Studies of glucose production in sheep using (6-3H)glucose and (U-14C)glucose.

Simultaneous primed-continuous intravenous infusions of [6-3H]glucose and [U-14C]glucose were performed on 13 fed, 4 fasted, and 4 dexamethasone-treated sheep. In 10 of the experiments on fed sheep, glucagon or insulin was infused intraportally for 2 h after control values were obtained. The 3H-labeled glucose gave glucose production values that were only 4.4 +/- 0.5, 5.4 +/- 1.0, and 5.8 +/- 0.8% higher than 14C-labeled glucose in the normal fed, fasted, and dexamethasone-treated sheep, respectively. Glucagon or insulin infusions did not significantly alter this recycling. It is condluced that a recycling of glucose carbon through metabolic intermediates is minimal in the sheep as compared with other species and also that it is not significantly altered by fasting or by hormones that affect glucose production.     

Adrenal glands are essential for activation of glucogenesis during undernutrition in fetal sheep near term

In adults, the adrenal glands are essential for the metabolic response to stress, but little is known about their role in fetal metabolism. This study examined the effects of adrenalectomizing fetal sheep on glucose and oxygen metabolism in utero in fed conditions and after maternal fasting for 48 h near term. Fetal adrenalectomy (AX) had little effect on the rates of glucose and oxygen metabolism by the fetus or uteroplacental tissues in fed conditions. Endogenous glucose production was negligible in both AX and intact, sham-operated fetuses in fed conditions. Maternal fasting reduced fetal glucose levels and umbilical glucose uptake in both groups of fetuses to a similar extent but activated glucose production only in the intact fetuses. The lack of fasting-induced glucogenesis in AX fetuses was accompanied by falls in fetal glucose utilization and oxygen consumption not seen in intact controls. The circulating concentrations of cortisol and total catecholamines, and the hepatic glycogen content and activities of key gluconeogenic enzymes, were also less in AX than intact fetuses in fasted animals. Insulin concentrations were also lower in AX than intact fetuses in both nutritional states. Maternal glucose utilization and its distribution between the fetal, uteroplacental, and nonuterine maternal tissues were unaffected by fetal AX in both nutritional states. Ovine fetal adrenal glands, therefore, have little effect on basal rates of fetal glucose and oxygen metabolism but are essential for activating fetal glucogenesis in response to maternal fasting. They may also be involved in regulating insulin sensitivity in utero.     

Effect of somatostatin suppression of glucagon secretion on glucose production in sheep.

The effect of somatostatin (SRIF) on glucagon and insulin secretion was examined in fed and fasted sheep. This was related to changes in glucose production. Infusion of SRIF at 80 micrograms/h caused a marked reduction in plasma glucagon concentrations. However, the insulin response to SRIF infusion was not consistent; its concentrations decreased occasionally, but often did not change. The depression of glucagon was not associated with a significant reduction in blood glucose concentrations in either fed or fasted sheep, but was associated with a reduction in glucose production by 12--15%. The inhibitory effect of insulin on glucose production was not markedly increased by glucagon deficiency. Infusion of insulin at 1.17 U/h with SRIF decreased glucose production only an additional 10%. Thus, it appears that under basal conditions pancreatic hormonal influences on hepatic glucose production were relatively small in sheep. This implies that under normal conditions in sheep, substrate supply has a much greater impact on hepatic glucogenesis than do hormones.     

Placental transfer of glucose

The rates of glucose transfer from maternal blood to pregnant uterus and from placenta to fetus were measured in eight sheep at spontaneously occurring glucose concentrations (control state) and while the fetus, the mother, or both were receiving a constant infusion of glucose. In addition two fetuses received insulin infusions. In the control state the net glucose flux from placenta to fetus was only 27 +/- 2.6% (SEM) of the net flux from the uterine circulation to the pregnant uterus. An empirical equation describing the relationship between placental glucose transfer and arterial plasma glucose concentrations was derived from the data and compared with equations constructed on the basis of methematical models of placental function. This analysis indicates that: (1) placental glucose transfer is mediated by carriers with Km approximately equal to 70 mg/dl; (2) the rate of glucose transfer from mother to fetus is limited primarily by the transport characteristics and glucose consumption rate of the placenta; (3) under normal conditions of placental perfusion, glucose transfer is approximately 15% less than it would be if placental blood flows were infinitely large.     

Effect of maternal exercise on fetal liver glycogen late in gestation in the rat

To determine the effect of maternal exercise on fetal liver glycogen content, fed and fasted rats that were pregnant for 20.5 or 21.5 days were run on a rodent treadmill for 60 min at 12 m/min with a 0% grade or 16 m/min up a 10% grade. The rats were anesthetized by intravenous injection of pentobarbital sodium, and fetal and maternal liver and plasma samples were collected and frozen. Fetal liver glycogenolysis did not occur as a result of maternal exercise. Fetal blood levels of lactate increased 22-60%, but glucose, plasma glucagon, and insulin were unchanged during maternal exercise. Maternal liver glycogen decreased as a result of exercise in all groups of rats except the fasted 20.5-day-pregnant group. Plasma free fatty acids increased in all groups and blood lactate increased in fed (20.5 days) and fasted (21.5 days) pregnant rats. Maternal glucose, glucagon, and insulin values remained constant during exercise. The fetus appears to be well-protected from metabolic stress during moderate-intensity maternal exercise.     

Maternal insulin and placental 3-O-methyl glucose transport

The effects of insulin in the maternal circulation on the placental clearance of 3-O-methyl glucose were investigated in 7 animals in the presence of a constant maternal glucose concentration. While maternal insulin concentration changed from 12 +/- 4 to 175 +/- 33 mu Units/ml, the placental clearance remained constant at 16.2 +/- 1.2 (control) and 15 +/- 1.3 ml/min per kg fetus under the influence of the insulin. To test the secondary hypothesis that in the control condition the hexose transport system was saturated, we performed a further series of experiments in 6 fasted animals. In these animals the control maternal plasma insulin concentration was 2 +/- 0.3 mu Units/ml and after the infusion of insulin it increased to 562 +/- 26 mu Units/ml. Under conditions of constant maternal and fetal plasma glucose concentrations, this massive elevation of plasma insulin did not change the placental clearance of 3MeG which was 15.2 +/- 1.6 in the control condition and 13.3 +/- ml/min per kg under the influence of high insulin. We conclude that maternal insulin ranging from 2 mu Units/ml to supraphysiologic doses does not effect a physiologically significant change in placental hexose transfer. Placental glucose transfer can probably therefore, be changed only be changing the concentration of glucose in the maternal and fetal plasma.     

Cortisol-mediated potentiation of uterine artery contractility: effect of pregnancy

During pregnancy, maternal plasma cortisol concentrations approximately double. Given that cortisol plays an important role in the regulation of vascular reactivity, the present study investigated the potential role of cortisol in potentiation of uterine artery (UA) contractility and tested the hypothesis that pregnancy downregulated the cortisol-mediated potentiation. In vitro cortisol treatment (3, 10, or 30 ng/ml for 24 h) produced a dose-dependent increase in norepinephrine (NE)-induced contractions in both nonpregnant and pregnant (138-143 days gestation) sheep UA. However, this cortisol-mediated response was significantly attenuated by approximately 50% in pregnant UA. The 11 beta-hydroxysteroid dehydrogenase (11-beta HSD) inhibitor carbenoxolone did not change the effect of cortisol in nonpregnant UA but abolished its effect in pregnant UA by increasing the NE pD(2) in control tissues from 6.20 +/- 0.05 to 6.59 +/- 0.11. The apparent dissociation constant value of NE alpha(1)-adrenoceptors was not changed by cortisol in pregnant UA but was decreased in nonpregnant UA. There was no difference in glucocorticoid receptor density between nonpregnant and pregnant UA. Cortisol significantly decreased endothelial nitric oxide (NO) synthase protein levels and NO release in both nonpregnant and pregnant UA, but the effect of cortisol was attenuated in pregnant UA by approximately 50%. Carbenoxolone alone had no effects on NO release in nonpregnant UA but was decreased in pregnant UA. These results suggest that cortisol potentiates NE-mediated contractions by decreasing NO release and increasing NE-binding affinity to alpha(1)-adrenoceptors in nonpregnant UA. Pregnancy attenuates UA sensitivity to cortisol, which may be mediated by increasing type-2 11-beta HSD activity in UA.     

Fetal metabolism and placental transfer of vasoactive intestinal peptide in sheep

Vasoactive Intestinal Peptide (VIP) is a 28-amino-acid putative neurotransmitter that may have a role in the regulation of myometrial blood flow and uterine contractility. The chronically cannulated fetal sheep preparation was used to examine the fetal clearance and placental transfer of VIP. Metabolic Clearance Rate (MCR) and placental transfer of VIP were measured by alternate steady-state infusion of VIP into the mother and fetus. Plasma concentrations of VIP were measured by radioimmunoassay. MCR was similar in the pregnant (45 +/- 10 ml/kg/min) and nonpregnant ewes (35 +/- 5 ml/kg/min). However, compared to both pregnant and nonpregnant ewes, fetal MCR was significantly increased at 77 +/- 15 ml/kg/min, indicating highly developed clearance mechanisms in the fetus. VIP did not cross the placenta in either direction. Both the placenta and fetal liver metabolized VIP and contributed to the elevated fetal clearance of VIP. The results show that VIP in fetal tissue is unlikely to influence maternal uterine activity with any VIP-mediated effects emanating from maternal and/or placental sources.     

The differential effect of insulin in vivo on the peripheral utilization of glucose and ketone bodies in the rat.

This in vivo study assessed the immediate effects of insulin on glucose and ketone body utilization in the fed, fasted, and diabetic ketoacidotic rat. The experimental design consisted of the functional removal of the liver (the site of glucose and ketone body production) and the pancreas from the anesthetized animals. This surgical procedure permitted the assessment of the effect of exogenously administered insulin on the rate of both glucose and ketone body utilization by peripheral tissues. Insulin exerted hypoglycemic activity in all three metabolic states studied. This hypoglycemic activity contrasted to the lack of demonstrable effect of this hormone on ketone body uptake by peripheral tissues. It was concluded that in the rat, the immediate effect of insulin, i.e. within 30 minutes, was to exert hypoglycemic activity without simultaneous hypoketoniemic activity.     

The effects of hypoxia on glucose turnover in the fetal sheep

The origin of the hypoxia-induced rise in fetal blood glucose concentration in fetal sheep of 124-135 days was investigated. Hypoxia was induced in pregnant sheep and fetuses with chronically implanted vascular catheters by causing the ewes to breathe 9% O2 and 3% CO2 in N2 for 60 min. The rise in fetal plasma glucose caused by a 60% reduction in maternal PaO2 was associated with a 50% fall in plasma insulin concentration. The fall in insulin and rise in glucose was prevented by the alpha-adrenergic blocking agent phentolamine but not by the beta-antagonist propranolol. Turnover of glucose in the fetus under these conditions was measured with [6-3H] and [U-14C] glucose. Hypoxia reduced fetal glucose consumption despite the hyperglycaemia. After 30 min of hypoxia there was no evidence of fetal production of glucose but by 60 min substantial production was evident. The reduced fetal consumption and increased production of glucose was inhibited by phentolamine but not by propranolol. It is concluded that in the fetal sheep hypoxia induced hyperglycaemia is first caused by reduced consumption of glucose and thus fetal glycogen stores are not depleted. If the hypoxia persists fetal blood glucose is elevated further by fetal production of glucose.     

Effects of maternal fasting on hepatic gluconeogenesis and glucose metabolism in fetal lambs.

In unstressed, normoglycaemic fetal lambs, the liver produces little glucose, and gluconeogenesis is insignificant. Indirect measurements have suggested that the fetus may produce glucose endogenously during hypoglycaemia induced by prolonged maternal starvation. In eight fetal lambs we directly measured total and radiolabelled substrate concentration differences across the liver to determine whether the fetal liver produces glucose after four days of fasting-induced hypoglycaemia. Simultaneously we measured umbilical glucose uptake and fetal glucose utilization. Glucose concentrations in ewes (1.78 +/- 0.44 mmol.-1) and fetuses (0.61 +/- 0.17 mmol.l-1) were decreased. Fetal glucose utilization rate (21.7 +/- 8.9 mumol.min-1.kg-1) was not significantly different from umbilical glucose uptake (17.2 +/- 8.9 mumol.min-1.kg-1). Hepatic glucose production (8.9 +/- 17.2 mumol.min-1.100 g-1) and gluconeogenesis (6.1 +/- 4.4 mumol.min-1.100 g-1) were present, but could account for only 13% and 8% of fetal glucose requirements, respectively. To determine whether glucose output by the fetal liver was limited by substrate availability, we infused lactate, acetate, and acetone into the umbilical veins of four fasted animals, increasing hepatic substrate delivery. Hepatic glucose output did not increase during infusion of gluconeogenic substrates, indicating that substrate availability did not limit gluconeogenesis. We conclude that the gluconeogenic pathway is intact in late-gestation fetal lambs and that the fetal liver is capable of gluconeogenesis. However, the primary change in fetal metabolism during maternal starvation is the reduction in fetal glucose utilization, obviating the need for substantial hepatic glucose production. The factors stimulating this modest increase in fetal hepatic glucose production remain to be elucidated.     

Nutrition and fetal brain maturation : II. Impact of maternal starvation on changing levels of acetylcholinesterase and enolase in vitro

The impact of maternal starvation during Days 17-20 of gestation was examined in 20-day fetal rat brain tissue cultured for 6 days in MEM and 10% adult rat serum. Acetylcholinesterase (AChE) activities were consistently greater in fetal brain cell cultures from starved mothers. When fetal tissues from starved mothers were continuously exposed to 72-h fasted serum, AChE activities increased from 1.03 +/- 0.14 to 1.59 +/- 0.21 mumol/h/mg protein (P less than 0.001). In fetal tissues from fed mothers, lower AChE activities were increased from 0.78 +/- 0.09 to 1.04 +/- 0.07 mumol/h/mg protein (P less than 0.05) when 72-h fasted serum was used to replace the fed serum during incubation. When fetal brain cell cultures from fed mothers were exposed for 6 days to graded concentrations of fed serum (2.5-15%), the activities of AChE fell reciprocally from 1.34 +/- 0.10 to 0.82 +/- 0.12 mumol/h/mg protein (P less than 0.05). The levels of AChE activity in tissues exposed to fasted serum were consistently greater, but fell similarly from 1.62 +/- 0.10 to 0.97 +/- 14 mumol/h/mg protein (P less than 0.01), when serum concentrations were increased from 2.5 to 15%. AChE activities were 30% higher in tissues incubated with cycloheximide 10(-3) M (P less than 0.02). Unlike AChE, fetal brain enolase activities were unaffected by maternal starvation. In fetal brain cell cultures from fed mothers, enolase fell from 1.85 +/- 0.10 to 1.37 +/- 0.12 mumol/min/mg protein following exposure to fasted instead of fed serum (P less than 0.02). In fetal cultures from starved mothers, enolase activities were depressed similarly from 1.76 +/- 0.08 to 1.41 +/- 0.09 mumol/min/mg protein when fasted replaced fed serum (P less than 0.02). Thus, the fetal brain cell cultures appear to maintain enzymatic realignments imposed by maternal starvation for at least 6 days. In addition, serum from fasted animals has significant growth inhibiting properties manifested by heightened activities of AChE and lower activities of enolase.     

Blood glucose and serum insulin levels in lean and genetically obese mice.

Fed and 24 hour fasted lean and genetically obese mice (ob/ob) were given a fixed glucose load per gm body weight by intraperitoneal and intragastric administration. Intraperitoneal glucose injection into the obese mice produced a prolonged elevated blood glucose level with a concomitant significant decrease of circulating insulin. Possible interpretations of this observation are discussed. In those obese animals in which glucose was administered intragastrically the fed obese mice had a blood glucose concentration of 450-500 mg% for a period of one hour but there was no increase in circulating insulin, however, in the fasted obese mice in which the glucose concentration was about 350 mg% for one hour, there was a significant increase in the circulating insulin levels. The fed and fasted lean mice showed normal glucose tolerance curves and the expected increase in circulating insulin following either intraperitoneal orintragastric glucose loads. It is concluded that hyperglycaemia in the ob/ob mice is unlikely to be the principal cause of hyperinsulinaemia.     

Relationships between glucose, insulin and glucagon during fasting in late gestation in the rat

Plasma glucose, insulin and glucagon were measured in pregnant and age-matched virgin rats in the fed state and after fasting 6, 48 or 120 hours during day 16–21 of gestation. The fed state in pregnancy was characterized by a metabolic setting favoring anabolism. The lower plasma glucose in the fed pregnant rats was associated with higher insulin, slightly lower glucagon and higher insulin/glucose and insulin/glucagon ratios than in virgin rats. During fasting, glucose fell to sustained hypoglycemic levels in the pregnant animals whereas glucose declined but did not achieve hypoglycemia at any point in the virgins. Despite the hypoglycemia, greater levels of plasma insulin persisted in the pregnant throughout the 120 hours of fasting and insulin/glucagon ratios did not differ significantly from the euglycemic virgins. Thus, “accelerated starvation” in pregnancy cannot be ascribed to relative glucagon excess. Rather, the preservation of normal insulin/glucagon ratios despite prevailing hypoglycemia, may provide a mechanism during fasting in pregnancy for restraining maternal protein catabolism in the face of the added fuel demands of the conceptus.     

Hepatic and muscle insulin action during late pregnancy in the dog

We evaluated the effects of physiologic increases in insulin on hepatic and peripheral glucose metabolism in nonpregnant (NP) and pregnant (P; 3rd trimester) conscious dogs (n = 9 each) using tracer and arteriovenous difference techniques during a hyperinsulinemic euglycemic clamp. Insulin was initially (-150 to 0 min) infused intraportally at a basal rate. During 0-120 min (Low Insulin), the rate was increased by 0.2 mU x kg(-1) x min(-1), and from 120 to 240 min (High Insulin) insulin was infused at 1.5 mU x kg(-1) x min(-1). Insulin concentrations were significantly higher in NP than P during all periods. Matched subsets (n = 5 NP and 6 P) were identified. In the subsets, insulin was 7 +/- 1, 9 +/- 1, and 28 +/- 3 microU/ml (basal, Low Insulin, and High Insulin, respectively) in NP, and 5 +/- 1, 7 +/- 1, and 27 +/- 3 microU/ml in P. Net hepatic glucose output was suppressed similarly in both subsets (> or =50% with Low Insulin, 100% with High Insulin), as was endogenous glucose rate of appearance. During High Insulin, NP dogs required more glucose (10.8 +/- 1.5 vs. 6.2 +/- 1.0 mg x kg(-1) x min(-1), P < 0.05), and hindlimb (primarily skeletal muscle) glucose uptake tended to be greater in NP than P (18.6 +/- 2.5 mg/min vs. 13.6 +/- 2.0 mg/min, P = 0.06). The normal canine liver remains insulin sensitive during late pregnancy. Differing insulin concentrations in pregnant and nonpregnant women and excessive insulin infusion rates may explain previous findings of hepatic insulin resistance in healthy pregnant women.     

Relationships between insulin and glucose metabolism and pituitary-ovarian functions in fasted heifers

The effects of fasting between Days 8 and 16 of the estrous cycle on plasma concentrations of luteinizing hormone (LH), progesterone, cortisol, glucose and insulin were determined in 4 fasted and 4 control heifers during an estrous cycle of fasting and in the subsequent cycle after fasting. Cortisol levels were unaffected by fasting. Concentrations of insulin and glucose, however, were decreased (p less than 0.05) by 12 and 36 h, respectively, after fasting was begun and did not return to control values until 12 h (insulin) and 4 to 7 days (glucose) after fasting ended. Concentrations of progesterone were greater (p less than 0.05) in fasted than in control heifers from Day 10 to 15 of the estrous cycle during fasting, while LH levels were lower (p less than 0.01) in fasted than in control heifers during the last 24 h of fasting. Concentrations of LH increased (p less than 0.01) abruptly in fasted heifers in the first 4 h after they were refed on Day 16 of the fasted cycle. Concentrations (means +/- SEM) of LH also were greater (p less than 0.05) in fasted (11.2 +/- 2.6 ng/ml) than in control (4.7 +/- 1.2 ng/ml) heifers during estrus of the cycle after fasting; this elevated LH was preceded by a rebound response in insulin levels in the fasted-refed heifers, with insulin increasing from 176 +/- 35 pg/ml to 1302 +/- 280 pg/ml between refeeding and estrus of the cycle after fasting. Concentrations of LH, glucose and insulin were similar in both groups after Day 2 of the postfasting cycle. Concentrations of progesterone in two fasted heifers and controls were similar during the cycle after fasting, whereas concentrations in the other fasted heifers were less than 1 ng/ml until Day 10, indicating delayed ovulation and (or) reduced luteal function. Thus, aberrant pituitary and luteal functions in fasted heifers were associated with concurrent fasting-induced changes in insulin and glucose metabolism.     

Fetal insulin and placental 3-O-methyl glucose clearance in near-term sheep

It is difficult, if not impossible, to measure the placental transfer of glucose directly because of placental glucose consumption and the low A-V glucose difference across the sheep placenta. We have approached the problem of quantifying placental hexose transfer by using a nonmetabolized glucose analogue (3-O-methyl glucose) which shares the glucose transport system. We have measured the clearance by using a multisample technique permitting least squares linear computing to avoid the errors implicit in the Fick principle. The placental clearance of 3-O-methyl glucose was measured in the control condition and after the administration of insulin to the fetal circulation. A glucose clamp technique was used to maintain constant transplacental glucose concentrations throughout the duration of the experiment. A control series was performed in which the only intervention was the infusion of normal saline. In these experiments the maternal and fetal glucose concentrations remained constant as did the volume of distribution of 3-O-methyl glucose in the fetus. The maternal insulin concentration remained constant and fetal insulin concentration changed from 11 +/- 2 microU/ml to 355 +/- 51 microU/ml (P less than 0.01). In the face of this large increase in fetal plasma insulin, there was no change in the placental clearance of 3-O-methyl glucose. In the control condition the clearance was 14.1 +/- 1.0 ml/min per kg and this was 13.8 +/- 1.0 ml/min per kg in the high insulin condition. Fetal insulin may change placental glucose flux by decreasing fetal plasma glucose concentrations but does not do so by changing the activity of the glucose transport system.     

Insulin induced fetal hypoglycaemia and fetal and maternal plasma prostaglandin concentrations in sheep in late gestation.

Fetal hypoglycaemia consequent on food withdrawal for 48 h in sheep in late pregnancy is accompanied by an increase in fetal PGE2 plasma concentrations and myometrial contractility. To assess the contribution of fetal hypoglycaemia and related cellular glucopenia in the increased production of fetal PGE2 we studied the effect of 48 h insulin infusion to the fetus. Fetal whole blood glucose was lowered from 19 +/- 2 to 9 +/- 1 mg.dl-1. This experimental regimen maintains glucose availability to those fetal cells in which insulin increases glucose uptake. Fetal umbilical venous and femoral arterial PGE2 concentrations and umbilical veno-arterial PGE2 difference were unchanged, but maternal uterine veno-arterial difference for PGFM increased during the insulin induced fetal hypoglycaemia. Myometrial activity was also unchanged. We conclude that the increased fetal PGE concentration previously reported during food withdrawal is due to a deficiency of glucose to specific insulin dependent cells within vascular beds served by the fetal cardiovascular system. In addition, the findings suggest a need for a supply of glucose of fetal origin for cells that are responsible for increased PGFM concentrations in the maternal uteroplacental circulation.     

Insulin-induced hypoglycemia in fed and fasted exercising rats.

To determine running performance and hormonal and metabolic responses during insulin-induced hypoglycemia, fed and fasted male rats (315 +/- 3 g) were infused with insulin (100 mU/ml, 1.5 ml/h) or saline (1.5 ml/h) for 60 min and then killed at rest or after running on the treadmill (21 m/min, 15% grade). Insulin-infused fed rats ran poorly during the second 10 min of a 20-min exercise test. They were capable of running a total of 43 +/- 5 min, compared with 138 +/- 6 min for saline-infused fed rats. Fasted insulin-infused rats were able to run only 12.8 +/- 0.8 min, compared with 122 +/- 15 min for fasted saline-infused rats. In fasted rats, blood glucose was 1.6 +/- 0.1 mM after 60 min of insulin infusion and 1.2 +/- 0.1 mM after running to exhaustion. Artificial increase of plasma free fatty acids had no effect on performance. Intravenous infusion of glucose at the time of fatigue produced an immediate recovery, allowing the formerly fatigued rats to run 20 min without development of fatigue. These results provide evidence that severe hypoglycemia can be a significant cause of fatigue, even if it occurs early in the course of an exercise bout.     

Evaluation ofin vivo insulin action and glucose metabolism in milk-fed rats

Milk diet has long been recommended in the management of gastrointestinal pathologies. Since milk feeding represents a high fat-low carbohydrate diet and it is acknowledged that insulin resistance is one of the consequences of high fat feeding, it is important to know whether or not chronic milk feeding leads to an impairment of the insulin-mediated glucose metabolism. To examine this question, adult female rats were given raw cow's milk (50% of total calories as lipids) for 18 days. They were compared to rats raised in parallel and fed the standard laboratory diet (15% of total calories as lipids). At the end of the 18 day period, body weight, daily caloric intake, basal plasma glucose and insulin levels in the milk-fed rats were similar to those in the control rats.In vivo insulin action was assessed with the euglycemichyperinsulinemic clamp technique in anesthetized animals. These studies were coupled with the 2-deoxyglucose technique allowing a measurement of glucose utilization by individual tissues. In the milk fed rats: 1) the basal rate of endogenous glucose production was significantly (p<0.01) reduced (by 20%); 2) their hepatic glucose production was however normally suppressed by hyperinsulinemia; 3) their basal glucose utilization rate was significantly (p<0.01) reduced (by 20%); 4) their glucose utilization rate by the whole-body mass or by individual tissues was normally increased by hyperinsulinemia. These results indicate that insulin action in adult rats is not grossly altered after chronic milk-feeding, at least under the present experimental conditions.