = 0.73, P = 0.002; and r = 0.52, P = 0.04, respectively), and inversely correlated with whole-body glucose disposal (r = -0.51, P = 0.05; and r = -0.50, P = 0.05). To determine whether the absence of the lipolytic response is limited to deficient catecholamine release, we perfused muscle and adipose tissue in situ with the selective beta(2)-agonist terbutaline during hyperinsulinemic euglycemia. Local addition of agonist increased glycerol and blood flow in both muscle and adipose (P < 0.01 and P < 0.05, respectively) similarly in T1DM and control subjects. We conclude that deficient release of (rather than impaired responsiveness to) catecholamines in T1DM prevents the local fat breakdown within muscle and adipose tissue that normally occurs during mild-moderate hypoglycemia. This defect within peripheral tissues may lead to a delayed increase in glucose disposal that could contribute to the severity of hypoglycemia when it is prolonged.

Marked heterogeneity of human skeletal muscle lipolysis at rest.

In this study, variations in lipolysis among different muscle groups were examined by measuring local net glycerol release in vivo in healthy, normal-weight subjects (n = 11) during rested, postabsorptive conditions. Microdialysis of the gastrocnemius, deltoid, and vastus lateralis muscle regions revealed that extracellular glycerol concentrations of these three muscle regions were 84.7 +/- 6.7, 59.7 + 7.3, and 56.4 +/- 7.5 micro mol/l, respectively, and the arterial plasma glycerol concentration was 44.8 +/- 2.3 micro mol/l (P = 0.0003-0.006, gastrocnemius vs. others). Local tissue blood flow, as measured by Xe clearance, did not differ among the regions. Net glycerol release was significantly higher in gastrocnemius muscle than in the two other regions. There were no regional differences in glycerol uptake when studied during glycerol infusion. Gastrocnemius muscle showed a dominance of type 1 fibers (70%), whereas the vastus lateralis muscle had equal distribution of fiber types (P = 0.02). No differences in intramuscular triaclyceride content, perimuscular fat, or the adipocyte-specific protein perilipin were observed among the muscle regions. Triglyceride turnover in the gastrocnemius muscle was 3.3 + 1.4% over 24 h, which is about 10 times more rapid than the turnover rate in subcutaneous adipose tissue (P < 0.01). Thus there were marked differences in lipolytic activity among skeletal muscle groups at rest, possibly reflecting variations in fiber type.

Assessment of skeletal muscle triglyceride content by (1)H nuclear magnetic resonance spectroscopy in lean and obese adolescents: relationships to insulin sensitivity, total body fat, and central adiposity.

The metabolism and composition of skeletal muscle tissue is of special interest because it is a primary site of insulin action and plays a key role in the pathogenesis of insulin resistance. Intramyocellular (IMCL) triglyceride stores are an accessible form of energy that may decrease skeletal muscle glucose utilization, thereby contributing to impaired glucose metabolism. Because of the invasive nature of muscle biopsies, there is limited, if any, information about intramuscular lipid stores in children. The development of (1)H nuclear magnetic resonance (NMR) spectroscopy provides a unique noninvasive alternative method that differentiates intracellular fat from intercellular fat in muscle tissue. The present study was performed to determine whether IMCL and extramyocellular (EMCL) lipid contents are increased early in the development of juvenile obesity and to explore the relationships between IMCL and EMCL to in vivo insulin sensitivity, independently of total body fat and central adiposity in obese and nonobese adolescents. Eight nonobese (BMI 21 kg/m(2), age 11-16 years) and 14 obese (BMI 35 +/- 1.5 kg/m(2), age 11-15 years) adolescents underwent 1) (1)H-NMR spectroscopy to noninvasively quantify IMCL and EMCL triglyceride content of the soleus muscle, 2) a 2-h euglycemic-hyperinsulinemic clamp (40 mU.m(-2).min(-1)) to assess insulin sensitivity, 3) a dual-energy X-ray absorptiometry scan to measure total percent body fat, and 4) magnetic resonance imaging to measure abdominal fat distribution. Both the IMCL and EMCL content of the soleus muscle were significantly greater in the obese adolescents than in the lean control subjects. A strong inverse correlation was found between IMCL and insulin sensitivity, which persisted and became even stronger after controlling for percent total body fat and abdominal subcutaneous fat mass (partial correlation r = -0.73, P < 0.01) but not when adjusting for visceral fat (r = - 0.54, P < 0.08). In obese adolescents, increase in total body fat and central adiposity were accompanied by higher IMCL and EMCL lipid stores. The striking relationships between both IMCL and EMCL with insulin sensitivity in childhood suggest that these findings are not a consequence of aging but occur early in the natural course of obesity.