Comparison of the direct and indirect effects of epinephrine on hepatic glucose production.

To determine the extent to which the effect of a physiologic increment in epinephrine (EPI) on glucose production (GP) arises indirectly from its action on peripheral tissues (muscle and adipose tissue), epinephrine was infused intraportally (EPI po) or peripherally (EPI pe) into 18-h-fasted conscious dogs maintained on a pancreatic clamp. Arterial EPI levels in EPI po and EPI pe groups rose from 97 +/- 29 to 107 +/- 37 and 42 +/- 12 to 1,064 +/- 144 pg/ml, respectively. Hepatic sinusoidal EPI levels in EPI po and EPI pe were indistinguishable (561 +/- 84 and 568 +/- 75 pg/ml, respectively). During peripheral epinephrine infusion, GP increased from 2.2 +/- 0.1 to 5.1 +/- 0.2 mg/kg x min (10 min). In the presence of the same rise in sinusoidal EPI, but with no rise in arterial EPI (during portal EPI infusion), GP increased from 2.1 +/- 0.1 to 3.8 +/- 0.6 mg/kg x min. Peripheral EPI infusion increased the maximal gluconeogenic rate from 0.7 +/- 0.4 to 1.8 +/- 0.5 mg/ kg x min. Portal EPI infusion did not change the maximal gluconeogenic rate. The estimated initial increase in glycogenolysis was approximately 1.7 and 2.3 mg/kg x min in the EPI pe and EPI po groups, respectively. Gluconeogenesis was responsible for 60% of the overall increase in glucose production stimulated by the increase in plasma epinephrine (EPI pe). Elevation of sinusoidal EPI per se had no direct gluconeogenic effect on the liver, thus its effect on glucose production was solely attributable to an increase in glycogenolysis. Lastly, the gluconeogenic effects of EPI markedly decreased (60-80%) its overall glycogenolytic action on the liver.

The role of fatty acids in mediating the effects of peripheral insulin on hepatic glucose production in the conscious dog.

We investigated the mechanism by which a selective increase in arterial insulin can suppress hepatic glucose production in vivo. Isotopic (3-3H-glucose) and arteriovenous difference methods were used in overnight-fasted, conscious dogs. A pancreatic clamp (somatostatin, basal portal insulin, and glucagon infusions) was used to control the endocrine pancreas. Equilibration (100 min) and basal (40 min) periods were followed by a 180-min test period. In control dogs (n = 5), basal insulin delivery was continued throughout the study. In the other two groups, peripheral insulin was selectively increased at the beginning of the test period by stopping the portal insulin infusion and infusing insulin peripherally at twice the basal portal rate. One group (INS + FAT; n = 6) received an infusion of 20% intralipid + heparin (0.5 U x kg(-1) x min(-1)) to clamp the nonesterified fatty acid (NEFA) levels during hyperinsulinemia; the other group (INS; n = 7) received only saline during the experimental period. In the INS group, a selective increase in peripheral insulin of 84 pmol/l was achieved (36 +/- 6 to 120 +/- 24 pmol/l, last 30 min) while portal insulin was unaltered (84 +/- 18 pmol/l). In the INS + FAT group, a similar increase in peripheral insulin was achieved (36 +/- 6 to 114 +/- 6 pmol/l, last 30 min); again, portal insulin was unaltered (96 +/- 12 pmol/l). In the control group, basal insulin did not change. Glucagon and glucose remained near basal values in all protocols. In the INS group, NEFA levels dropped from 700 +/- 90 (basal) to 230 +/- 65 micromol/l (last 30 min; P > 0.05), but in the INS + FAT group changed minimally (723 +/- 115 [basal] to 782 +/- 125 micromol/l [last 30 min]). In the INS group, net hepatic glucose output dropped by 6.7 micromol x kg(-1) x min(-1) (P < 0.05), whereas in the INS + FAT group it dropped by 3.9 micromol x kg(-1) x min(-1) (P < 0.05). When insulin levels were not increased (i.e., in the control group), net hepatic glucose output dropped 1.7 micromol x kg(-1) x min(-1) (P < 0.05). In all groups, the net hepatic glucose output data were confirmed by the tracer-determined glucose production data. In the INS group, net hepatic gluconeogenic substrate uptake (alanine, glutamine, glutamate, glycerol, glycine, lactate, threonine, and serine) fell slightly (10.4 +/- 1.3 [basal] to 7.2 +/- 1.3 micromol x kg(-1) x min(-1) [last 30 min]), whereas in the INS + FAT group it did not change (7.3 +/- 1.5 [basal] to 7.4 +/- 0.6 micromol x kg(-1) x min(-1) [last 30 min]), and in the control group it increased slightly (9.6 +/- 1.3 [basal] to 10.3 +/- 1.4 micromol x kg(-1) x min(-1) [last 30 min). These results indicate that peripheral insulin's ability to regulate hepatic glucose production is partially linked to its inhibition of lipolysis. When plasma NEFA levels were prevented from falling during a selective arterial hyperinsulinemia, approximately 55% of insulin's inhibition of net hepatic glucose output (NHGO) was eliminated. The fall in NEFA levels brings about a redirection of glycogenolytically derived carbon within the hepatocyte such that there is an increase in lactate efflux and a corresponding decrease in NHGO.

Basal hepatic glucose production is regulated by the portal vein insulin concentration.

The ability of portal vein insulin to control hepatic glucose production (HGP) is debated. The aim of the present study was to determine, therefore, if the liver can respond to a selective decrease in portal vein insulin. Isotopic ([3H]glucose) and arteriovenous difference methods were used to measure HGP in conscious overnight fasted dogs. A pancreatic clamp (somatostatin plus basal portal insulin and glucagon) was used to control the endocrine pancreas. A 40-min control period was followed by a 180-min test period. During the latter, the portal vein insulin level was selectively decreased while the arterial insulin level was not changed. This was accomplished by stopping the portal insulin infusion and giving insulin peripherally at half the basal portal rate (PID, n=5). In a control group (n=5), the portal insulin infusion was not changed and glucose was infused to match the hyperglycemia that occurred in the PID group. A selective decrease of 120 pmol/l in portal vein insulin was achieved (basal, 150+/-36 to last 30 min, 30+/-12 pmol/l) in the absence of a change in the arterial insulin level (basal, 30+/-3 to last 30 min, 36+/-4 pmol/l). Neither arterial nor portal insulin levels changed in the control group (30+/-6 and 126+/-30 pmol/l, respectively). In response to the selective decrease in portal vein insulin, net hepatic glucose output (NHGO) increased significantly, from 8+/-1 (basal) to 30+/-6 and 14+/-2 micromol x kg(-1) x min(-1) by 15 min and the last 30 min (P < 0.05) of the experimental period, respectively. Arterial plasma glucose increased from 5.9+/-0.2 (basal) to 10.5+/-0.4 micromol/l (last 30 min). Three-carbon gluconeogenic precursor uptake fell from 11.2+/-2.9 (basal) to 5.9+/-0.7 micromol x kg(-1) x min(-1) (last 30 min), and thus a change in gluconeogenesis could not account for any of the increase in NHGO. With matched hyperglycemia (basal, 5.5+/-0.3 to last 30 min, 10.5+/-0.8 micromol/l) but no change in insulin, NHGO decreased from 12+/-1 (basal) to 0 (-1+/-6 micromol x kg(-1) x min(-1), last 30 min, P < 0.05) and hepatic gluconeogenic precursor uptake did not change (basal, 8.0+/-1.7 to last 30 min, 8.9+/-2.2 micromol x kg[-1] x min[-1]). Thus, the liver responds rapidly to a selective decrease in portal vein insulin by markedly increasing HGP as a result of increased glycogenolysis. These studies indicate that after an overnight fast, basal HGP (glycogenolysis) is highly sensitive to the hepatic sinusoidal insulin level.

A comparison of the effects of selective increases in peripheral or portal insulin on hepatic glucose production in the conscious dog.

We investigated the mechanisms by which peripheral or portal insulin can independently alter liver glucose production. Isotopic ([3-3H]glucose) and arteriovenous difference methods were used in conscious overnight-fasted dogs. A pancreatic clamp (somatostatin plus basal insulin and basal glucagon infusions) was used to control the endocrine pancreas. After a 40-min basal period, a 180-min experimental period followed in which selective increases in peripheral (PERI group, n = 5) or portal-vein (PORT group, n = 5) insulin were induced. In control dogs (CONT group, n = 10), insulin was not increased. Glucagon levels were fixed in all studies, and basal euglycemia was maintained by peripheral glucose infusion in the two experimental groups. In the PERI group, arterial insulin rose from 36 +/- 12 to 120 +/- 12 pmol/l, while portal insulin was unaltered. In the PORT group, portal insulin rose from 108 +/- 42 to 192 +/- 42 pmol/l, while arterial insulin was unaltered. Neither arterial nor portal insulin changed from basal in the CONT group. With a selective rise in peripheral insulin, the net hepatic glucose output (NHGO; basal, 11.8 +/- 0.7 micromol x kg-1 x min-1) did not change initially (11.8 +/- 2.1 micromol x kg-1 x min-1, 30 min after the insulin increase), but eventually fell (P < 0.05 ) to 6.1 +/- 0.9 micromol x kg-1 x min-1 (last 30 min). With a selective rise in portal insulin, NHGO dropped quickly (P < 0.05) from 10.0 +/- 0.9 to 5.6 +/- 0.6 micromol x kg-1 x min-1 (30 min after the insulin increase) and eventually reached 3.1 +/- 1.1 micromol x kg-1 x min-1 (last 30 min). When insulin levels were not increased (CONT group), NHGO dropped progressively from 10.1 +/- 0.6 to 8.3 +/- 0.6 micromol x kg-1 x min-1 (last 30 min). Conclusions drawn from the net hepatic glucose balance data were confirmed by the tracer data. Net hepatic gluconeogenic substrate uptake (three carbon precursors) fell 2.0 micromol x kg-1 x min-1 in the PERI group, but rose 1.2 micromol x kg-1 x min-1 in the PORT group and 1.2 micromol x kg-1 x min-1 in the CONT group. A selective 84 pmol/l rise in arterial insulin was thus associated with a fall in NHGO of approximately 50%, which took 1 h to manifest. Conversely, a selective 84 pmol/l rise in portal insulin was associated with a 50% fall in NHGO, which occurred quickly (15 min). From the control data, it is evident that in either case approximately 30% of the fall in NHGO was due to a drift down in baseline and that 70% was due to the rise in insulin. In conclusion, an increment in portal insulin had a rapid inhibitory effect on NHGO, caused by the suppression of glycogenolysis, while an equal increment in arterial insulin produced an equally potent but slower effect that resulted from a small increase in hepatic sinusoidal insulin, from a suppression of gluconeogenic precursor uptake by the liver, and from a redirection of glycogenolytic carbon to lactate rather than glucose.

Low plasma leptin levels contribute to diabetic hyperphagia in rats.

The adipocyte hormone leptin reduces food intake in normal animals. During uncontrolled type 1 diabetes, plasma leptin levels fall, whereas food intake increases. To test the hypothesis that low leptin levels contribute to diabetic hyperphagia, we investigated the effect on food intake of replacement of leptin at basal plasma concentrations for 7 days in Long-Evans rats with uncontrolled diabetes induced by streptozotocin (STZ). One group of STZ diabetic rats received saline (STZ + Sal) (n = 11), while the other group (STZ + Lep) (n = 15) received a subcutaneous infusion of recombinant rat leptin (100 microg x kg(-1) x day(-1)) via osmotic minipumps. A nondiabetic control group (Con) (n = 11) received saline only. In the STZ + Sal group, plasma leptin levels decreased by 75% (P < 0.05) from 2.4+/-0.5 on the day before STZ/citrate buffer vehicle (Veh) injection (day 0) to 0.6+/-0.2 ng/ml on day 7. In contrast, plasma leptin levels on days 3-7 were comparable to pretreatment values in both the STZ + Lep group (day 0: 2.6+/-0.4 vs. day 7: 2.5+/-0.3 ng/ml, NS) and the Con group (day 0: 3.8+/-0.4 vs. day 7: 2.9+/-1.0 ng/ml, NS). In the STZ + Sal group, daily food intake increased gradually to values 43% above basal by day 7 (day 0: 24+/-2 to day 7: 33+/-3 g, P < 0.05), whereas food intake did not increase in either the STZ + Lep group (day 0: 24+/-1 vs. day 7: 21+/-2 g, NS), or the Con group (day 0: 23+/-1 vs. day 7: 23+/-2 g). Plasma glucose levels exceeded nondiabetic control values (7.7+/-0.2 mmol/l) in both diabetic groups, but were lower in the STZ + Lep group (17.2+/-1.8 mmol/l) than in the STZ + Sal group (24.3+/-1.1 mmol/l, P < 0.05). To determine if sensitivity to leptin-induced anorexia was affected by STZ treatment, a second experiment was performed in which the effect of intracerebroventricular leptin injection (at doses of 0.35, 1.0, or 3.5 microg) on food intake was measured 10 days after STZ or Veh treatment. Leptin suppressed both 4- and 24-h food intake in the two groups to an equal extent at every dose (by 15, 22, and 35%, respectively). These findings support the hypothesis that the effect of uncontrolled diabetes to lower leptin levels contributes to diabetic hyperphagia and that this effect is not due to altered leptin sensitivity.

Effects of streptozotocin-induced diabetes and insulin treatment on the hypothalamic melanocortin system and muscle uncoupling protein 3 expression in rats.

Hypothalamic melanocortins are among several neuropeptides strongly implicated in the control of food intake. Agonists for melanocortin 4 (MC-4) receptors such as alpha-melanocyte-stimulating hormone (alpha-MSH), a product of proopiomelanocortin (POMC), reduce food intake, whereas hypothalamic agouti-related protein (AgRP) is a MC-4 receptor antagonist that increases food intake. To investigate whether reduced melanocortin signaling contributes to hyperphagia induced by uncontrolled diabetes, male Sprague-Dawley rats were studied 7 days after administration of streptozotocin (STZ) or vehicle. In addition, we wished to determine the effect of diabetes on muscle uncoupling protein 3 (UCP-3), a potential regulator of muscle energy metabolism. STZ diabetic rats were markedly hyperglycemic (31.3 +/- 1.0 mmol/l; P < 0.005) compared with nondiabetic controls (9.3 +/- 0.2 mmol/l). Insulin treatment partially corrected the hyperglycemia (18.8 +/- 2.5 mol/l; P < 0.005). Plasma leptin was markedly reduced in STZ diabetic rats (0.4 +/- 0.1 ng/ml; P < 0.005) compared with controls (3.0 +/- 0.4 ng/ml), an effect that was also partially reversed by insulin treatment (1.8 +/- 0.3 ng/ml). Untreated diabetic rats were hyperphagic, consuming 40% more food (48 +/- 1 g/day; P < 0.005) than controls (34 +/- 1 g/day). Hyperphagia was prevented by insulin treatment (32 +/- 2 g/day). In untreated diabetic rats, hypothalamic POMC mRNA expression (measured by in situ hybridization) was reduced by 80% (P < 0.005), whereas AgRP mRNA levels were increased by 60% (P < 0.01), suggesting a marked decrease of hypothalamic melanocortin signaling. The change in POMC, but not in AgRP, mRNA levels was partially reversed by insulin treatment. By comparison, the effects of diabetes to increase hypothalamic neuropeptide Y (NPY) expression and to decrease corticotropin-releasing hormone (CRH) expression were normalized by insulin treatment, whereas the expression of mRNA encoding the long form of the leptin receptor in the arcuate nucleus was unaltered by diabetes or insulin treatment. UCP-3 mRNA expression in gastrocnemius muscle from diabetic rats was increased fourfold (P < 0.005), and the increase was prevented by insulin treatment. The effect of uncontrolled diabetes to decrease POMC, while increasing AgRP gene expression, suggests that reduced hypothalamic melanocortin signaling, along with increased NPY and decreased CRH signaling, could contribute to diabetic hyperphagia. These responses, in concert with increased muscle UCP-3 expression, may also contribute to the catabolic effects of uncontrolled diabetes on fuel metabolism in peripheral tissues.

Leptin deficiency induced by fasting impairs the satiety response to cholecystokinin.

Leptin administration potentiates the satiety response to signals such as cholecystokinin (CCK), that are released from the gut during a meal. To investigate the physiological relevance of this observation, we hypothesized that leptin deficiency, induced by fasting, attenuates the satiety response to CCK. To test this hypothesis, 48-h-fasted or fed rats were injected with i.p. saline or CCK. Fasting blunted the satiety response to 3.0 microg/kg CCK, such that 30-min food intake was suppressed by 65.1% (relative to saline-treated controls) in fasted rats vs. 85.9% in the fed state (P < 0.05). In a subsequent experiment, rats were divided into three groups: 1) vehicle/fed; 2) vehicle/fasted; and 3) leptin-replaced/fasted; and each group received 3.0 microg/kg i.p. CCK. As expected, the satiety response to CCK was attenuated by fasting in vehicle-treated rats (30-min food intake: vehicle/fed, 0.3 +/- 0.1 g; vehicle/fasted, 1.7 +/- 0.4 g; P < 0.01), and this effect was prevented by leptin replacement (0.7 +/- 0.2 g, P < 0.05 vs. vehicle/fasted; P = not significant vs. vehicle/fed). To investigate whether elevated neuropeptide Y (NPY) signaling plays a role in the effect of leptin deficiency to impair the response to CCK, we measured the response to 3.0 microg/kg i.p. CCK after treatment with 7.5 microg intracerebroventricular NPY. We found that both CCK-induced satiety and its ability to increase c-Fos-like-immunoreactivity in key brainstem-feeding centers were attenuated by NPY pretreatment. We conclude that an attenuated response to meal-related satiety signals is triggered by leptin deficiency and may contribute to increased food intake.

Portal adrenergic blockade does not inhibit the gluconeogenic effects of circulating catecholamines on the liver.

This study was undertaken to determine the impact of portal adrenergic blockade on the gluconeogenic effects of epinephrine (EPI) and norepinephrine (NE). Experiments were performed on 18-hour fasted conscious dogs and consisted of a 100-minute equilibration, a 40-minute basal, and two 90-minute test periods. A pancreatic clamp was used to fix insulin and glucagon levels at basal values. Propranolol (1 microgram/kg.min) and phentolamine (2 micrograms/kg.min) were infused intraportally during both test periods. Portal infusion of alpha- and beta-adrenergic blockers alone (first test period) slightly increased hepatic glucose production from 2.4 +/- 0.4 to 2.8 +/- 0.5 mg/kg.min (nonsignificant [NS]) NE (500 ng/kg.min) and EPI (180 ng/kg.min) were infused peripherally during the second test period. Arterial NE and EPI increased from 186 +/- 63 to 6,725 +/- 913 pg/mL and 76 +/- 25 to 2,674 +/- 344 pg/mL, respectively. Portal NE and EPI increased from 135 +/- 32 to 4,082 +/- 747 pg/mL and 28 +/- 8 to 1,114 +/- 174 pg/mL, respectively. Hepatic glucose production, the maximal gluconeogenic rate, and gluconeogenic efficiency increased from 2.8 +/- 0.5 to 3.8 +/- 0.4 mg/kg.min (P < .05), 0.7 +/- 0.3 to 2.1 +/- 0.6 mg/kg.min (P < .05), and 21% +/- 8% to 60% +/- 13% (P < .05), respectively, in response to catecholamine infusion. Net hepatic lactate balance changed from output (1.5 +/- 3.3 mumol/kg.min) to uptake (-11.0 +/- 3.8 mumol/kg.min, P < .05). Net hepatic glycerol uptake increased from -1.5 +/- 0.7 to -5.5 +/- 2.0 mumol/kg.min (P < .05). Net hepatic uptake of gluconeogenic amino acids did not change significantly. Similarly, hepatic glycogenolysis did not increase during catecholamine infusion. In conclusion, portal delivery of adrenergic blockers selectively inhibits the glycogenolytic effects of EPI and NE on the liver, but allows a marked gluconeogenic response to the catecholamines.

Hepatic and gut clearance of catecholamines in the conscious dog.

Our aim was to assess hepatic and gut catecholamine clearance under normal and simulated stress conditions. Following a 90-minute saline infusion period, epinephrine ([EPI] 180 ng/kg x min) and norepinephrine ([NE] 500 ng/kg x min) were infused peripherally for 90 minutes into five 18-hour fasted, conscious dogs undergoing a pancreatic clamp (somatostatin plus basal insulin and glucagon). Arterial plasma levels of EPI and NE increased from 44 +/- 9 to 2,961 +/- 445 and 96 +/- 6 to 6,467 +/- 571 pg/mL, respectively (both P < .05). Portal vein plasma levels of EPI and NE increased from 23 +/- 8 to 1,311 +/- 173 and 79 +/- 10 to 3,477 +/- 380 pg/mL, respectively (both P < .05). Hepatic vein plasma levels of EPI and NE increased from 5 +/- 2 to 117 +/- 33 and 48 +/- 10 to 448 +/- 59 pg/mL, respectively (both P < .05). Net hepatic and gut EPI uptake increased from 0.5 +/- 0.1 to 30.0 +/- 3.0 and 0.4 +/- 0.1 to 26.3 +/- 4.0 ng/kg x min, respectively (both P < .05). Net hepatic and gut NE uptake increased from 1.5 +/- 0.4 to 74.7 +/- 8.4 and 0.8 +/- 0.2 to 57.9 +/- 7.6 ng/kg x min, respectively (both P < .05). Neither the net hepatic (0.86 +/- 0.05 to 0.93 +/- 0.02) nor gut (0.45 +/- 0.10 to 0.55 +/- 0.04) fractional extraction of EPI changed significantly during the simulated stress condition. Net hepatic and gut spillover of NE increased from 0.8 +/- 0.2 to 3.5 +/- 1.3 and 0.6 +/- 0.2 to 8.8 +/- 2.0 ng/kg x min, respectively, during catecholamine infusion (both P < .05). These results indicate that (1) approximately 30% of circulating catecholamines are cleared by the splanchnic bed (16% and 14% by the liver and gut, respectively); (2) the liver and gut remove a large proportion (approximately 86% to 93% and 45% to 55%, respectively) of the catecholamines delivered to them on first pass; and (3) high levels of plasma catecholamines increase NE spillover from both the liver and gut, suggesting that the percentage of NE released from the presynaptic neuron that escapes the synaptic cleft is increased in the presence of high circulating catecholamine levels.

Insulin action on the liver in vivo.

Insulin has a potent inhibitory effect on hepatic glucose production by direct action at hepatic receptors. The hormone also inhibits glucose production by suppressing both lipolysis in the fat cell and secretion of glucagon by the alpha-cell. Neural sensing of insulin levels appears to participate in control of hepatic glucose production in rodents, but a role for brain insulin sensing has not been documented in dogs or humans. The primary effect of insulin on the liver is its direct action.

Interaction of equal increments in arterial and portal vein insulin on hepatic glucose production in the dog.

We have previously shown that a selective increase of 84 pmol/l in either arterial or portal vein insulin (independent of a change in insulin in the other vessel) can suppress tracer-determined glucose production (TDGP) and net hepatic glucose output (NHGO) by approximately 50%. In the present study we investigated the interaction between equal increments in arterial and portal vein insulin in the suppression of TDGP and NHGO. Isotopic ([3-3H]glucose) and arteriovenous difference methods were used in conscious overnight fasted dogs. A pancreatic clamp was used to control the endocrine pancreas. A 40-min basal period was followed by a 180-min test period, during which arterial and portal vein insulin levels were simulataneously and equally increased 102 pmol/l. Hepatic sinusoidal glucagon levels remained unchanged, and euglycemia was maintained by peripheral glucose infusion. TDGP was suppressed approximately 60% by the last 30 min of the experimental period. In contrast, NHGO was suppressed 100% by that time. Coincidentally, hepatic glucose uptake (net hepatic [3H]glucose balance) increased significantly (approximately 4 mumol.kg-1.min-1). The effects of simultaneous equal increases in peripheral and portal venous insulin were not additive in the suppression of TDGP. However, they were additive in decreasing NHGO as a result of an increase in the uptake of glucose by the liver.

Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.

The effects of catecholamines (CATS) infused into the hepatic portal vein were studied in ten 18-h-fasted conscious dogs. Glucose production (GP) and gluconeogenesis (GNG) were assessed using tracer ([3H]glucose, [14C]alanine) and arteriovenous difference techniques. Each experiment consisted of a 100-min equilibration, a 40-min basal, and two 90-min test periods. A pancreatic clamp (somatostatin + basal portal insulin and glucagon) was used to fix insulin and glucagon at basal levels. Propranolol (1 microgram.kg-1.min-1) and phentolamine (2 micrograms.kg-1.min-1) were infused intraportally during both test periods of the blockade group while a carrier solution was infused in the control group. Norepinephrine (NE; 100 ng.kg-1.min-1) and epinephrine (Epi; 40 ng.kg-1.min-1) were infused intraportally during the second test period of both protocols. Portal NE (70 +/- 46 to 8,404 +/- 674 and 162 +/- 57 to 6,530 +/- 624 pg/ml, respectively) and portal Epi (21 +/- 11 to 3,587 +/- 309 and 29 +/- 6 to 2,989 +/- 406 pg/ml, respectively) rose in the control and adrenergic blockade groups, respectively. The increases in arterial NE and Epi were modest in both groups. Intraportal infusion of CATS increased GP from 2.1 +/- 0.2 to 6.2 +/- 1.0 mg.kg-1.min-1 in the control group but did not change it (2.7 +/- 0.4 to 2.7 +/- 0.3 mg.kg-1.min-1) in the blockade group. Portal CATS had no effect on GNG in the presence or absence of adrenergic blockade (GNG rose from 0.7 +/- 0.2 to 0.9 +/- 0.2 and 0.8 +/- 0.2 to 1.0 +/- 0.2 mg.kg-1.min-1 in the control and blockade groups, respectively). In conclusion, portal infusion of catecholamines significantly augmented GP by selectively stimulating glycogenolysis. The increase in hepatic GP could be completely inhibited by intraportal adrenergic blockade.

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.

Effect of a selective rise in sinusoidal norepinephrine on HGP is due to an increase in glycogenolysis.

To determine the effect of a selective rise in liver sinusoidal norepinephrine (NE) on hepatic glucose production (HGP), norepinephrine (50 ng.kg-1.min-1) was infused intraportally (Po-NE) for 3 h into five 18-h-fasted conscious dogs with a pancreatic clamp. In the control protocol, NE (0.2 ng.kg-1.min-1) and glucose were infused peripherally to match the arterial NE and blood glucose levels in the Po-NE group. Hepatic sinusoidal NE levels rose approximately 30-fold in the Po-NE group but did not change in the control group. The arterial NE levels did not change significantly in either group. During the portal NE infusion, HGP increased from 1.9 +/- 0.2 to 3.5 +/- 0.4 mg.kg-1.min-1 (15 min; P < 0.05) and then gradually fell to 2.4 +/- 0.4 mg.kg-1.min-1 by 3 h. HGP in the control group did not change (2.0 +/- 0.2 to 2.0 +/- 0.2 mg.kg-1.min-1) for 15 min but then gradually fell to 1.1 +/- 0.2 mg.kg-1.min-1 by the end of the study. Because the fall in HGP from 15 min on was parallel in the two groups, the effect of NE on HGP (the difference between HGP in the two groups) did not decline over time. Gluconeogenesis did not change significantly in either group. In conclusion, elevation in hepatic sinusoidal NE significantly increases HGP by selectively stimulating glycogenolysis. Compared with the previously determined effects of epinephrine or glucagon on HGP, the effect of NE is, on a molar basis, less potent but more sustained over time.

Effect of a selective rise in hepatic artery insulin on hepatic glucose production in the conscious dog.

In the present study we compared the hepatic effects of a selective increase in hepatic sinusoidal insulin brought about by insulin infusion into the hepatic artery with those resulting from insulin infusion into the portal vein. A pancreatic clamp was used to control the endocrine pancreas in conscious overnight-fasted dogs. In the control period, insulin was infused via peripheral vein and the portal vein. After the 40-min basal period, there was a 180-min test period during which the peripheral insulin infusion was stopped and an additional 1.2 pmol. kg-1. min-1 of insulin was infused into the hepatic artery (HART, n = 5) or the portal vein (PORT, n = 5, data published previously). In the HART group, the calculated hepatic sinusoidal insulin level increased from 99 +/- 20 (basal) to 165 +/- 21 pmol/l (last 30 min). The calculated hepatic artery insulin concentration rose from 50 +/- 8 (basal) to 289 +/- 19 pmol/l (last 30 min). However, the overall arterial (50 +/- 8 pmol/l) and portal vein insulin levels (118 +/- 24 pmol/l) did not change over the course of the experiment. In the PORT group, the calculated hepatic sinusoidal insulin level increased from 94 +/- 30 (basal) to 156 +/- 33 pmol/l (last 30 min). The portal insulin rose from 108 +/- 42 (basal) to 192 +/- 42 pmol/l (last 30 min), whereas the overall arterial insulin (54 +/- 6 pmol/l) was unaltered during the study. In both groups hepatic sinusoidal glucagon levels remained unchanged, and euglycemia was maintained by peripheral glucose infusion. In the HART group, net hepatic glucose output (NHGO) was suppressed from 9.6 +/- 2.1 micromol. kg-1. min-1 (basal) to 4.6 +/- 1.0 micromol. kg-1. min-1 (15 min) and eventually fell to 3.5 +/- 0.8 micromol. kg-1. min-1 (last 30 min, P < 0.05). In the PORT group, NHGO dropped quickly (P < 0.05) from 10.0 +/- 0.9 (basal) to 7.8 +/- 1.6 (15 min) and eventually reached 3.1 +/- 1.1 micromol. kg-1. min-1 (last 30 min). Thus NHGO decreases in response to a selective increase in hepatic sinusoidal insulin, regardless of whether it comes about because of hyperinsulinemia in the hepatic artery or portal vein.