The Acute Effects of Low-Dose TNF-αon Glucose Metabolism andβ-Cell Function in Humans

Type 2 diabetes is characterized by increased insulin resistance and impaired insulin secretion. Type 2 diabetes is also associated with low-grade inflammation and increased levels of proinflammatory cytokines such as TNF-α. TNF-αhas been shown to impair peripheral insulin signalingin vitroandin vivo. However, it is unclear whether TNF-αmay also affect endogenous glucose production (EGP) during fasting and glucose-stimulated insulin secretion (GSIS)in vivo. We hypothesized that low-dose TNF-αwould increase EGP and attenuate GSIS. Recombinant human TNF-αor placebo was infused in healthy, nondiabetic young men (n=10) during a 4-hour basal period followed by an intravenous glucose tolerance test (IVGTT). TNF-αlowered insulin levels by 12% during the basal period (P<0.05). In response to the IVGTT, insulin levels increased markedly in both trials, but there was no difference between trials. Compared to placebo, TNF-αdid not affect EGP during the basal period. Our results indicate that TNF-αacutely lowers basal plasma insulin levels but does not impair GSIS. The mechanisms behind this are unknown but we suggest that it may be due to TNF-αincreasing clearance of insulin from plasma without impairing beta-cell function or hepatic insulin sensitivity.

Acute peripheral tissue effects of ghrelin on interstitial levels of glucose, glycerol, and lactate: a microdialysis study in healthy human subjects

Ghrelin is a gut-derived peptide and an endogenous ligand for the ghrelin receptor. Intravenous infusion of ghrelin induces insulin resistance and hyperglycemia and increases circulating levels of nonesterified free fatty acids. Our objective was to investigate whether the metabolic effects are mediated directly by ghrelin in skeletal muscle and adipose (peripheral and central) tissues. Ten healthy men (24.9 ± 1.3 yr) received 300 min of supraphysiological ghrelin administration by microdialysis catheters in skeletal muscle and adipose tissues in a randomized, single-blind, and placebo-controlled study. Microdialysis perfusates were analyzed every 30 min for glucose, glycerol, and lactate during both a basal period and a hyperinsulinemic euglycemic clamp. The primary outcome measures were interstitial concentrations of glucose, glycerol, and lactate in skeletal muscle and adipose tissues. Interstitial concentrations of glucose were similar in skeletal muscle, peripheral, and central adipose tissue in the basal period. During hyperinsulinemia, interstitial concentrations of glucose in skeletal muscle decreased in response to ghrelin exposure [2.84 ± 0.25 (ghrelin) vs. 3.06 ± 0.26 mmol/l (placebo), P = 0.04]. Ghrelin exposure did not impact on interstitial concentrations of glycerol and lactate. We conclude that ghrelin administration into skeletal muscle decreases interstitial concentrations of glucose during euglycemic hyperinsulinemia, which is indicative of increased insulin sensitivity without any effects on interstitial glycerol levels in either muscle or adipose tissue. These data contrast with the metabolic effects of ghrelin observed after systemic exposure and suggest the existence of a second messenger that remains to be identified.

Effects of 11β-hydroxysteroid dehydrogenase-1 inhibition on hepatic glycogenolysis and gluconeogenesis

The aim of this study was to determine the effect of prolonged 11β-hydroxysteroid dehydrogenase-1 (11β-HSD1) inhibition on basal and hormone-stimulated glucose metabolism in fasted conscious dogs. For 7 days prior to study, either an 11β-HSD1 inhibitor (HSD1-I; n = 6) or placebo (PBO; n = 6) was administered. After the basal period, a 4-h metabolic challenge followed, where glucagon (3×-basal), epinephrine (5×-basal), and insulin (2×-basal) concentrations were increased. Hepatic glucose fluxes did not differ between groups during the basal period. In response to the metabolic challenge, hepatic glucose production was stimulated in PBO, resulting in hyperglycemia such that exogenous glucose was required in HSD-I ( P < 0.05) to match the glycemia between groups. Net hepatic glucose output and endogenous glucose production were decreased by 11β-HSD1 inhibition ( P < 0.05) due to a reduction in net hepatic glycogenolysis ( P < 0.05), with no effect on gluconeogenic flux compared with PBO. In addition, glucose utilization ( P < 0.05) and the suppression of lipolysis were increased ( P < 0.05) in HSD-I compared with PBO. These data suggest that inhibition of 11β-HSD1 may be of therapeutic value in the treatment of diseases characterized by insulin resistance and excessive hepatic glucose production.

Impact of Growth Hormone Receptor Blockade on Substrate Metabolism during Fasting in Healthy Subjects

Context: Experimental studies in GH-deficient patients and in healthy subjects receiving somatostatin-infusion suggest that GH is an important regulator of substrate metabolism during fasting. These models may not adequately reflect the selective effects of GH, and GH receptor (GHR) blockade offers a new model to define the metabolic role of GH. Objective: The aim of this study was to investigate the impact of GHR blockade on substrate metabolism and insulin sensitivity during fasting. Design: We conducted a randomized, placebo-controlled, crossover study in 10 healthy young men. Intervention: After 36 h of fasting with saline or pegvisomant (GHR blockade), the subjects were studied during a 4-h basal period and 2.5-h hyperinsulinemic euglycemic clamp. Main Outcome: We measured whole-body and forearm glucose, lipid, and protein metabolism, peripheral insulin sensitivity, and acyl and desacyl ghrelin. Results: GHR blockade significantly suppressed circulating free fatty acids (1226 ± 83 vs. 1074 ± 65 μmol/liter; P = 0.03) and ketone bodies (3080 ± 271 vs. 2015 ± 235 μmol/liter; P ≤ 0.01), as well as forearm uptake of free fatty acids (0.341 ± 0.150 vs. 0.004 ± 0.119 μmol/100 ml · min; P &lt; 0.01) and lipid oxidation (1.3 ± 0.1 vs. 1.2 ± 0.1 mg/kg · min; P = 0.03) in the basal period. By contrast, IGF-I levels in either serum or peripheral tissues were not impacted by GHR blockade, and protein metabolism was also unaffected. Basal glucose levels were elevated by GHR blockade, but insulin sensitivity was similar; this was associated with an increased acyl/desacyl ghrelin ratio. Conclusion: GHR blockade, without changes in circulating or tissue IGF-I levels, selectively suppresses lipid mobilization and oxidation after short-term fasting. This supports the notion that stimulation of lipolysis is a primary and important effect of GH. GH receptor blockade during fasting in healthy subjects suppresses lipid metabolism without a change in insulin sensitivity or protein metabolism.

Prenatal programming of hypernatremia and hypertension in neonatal lambs

Maternal water restriction and the accompanying dehydration-induced anorexia may induce long-term physiological changes in offspring. We determined the impact of prenatal hypertonicity (Pre-Dehy) on offspring cardiovascular and osmoregulatory function. Pre-Dehy lambs were exposed to in utero hypernatremia (8- to 10-meq increase; 110–150 days of gestation) induced by maternal water restriction. Control lambs were born to ewes provided ad libitum water and food throughout gestation. After delivery, all ewes were provided ad libitum water and all newborns were allowed ad libitum nursing. Lambs were prepared with vascular and bladder catheters at 15 ± 2 days of age and studied at 21 ± 2 days. After a 2-h basal period, lambs received an infusion of hypotonic (0.075 M) NaCl (0.15 ml·kg−1·h−1 iv) for 2 h. Lamb arterial blood pressure was monitored, and blood samples were obtained before, during, and after infusion. During the neonatal basal period, Pre-Dehy lambs had significantly increased plasma osmolality (302 ± 1 vs. 294 ± 1 mosmol/kgH2O, P < 0.01), sodium levels (144 ± 1 vs. 140 ± 1 meq/l, P < 0.01), hematocrit (28 ± 1% vs. 25 ± 1%, P < 0.05), and mean arterial blood pressure (79 ± 2 vs. 68 ± 1 mmHg, P < 0.001) compared with control lambs. Despite the infusion of hypotonic saline, Pre-Dehy lambs maintained relative hypertonicity, hypernatremia, and hypertension. However, plasma arginine vasopressin, glomerular filtration rate, and urinary osmolar and sodium excretion and clearance (per kg body wt) were similar in the groups. Offspring of prenatally water-restricted ewes exhibit hypernatremia, hypertonicity, and hypertension, which persist despite hypotonic saline infusion. In utero hypertonicity and perhaps maternal nutrient stress may program offspring osmoregulation and systemic arterial hypertension.

P-Glycoprotein-Mediated Drug Secretion in Mouse Proximal Tubule Perfused In Vitro

To examine the functional significance of durg-transporting P-glycoprotein (P-gp), studies were conducted in the isolated and perfused proximal tubule S2 segment from mice disrupting both mdr1a and mdr1b genes [mdr1a/1b(—)(—)] and their wild-type mice. Efflux of the intracellular fluorescence of rhodamine 123, a fluorescence substrate of P-gp, into the lumen was measured, and the decay half-time of the intracellular fluorescence (T1/2) as an index of the drug-transporting P-gp activity was regarded. In the wild-type mice, the T1/2 was 34 ± 4 s (n = 36) at the basal period and was increased to 434 ± 41 s by the addition of luminal verapamil, a P-gp inhibitor. In the mdr1a/1b(—)(—) mice, the T1/2 was 407 ± 16 s (n = 10) at the basal period and was no longer affected by the luminal addition of verapamil. The digoxin content in the kidney after a repeated administration of the drug was markedly elevated in the mdr1a/1b(—)(—) mice. In conclusion, P-gp—mediated drug efflux capacity indeed exists in the apical membrane of proximal tubule cells from the wild-type mice, whereas it is absent in that of the mdr1a/1b(—)(—) mice.

Role of a negative arterial-portal venous glucose gradient in the postexercise state

Prior exercise stimulates muscle and liver glucose uptake. A negative arterial-portal venous glucose gradient (a-pv grad) stimulates resting net hepatic glucose uptake (NHGU) but reduces muscle glucose uptake. This study investigates the effects of a negative a-pv grad during glucose administration after exercise in dogs. Experimental protocol: exercise (−180 to −30 min), transition (−30 to −20 min), basal period (−20 to 0 min), and experimental period (0 to 100 min). In the experimental period, 130 mg/dl arterial hyperglycemia was induced via vena cava (Pe, n = 6) or portal vein (Po, n = 6) glucose infusions. Insulin and glucagon were replaced at fourfold basal and basal rates. During the experimental period, the a-pv grad (mg/dl) was 3 ± 1 in Pe and −10 ± 2 in Po. Arterial insulin and glucagon were similar in the two groups. In Pe, net hepatic glucose balance (mg ⋅ kg−1⋅ min−1, negative = uptake) was 4.2 ± 0.3 (basal period) and −1.2 ± 0.3 (glucose infusion); in Po it was 4.1 ± 0.5 and −3.2 ± 0.4, respectively ( P < 0.005 vs. Pe). Total glucose infusion (mg ⋅ kg−1⋅ min−1) was 11 ± 1 in Po and 8 ± 1 in Pe ( P < 0.05). Net hindlimb and whole body nonhepatic glucose uptakes were similar. Conclusions: the portal signal independently stimulates NHGU after exercise. Conversely, prior exercise eliminates the inhibitory effect of the portal signal on glucose uptake by nonhepatic tissues. The portal signal therefore increases whole body glucose disposal after exercise by an amount equal to the increase in NHGU.

Effect of short-term exposure to high ambient temperatures on gas exchange and heat production in boars of different breeds

The effects of short-term exposure to high ambient temperatures on gas exchange, heat production (HE), respiration rate (RR) and rectal temperature were evaluated individually with boars of approximately 100 kg live weight. The boars were of different breeds with four of Yorkshire (YS), eight of Danish Landrace (DL), out of which three were found stress susceptible by the halothane test (DLH+), eight of Duroc (DR) and eight of Hampshire (HS) breeds. After 1 h rest in the respiration chamber at 17·0°C the gas exchange measurements started with al-h basal period at 17 °C, followed by 2h of heating during which temperature increased to 35·0 °C (period I) and then further to 39·7X1 (period II). Then cooling of the chamber started, and after 1 h (period III) temperature had decreased to 21·8°C, and after the 2nd h of cooling (period IV) temperature was 18·2 °C. The gas exchange was measured for each hour from 09.00 h (basal period) until 14.00 h (period IV). RR was recorded every 15 min. Rectal temperatures were measured when the animals were removed from the chamber. The gas exchange and HE increased slowly during period I but rapidly in period II, followed by decreasing values in the cooling periods. HS and DLH+ had considerably higher gas exchange and HE than other breeds in these two periods and the values remained high during period III. In period IV all breeds had gas exchange rates and HE below those of the basal period. RR increased slightly in period I and then a sharp increase followed during period II. Maximum RR was recorded in period III with an average of 183 breaths per min for all breeds. RR increased earlier and more steeply in HS and reached the highest mean value of 236 breaths per min. Four HS boars salivated heavily during heat stress and rectal temperatures of these animals were 39·7 °C when removed from the chamber compared with close to 39·0 °C for all other breeds. It was concluded that there were considerable breed differences in response to heat stress and that DLH+ and HS were more severely stressed than boars ofYS, DL and DR.

Effect of Acute Hyperglycaemia on Basal and Fat-Induced Exocrine Pancreatic Secretion in Humans

1. We have investigated the effect of acute hyperglycaemia on pancreatico-biliary secretion in healthy subjects. Duodenal outputs of trypsin, lipase, amylase, bicarbonate and bilirubin were measured for 90 min under basal conditions and for 90 min in response to intrajejunal fat administration (1 g/h) on 2 separate days: during normoglycaemia (blood glucose 5 mmol/l) and during acute hyperglycaemia aimed at 15 mmol/l. Plasma cholecystokinin levels, as the major hormonal stimulus of pancreatic and biliary secretion, and plasma pancreatic polypeptide levels, as an indirect measure of vagal-cholinergic tone, were determined at regular intervals. 2. In the basal period pancreatico-biliary secretion was significantly (P < 0.05) reduced during hyperglycaemia compared with normoglycaemia. During normoglycaemia and hyperglycaemia intrajejunal fat significantly (P < 0.05) stimulated pancreaticobiliary secretion. However, during hyperglycaemia, fat-stimulated 90 min pancreatico-biliary secretion was significantly (P < 0.05) reduced compared with normoglycaemia: trypsin (23 ± 7 units versus 66 ± 20 units), lipase (36 ± 8 k-units versus 74 ± 18 k-units), amylase (8 ± 2 k-units versus 18 ± 5 k-units) and bilirubin (32 ± 8 μmol versus 71 ± 14 μmol). Plasma cholecystokinin levels increased significantly (P < 0.05) during fat administration and were not different between the two experiments. Plasma pancreatic polypeptide levels were significantly (P < 0.05) reduced during hyperglycaemia both in the basal period and during intrajejunal fat administration. 3. It is concluded that basal and fat-stimulated pancreatico-biliary secretion are significantly reduced during acute hyperglycaemia. Acute hyperglycaemia does not affect intrajejunal fat-stimulated cholecystokinin secretion. Acute hyperglycaemia inhibits basal and stimulated pancreatic polypeptide secretion suggesting vagal-cholinergic inhibition of pancreatico-biliary secretion during hyperglycaemia.