Small increases in insulin inhibit hepatic glucose production solely caused by an effect on glycogen metabolism.

Based on our earlier work, a 2.5-fold increase in insulin secretion should completely inhibit hepatic glucose production through the hormone's direct effect on hepatic glycogen metabolism. The aim of the present study was to test the accuracy of this prediction and to confirm that gluconeogenic flux, as measured by three independent techniques, was unaffected by the increase in insulin. A 40-min basal period was followed by a 180-min experimental period in which an increase in insulin was induced, with euglycemia maintained by peripheral glucose infusion. Arterial and hepatic sinusoidal insulin levels increased from 10 +/- 2 to 19 +/- 3 and 20 +/- 4 to 45 +/- 5 microU/ml, respectively. Net hepatic glucose output decreased rapidly from 1.90 +/- 0.13 to 0.23 +/- 0.16 mg. kg(-1). min(-1). Three methods of measuring gluconeogenesis and glycogenolysis were used: 1) the hepatic arteriovenous difference technique (n = 8), 2) the [(14)C]phosphoenolpyruvate technique (n = 4), and 3) the (2)H(2)O technique (n = 4). The net hepatic glycogenolytic rate decreased from 1.72 +/- 0.20 to -0.28 +/- 0.15 mg. kg(-1). min(-1) (P < 0.05), whereas none of the above methods showed a significant change in hepatic gluconeogenic flux (rate of conversion of phosphoenolpyruvate to glucose-6-phosphate). These results indicate that liver glycogenolysis is acutely sensitive to small changes in plasma insulin, whereas gluconeogenic flux is not.

Alpha- and beta-cell responses to small changes in plasma glucose in the conscious dog.

The responses of the pancreatic alpha- and beta-cells to small changes in glucose were examined in overnight-fasted conscious dogs. Each study consisted of an equilibration (-140 to -40 min), a control (-40 to 0 min), and a test period (0 to 180 min), during which BAY R3401 (10 mg/kg), a glycogen phosphorylase inhibitor, was administered orally, either alone to create mild hypoglycemia or with peripheral glucose infusion to maintain euglycemia or create mild hyperglycemia. Drug administration in the hypoglycemic group decreased net hepatic glucose output (NHGO) from 8.9 +/- 1.7 (basal) to 6.0 +/- 1.7 and 5.8 +/- 1.0 pmol x kg(-1) x min(-1) by 30 and 90 min. As a result, the arterial plasma glucose level decreased from 5.8 +/- 0.2 (basal) to 5.2 +/- 0.3 and 4.4 +/- 0.3 mmol/l by 30 and 90 min, respectively (P < 0.01). Arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin decreased (P < 0.01) from 78 +/- 18 and 90 +/- 24 to 24 +/- 6 and 12 +/- 12 pmol/l over the first 30 min of the test period and decreased to 18 +/- 6 and 0 pmol/l by 90 min, respectively. The arterial glucagon levels and the hepatic portal-arterial difference in plasma glucagon increased from 43 +/- 5 and 4 +/- 2 to 51 +/- 5 and 10 +/- 5 ng/l by 30 min (P < 0.05) and to 79 +/- 16 and 31 +/- 15 ng/l by 90 min (P < 0.05), respectively. In euglycemic dogs, the arterial plasma glucose level remained at 5.9 +/- 0.1 mmol/l, and the NHGO decreased from 10 +/- 0.6 to -3.3 +/- 0.6 pmol x kg(-1) x min(-1) (180 min). The insulin and glucagon levels and the hepatic portal-arterial differences remained constant. In hyperglycemic dogs, the arterial plasma glucose level increased from 5.9 +/- 0.2 to 6.2 +/- 0.2 mmol/l by 30 min, and the NHGO decreased from 10 +/- 1.7 to 0 pmol x kg(-1) x min(-1) by 30 min. The arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin increased from 60 +/- 18 and 78 +/- 24 to 126 +/- 30 and 192 +/- 42 pmol/l by 30 min, after which they averaged 138 +/- 24 and 282 +/- 30 pmol/l, respectively. The arterial plasma glucagon levels and the hepatic portal-arterial difference in plasma glucagon decreased slightly from 41 +/- 7 and 4 +/- 3 to 34 +/- 7 and 3 +/- 2 ng/l during the test period. These data show that the alpha- and beta-cells of the pancreas respond as a coupled unit to very small decreases in the plasma glucose level.

The direct effects of catecholamines on hepatic glucose production occur via alpha(1)- and beta(2)-receptors in the dog.

The role of alpha- and beta-adrenergic receptor subtypes in mediating the actions of catecholamines on hepatic glucose production (HGP) was determined in sixteen 18-h-fasted conscious dogs maintained on a pancreatic clamp with basal insulin and glucagon. The experiment consisted of a 100-min equilibration, a 40-min basal, and two 90-min test periods in groups 1 and 2, plus a 60-min third test period in groups 3 and 4. In group 1 [alpha-blockade with norepinephrine (alpha-blo+NE)], phentolamine (2 microg x kg(-1) x min(-1)) was infused portally during both test periods, and NE (50 ng x kg(-1) x min(-1)) was infused portally at the start of test period 2. In group 2, beta-blockade with epinephrine (beta-blo+EPI), propranolol (1 microg x kg(-1) x min(-1)) was infused portally during both test periods, and EPI (8 ng x kg(-1) x min(-1)) was infused portally during test period 2. In group 3 (alpha(1)-blo+NE), prazosin (4 microg x kg(-1) x min(-1)) was infused portally during all test periods, and NE (50 and 100 ng x kg(-1) x min(-1)) was infused portally during test periods 2 and 3, respectively. In group 4 (beta(2)-blo+EPI), butoxamine (40 microg x kg(-1) x min(-1)) was infused portally during all test periods, and EPI (8 and 40 ng x kg(-1) x min(-1)) was infused portally during test periods 2 and 3, respectively. In the presence of alpha- or alpha(1)-adrenergic blockade, a selective rise in hepatic sinusoidal NE failed to increase net hepatic glucose output (NHGO). In a previous study, the same rate of portal NE infusion had increased NHGO by 1.6 +/- 0.3 mg x kg(-1) x min(-1). In the presence of beta- or beta(2)-adrenergic blockade, the selective rise in hepatic sinusoidal EPI caused by EPI infusion at 8 ng x kg(-1) x min(-1) also failed to increase NHGO. In a previous study, the same rate of EPI infusion had increased NHGO by 1.6 +/- 0.4 mg x kg(-1) x min(-1). In conclusion, in the conscious dog, the direct effects of NE and EPI on HGP are predominantly mediated through alpha(1)- and beta(2)-adrenergic receptors, respectively.

Portal glucose infusion increases hepatic glycogen deposition in conscious unrestrained rats.

It has been demonstrated in the conscious dog that portal glucose infusion creates a signal that increases net hepatic glucose uptake and hepatic glycogen deposition. Experiments leading to an understanding of the mechanism by which this change occurs will be facilitated if this finding can be reproduced in the rat. Rats weighing 275-300 g were implanted with four indwelling catheters (one in the portal vein, one in the left carotid artery, and two in the right jugular vein) that were externalized between the scapulae. The rats were studied in a conscious, unrestrained condition 7 days after surgery, following a 24-h fast. Each experiment consisted of a 30- to 60-min equilibration, a 30-min baseline, and a 120-min test period. In the test period, a pancreatic clamp was performed by using somatostatin, insulin, and glucagon. Glucose was given simultaneously either through the jugular vein to clamp the arterial blood level at 220 mg/dl (Pe low group) or at 250 mg/dl (Pe high group), or via the hepatic portal vein (Po group; 6 mg. kg(-1). min(-1)) and the jugular vein to clamp the arterial blood glucose level to 220 mg/dl. In the test period, the arterial plasma glucagon and insulin levels were not significantly different in the three groups (36 +/- 2, 33 +/- 2, and 30 +/- 2 pg/ml and 1.34 +/- 0.08, 1. 37 +/- 0.18, and 1.66 +/- 0.11 ng/ml in Po, Pe low, and Pe high groups, respectively). The arterial blood glucose levels during the test period were 224 +/- 4 mg/dl for Po, 220 +/- 3 for Pe low, and 255 +/- 2 for Pe high group. The liver glycogen content (micromol glucose/g liver) in the two Pe groups was not statistically different (51 +/- 7 and 65 +/- 8, respectively), whereas the glycogen level in the Po group was significantly greater (93 +/- 9, P < 0.05). Because portal glucose delivery also augments hepatic glycogen deposition in the rat, as it does in the dogs, mechanistic studies relating to its function can now be undertaken in this species.

Rapid reversal of the effects of the portal signal under hyperinsulinemic conditions in the conscious dog.

Experiments were performed on two groups of 42-h-fasted conscious dogs (n = 6/group). Somatostatin was given peripherally with insulin (4-fold basal) and glucagon (basal) intraportally. In the first experimental period, glucose was infused peripherally to double the hepatic glucose load (HGL) in both groups. In the second experimental period, glucose (21.8 micromol. kg-1. min-1) was infused intraportally and the peripheral glucose infusion rate (PeGIR) was reduced to maintain the precreating HGL in the portal signal (PO) group, whereas saline was given intraportally in the control (CON) group and PeGIR was not changed. In the third period, the portal glucose infusion was stopped in the PO group and PeGIR was increased to sustain HGL. PeGIR was continued in the CON group. The glucose loads to the liver did not differ in the CON and PO groups. Net hepatic glucose uptake was 9.6 +/- 2.5, 11.6 +/- 2.6, and 15.5 +/- 3.2 vs. 10.8 +/- 1.8, 23.7 +/- 3.0, and 15.5 +/- 1.1 micromol. kg-1. min-1, and nonhepatic glucose uptake (non-HGU) was 29.8 +/- 1.1, 40.1 +/- 4.5, and 49.5 +/- 4.0 vs. 26.6 +/- 4.3, 23.2 +/- 4.0, and 40.4 +/- 3.1 micromol. kg-1. min-1 in the CON and PO groups during the three periods, respectively. Cessation of the portal signal shifted NHGU and non-HGU to rates similar to those evident in the CON group within 10 min. These results indicate that even under hyperinsulinemic conditions the effects of the portal signal on hepatic and peripheral glucose uptake are rapidly reversible.

Low cost of locomotion in the banded Gecko: a test of the nocturnality hypothesis.

This study tested the hypothesis that there has been an evolutionary increase in locomotor performance capacity at low temperature in nocturnal lizards. Nocturnal lizards are often active at low and suboptimal body temperatures. An evolutionary decrease in the minimum cost of locomotion could increase endurance capacity at low temperature, partially offsetting the thermal handicap of nocturnality. In support of the nocturnality hypothesis, we discovered that minimum cost of locomotion of a nocturnal gecko, Coleonyx variegatus (4.2 g), was only 58% of the minimum cost of locomotion of Phrynosoma douglassii, a diurnal lizard (4.5 g). As a result, maximum aerobic speed was 2.3 times as great in the nocturnal lizard compared to the diurnal lizard. By using the method of phylogenetically independent contrasts at the species level, we showed that the relationship between mass and minimum cost of locomotion in diurnal lizards was similar to that of the ahistorical standard allometry and that low minimum cost of locomotion in geckos represents a significant evolutionary change from the ancestral diurnal pattern. The decrease in the minimum cost of locomotion concordant with the evolution of nocturnality suggests that geckos evolved a greater capacity for sustained locomotion at low temperature.

Efficiency of uphill locomotion in nocturnal and diurnal lizards

Nocturnal geckos can walk on level ground more economically than diurnal lizards. One hypothesis for why nocturnal geckos have a low cost of locomotion is that they can perform mechanical work during locomotion more efficiently than other lizards. To test this hypothesis, we compared the efficiency of the nocturnal gecko Coleonyx variegatus (average body mass 4.2 g) and the diurnal skink Eumeces skiltonianus (average body mass 4.8 g) when they performed vertical work during uphill locomotion. We measured the rate of oxygen consumption when each species walked on the level and up a 50 slope over a range of speeds. For Coleonyx variegatus, the energetic cost of traveling a unit distance (the minimum cost of transport, Cmin) increased from 1.5 to 2.7 ml O2 kg-1 m-1 between level and uphill locomotion. For Eumeces skiltonianus, Cmin increased from 2.5 to 4.7 ml O2 kg-1 m-1 between level and uphill locomotion. By taking the difference between Cmin for level and uphill locomotion, we found that the efficiency of performing vertical work during locomotion was 37 % for Coleonyx variegatus and 19 % for Eumeces skiltonianus. The similarity between the 1.9-fold difference in vertical efficiency and the 1.7-fold difference in the cost of transport on level ground is consistent with the hypothesis that nocturnal geckos have a lower cost of locomotion than other lizards because they can perform mechanical work during locomotion more efficiently.