Epinephrine plasma thresholds for lipolytic effects in man: measurements of fatty acid transport with [l-13C]palmitic acid

Lipolysis during fasting. Decreased suppression by insulin and increased stimulation by epinephrine

These studies were designed to determine whether the insulin resistance of fasting extends to its antilipolytic effects and whether fasting enhances the lipolytic effects of adrenergic stimulation independent of changes in plasma hormone and substrate concentrations. Palmitate flux was determined isotopically ([1-14C]palmitate) before and during epinephrine infusion in normal volunteers after a 14-h (day 1) and an 84-h (day 4) fast. Using a pancreatic clamp, constant plasma hormone and glucose concentrations were achieved on both study days in seven subjects. Six subjects were infused with saline and served as controls. During the pancreatic clamp, palmitate flux was greater (P less than 0.01) on day 4 than day 1, despite similar plasma insulin, glucagon, growth hormone, cortisol, epinephrine, norepinephrine, and glucose concentrations. The lipolytic response to epinephrine was greater (P less than 0.05) on day 4 than day 1 in both groups of subjects. In conclusion, lipolysis during fasting is less completely suppressed by insulin and more readily stimulated by epinephrine.

Palmitate and glycerol kinetics during brief starvation in normal weight young adult and elderly subjects

Data obtained in vitro suggest that the ability to mobilize fat decreases with age. We determined lipolytic rates in vivo in normal weight young adult (22-33 yr) and elderly (65-77 yr) subjects using a simultaneous infusion of [1,2-13C2]palmitate and [2H5]glycerol. The subjects were studied after a 12-h fast and again after 60-82 h of fasting. When lipolysis was expressed per unit of adipose tissue the values for the young adults were more than double those for the elderly (P less than 0.05). However, the amount of body fat in the elderly was twice that of the young adults, so that lipolysis per unit of body weight was similar in both groups. These results demonstrate that lipolysis per unit of adipose tissue is lower in elderly subjects. This may be due to their increase in body fat, however, since the total amount of potential energy mobilized from adipose tissue was similar to that of the young adults.

Free fatty acid metabolism and obesity in man: in vivo in vitro comparisons

We have examined the relationship of free fatty acid (FFA) turnover and lipid oxidation rates in vivo to the size of body triglyceride stores and compared these findings with the in vitro lipolytic rates of isolated abdominal fat cells. The studies were performed in 20 Pima Indian women 18 to 35 years of age, both lean and obese. FFA turnover rate was measured using a 1-14C-palmitate infusion, lipid oxidation rate by indirect calorimetry using a ventilated hood, body composition by underwater weighing with correction for residual lung volume, and fat cell lipolytic rates in vitro by published methods. Both FFA turnover and lipid oxidation rates, expressed per kg of body fat, decreased with increasing degree of obesity (as measured by percent body fat) (r = -0.90, and r = -0.75, P less than or equal to 0.0001, respectively). In contrast, the rate of lipolysis determined in vitro, expressed per kg of fat, increased with increasing degree of obesity (r = 0.58, P less than 0.01). A ratio of FFA turnover/lipolysis, which directly compares these in vivo and in vitro measurements, decreased significantly with increases in the degree of obesity (r = -0.81, P less than or equal to 0.0001). Furthermore, there were no positive correlations between the measures of in vivo FFA metabolism and in vitro lipolysis when both were expressed per fat mass, per fat cell number, or per fat cell surface area. The in vivo data also demonstrated that lipid oxidation could only account for 50% of the FFA disappearance rate. While lipid oxidation rate adjusted to the metabolic size increased with increasing plasma FFA concentration (r = 0.75, P less than 0.0003), the nonoxidative component of the FFA turnover failed to increase with increases in plasma FFA concentration (P = 0.5). We conclude that FFA is not available in vivo in proportion to the size of the triglyceride stores. The reason for this is not due to an inability of fat cells to release their stored triglyceride as assessed in vitro. Hence, in vitro measurements of fat cell lipolysis cannot be used to directly predict in vivo FFA metabolism. The large nonoxidative FFA disposal is likely to be important in the regulation of plasma FFA concentrations.

Ketone body transport in the human neonate and infant

Using a continuous intravenous infusion of D-(-)-3-hydroxy[4,4,4-2H3]butyrate tracer, we measured total ketone body transport in 12 infants: six newborns, four 1-6-mo-olds, one diabetic, and one hyperinsulinemic infant. Ketone body inflow-outflow transport (flux) averaged 17.3 +/- 1.4 mumol kg-1 min-1 in the neonates, a value not different from that of 20.6 +/- 0.9 mumol kg-1 min-1 measured in the older infants. This rate was accelerated to 32.2 mumol kg-1 min-1 in the diabetic and slowed to 5.0 mumol kg-1 min-1 in the hyperinsulinemic child. As in the adult, ketone turnover was directly proportional to free fatty acid and ketone body concentrations, while ketone clearance declined as the circulatory content of ketone bodies increased. Compared with the adult, however, ketone body turnover rates of 12.8-21.9 mumol kg-1 min-1 in newborns fasted for less than 8 h, and rates of 17.9-26.0 mumol kg-1 min-1 in older infants fasted for less than 10 h, were in a range found in adults only after several days of total fasting. If the bulk of transported ketone body fuels are oxidized in the infant as they are in the adult, ketone bodies could account for as much as 25% of the neonate's basal energy requirements in the first several days of life. These studies demonstrate active ketogenesis and quantitatively important ketone body fuel transport in the human infant. Furthermore, the qualitatively similar relationships between the newborn and the adult relative to free fatty acid concentration and ketone inflow, and with regard to ketone concentration and clearance rate, suggest that intrahepatic and extrahepatic regulatory systems controlling ketone body metabolism are well established by early postnatal life in humans.

Enhanced glycemic responsiveness to epinephrine in insulin-dependent diabetes mellitus is the result of the inability to secrete insulin. Augmented insulin secretion normally limits the glycemic, but not the lipolytic or ketogenic, response to epinephrine in humans

To determine if the enhanced glycemic response to epinephrine in patients with insulin-dependent diabetes mellitus (IDDM) is the result of increased adrenergic sensitivity per se, increased glucagon secretion, decreased insulin secretion, or a combination of these, plasma epinephrine concentration-response curves were determined in insulin-infused (initially euglycemic) patients with IDDM and nondiabetic subjects on two occasions: once when insulin and glucagon were free to change (control study), and again when insulin and glucagon were held constant (islet clamp study). During the control study, plasma C-peptide doubled, and glucagon did not change in the nondiabetic subjects, whereas plasma C-peptide did not change but glucagon increased in the patients. The patients with IDDM exhibited threefold greater increments in plasma glucose, largely the result of greater increments in glucose production. This enhanced glycemic response was apparent with 30-min increments in epinephrine to plasma concentrations as low as 100-200 pg/ml, levels that occur commonly under physiologic conditions. During the islet clamp study (somatostatin infusion with insulin and glucagon replacement at fixed rates), the heightened glycemic response was unaltered in the patients with IDDM, but the nondiabetic subjects exhibited an enhanced glycemic response to epinephrine indistinguishable from that of patients with IDDM. In contrast, the FFA, glycerol, and beta-hydroxybutyrate responses were unaltered. Thus, we conclude the following: Short, physiologic increments in plasma epinephrine cause greater increments in plasma glucose in patients with IDDM than in nondiabetic subjects, a finding likely to be relevant to glycemic control during the daily lives of such patients as well as during the stress of intercurrent illness. Enhanced glycemic responsiveness of patients with IDDM to epinephrine is not the result of increased sensitivity of adrenergic receptor-effector mechanisms per se nor of their increased glucagon secretory response; rather, it is the result of their inability to augment insulin secretion. Augmented insulin secretion, albeit restrained, normally limits the glycemic response, but not the lipolytic or ketogenic responses, to epinephrine in humans.

Adrenergic regulation of lipolysis in abdominal adipocytes of obese subjects during caloric restriction: reversal of catecholamine action caused by relief of endogenous inhibition

The effects of adrenaline, noradrenaline, and of the alpha 2- and beta-selective agonists clonidine and isoproterenol were studied in fifteen obese subjects before and after 4 weeks of caloric restriction (300 cal day-1). Basal glycerol release averaged 1.4 mumol (10(6) cells)-1 (180 min)-1 before starvation and 2.8 mumol (10(6) cells)-1 (180 min)-1 during starvation (P less than or equal to 0.1). Before starvation adrenaline and noradrenaline caused a 2-3-fold increase of glycerol release. This lipolytic effect disappeared during starvation. An inhibitory effect of adrenaline was observed instead which was maximal at an adrenaline concentration of 1 mumol 1(-1) (P less than or equal to 0.05). The dose-response relationships of the alpha 2- and beta-selective agents clonidine and isoproterenol were not appreciably changed by caloric restriction. The increase of basal lipolytic rate and the reversal of adrenaline action seen during caloric restriction could be mimicked by removal of endogenous adenosine using adenosine deaminase (1.6 microgram ml-1). In addition, inclusion of N6-phenylisopropyladenosine (1 mumol 1(-1)) into the medium reverted the adrenaline-induced inhibition seen during caloric restriction. The results suggest that local modulators such as adenosine are of primary importance for the apparent change of responsiveness to adrenaline and noradrenaline seen during starvation of human fat cells in vitro.

Mechanisms of catecholamine effects on ketogenesis

Ketogenesis may be controlled at several sites. Lipolysis with release of plasma nonesterified fatty acid (NEFA) substrate is the first step. Plasma NEFA are taken up by the liver in a concentration-dependent fashion and, after conversion to the acyl-CoA derivative, may either be reesterified or enter the mitochondria via the carnitine shuttle. After beta-oxidation the resultant acetyl-CoA may either be converted to ketone bodies that are then released into the circulation or be condensed with oxaloacetate and enter the tricarboxylic acid cycle, the third potential control point. In humans, infusion of epinephrine causes a transient two- to threefold increase in fatty acids, glycerol, and ketone bodies. Insulin levels show a small absolute increase. Norepinephrine has similar effects, although insulin levels tend to be suppressed and glucagon levels rise somewhat. If somatostatin is added simultaneously, the lipolytic and ketogenic effects are accentuated and prolonged. Dopamine, in a high dose, has no effect on ketone bodies alone but shows small increases in NEFA and ketone bodies in the presence of somatostatin and may play a modulatory role in ketogenesis. The ketogenic effect of catecholamines could thus be in the adipocyte or in the liver. Studies with perfused liver or hepatocytes showed only trivial effects on ketogenesis even with supraphysiological doses of catecholamines. Furthermore infusion studies in rats showed decreased rather than increased ketogenesis with no change in NEFA levels. The data suggest that a) there are species differences, and b) in humans epinephrine- and norepinephrine-induced increases in ketogenesis are secondary to increases in NEFA substrate supply.

Sympathoadrenal system and regulation of thermogenesis

The sympathetic nervous system (SNS) plays a critical role in the regulation of mammalian thermogenic responses to cold exposure and dietary intake. Catecholamine-stimulated thermogenesis is mediated by the beta-adrenergic receptor. In the rat brown adipose tissue is the major site of metabolic heat production in response to both cold (nonshivering thermogenesis) and diet (diet-induced thermogenesis). Measurements of norepinephrine turnover rate in interscapular brown adipose tissue of the rat demonstrate increased sympathetic activity in response to both cold exposure and overfeeding. In adult humans, a physiologically significant role for brown adipose tissue has not been established but cannot be excluded. It appears likely that dietary changes in SNS activity are related, at least in part, to the changes in metabolic rate that occur in association with changes in dietary intake.

Pharmacokinetic and pharmacodynamic considerations in drug therapy of cardiac emergencies

In the drug therapy of cardiac emergencies, it is necessary to rapidly achieve therapeutic drug concentrations and adjust drug dose as the patient's clinical status changes. Cardiac dysfunction is often present and may alter drug pharmacokinetics. Circulatory failure causes sympathetically mediated vasoconstriction in most tissues, with relative sparing of the brain and heart due to autoregulation. Blood flow to vasoconstricted tissues is reduced, and the available cardiac output is redistributed so that the heart and brain receive a greater fraction. Drug distribution to tissues is therefore slowed, and the initial concentration of drug in blood is higher when circulatory failure is present than when it is absent. This higher blood concentration is reflected by higher concentrations of drug in the brain and heart, which are relatively well perfused. Initial doses of many drugs need to be reduced in patients with circulatory failure to prevent cardiac or central nervous system toxicity. Cardiac output is markedly diminished during cardiopulmonary resuscitation (CPR), but blood flow distribution is qualitatively similar to that of circulatory failure with spontaneous circulation. Pneumatic trousers increase lower extremity vascular resistance and may produce a similar redistribution of blood flow. Drug distribution during the use of CPR or pneumatic trousers should be similar to that of circulatory failure with spontaneous circulation, but few data are available to guide drug dosing during the use of these interventions. Animal data suggest that the central volume of distribution of some drugs during CPR may be as small as one-tenth of normal. Drug metabolism in circulatory failure may be impaired by reduced hepatic blood flow resulting in decreased clearance of highly extracted drugs, or by hepatocellular dysfunction resulting in decreased clearance of poorly extracted drugs. Drug excretion may be impaired by reduced renal blood flow resulting in decreased filtration or secretion and increased reabsorption. The maintenance dose of many drugs must therefore be reduced in the presence of circulatory failure. Intravenous drug administration is preferred in patients with circulatory failure. The central intravenous route is often convenient but must be used cautiously when administering potentially cardiotoxic drugs. Intratracheal administration appears to be a promising alternative for some drugs, such as adrenaline (epinephrine). Intracardiac injections are hazardous and offer no demonstrated advantage over other routes.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucose and ketone body kinetics in diabetic ketoacidosis

The hyperglycaemia and hyperketonaemia of diabetic ketoacidosis are initiated primarily by overproduction of these substrates; subsequent maintenance of hyperglycaemia occurs, in large part, due to impaired utilization of glucose, whereas overproduction of ketone bodies continues to be the major mechanism for maintenance of hyperketonaemia. Insulin deficiency results in increased rates of lipolysis and provides increased substrate (free fatty acids) for ketogenesis. Hyperglucagonaemia can augment ketogenesis further in the setting of insulin deficiency. It is likely that other counter-insulin hormones (growth hormone, catecholamines) also contribute to the pathogenesis of DKA, though their role is less well defined. Insulin corrects DKA largely via suppression of lipolysis (and thus ketone body production); insulin suppresses glucose production at lower levels than it does ketone body production.

Neuroendocrine responses to glucose ingestion in man. Specificity, temporal relationships, and quantitative aspects

The mechanisms of postprandial glucose counterregulation-those that blunt late decrements in plasma glucose, prevent hypoglycemia, and restore euglycemia-have not been fully defined. To begin to clarify these mechanisms, we measured neuroendocrine and metabolic responses to the ingestion of glucose (75 g), xylose (62.5 g), mannitol (20 g), and water in ten normal human subjects to determine for each response the magnitude, temporal relationships, and specificity for glucose ingestion. Measurements were made at 10-min intervals over 5 h. By multivariate analysis of variance, the plasma glucose (P < 0.0001), insulin (P < 0.0001), glucagon (P < 0.03), epinephrine (P < 0.0004), and growth hormone (P < 0.01) curves, as well as the blood lactate (P < 0.0001), glycerol (P < 0.001), and beta-hydroxybutyrate (P < 0.0001) curves following glucose ingestion differed significantly from those following water ingestion. However, the growth hormone curves did not differ after correction for differences at base line. In contrast, the plasma norepinephrine (P < 0.31) and cortisol (P < 0.24) curves were similar after ingestion of all four test solutions, although early and sustained increments in norepinephrine occurred after all four test solutions. Thus, among the potentially important glucose regulatory factors, only transient increments in insulin, transient decrements in glucagon, and late increments in epinephrine are specific for glucose ingestion. They do not follow ingestion of water, xylose, or mannitol. Following glucose ingestion, plasma glucose rose to peak levels of 156+/-6 mg/dl at 46+/-4 min, returned to base line at 177+/-4 min, reached nadirs of 63+/-3 mg/dl at 232+/-12 min, and rose to levels comparable to base line at 305 min, which was the final sampling point. Plasma insulin rose to peak levels of 150+/-17 muU/ml (P < 0.001) at 67+/-8 min. At the time glucose returned to base line, insulin levels (49+/-12 muU/ml) remained fourfold higher than base line (P < 0.01); thereafter they declined but never fell below base line. Plasma glucagon decreased from 95+/-14 pg/ml to nadirs of 67+/-11 pg/ml (P < 0.001) at 84+/-9 min and then rose progressively to peak levels of 114+/-17 pg/ml (P < 0.001 vs. nadirs) at 265+/-12 min. Plasma epinephrine, which was 18+/-4 pg/ml at base line, did not change initially and then rose to peak levels of 119+/-20 pg/ml (P < 0.001) at 271+/-13 min. These data indicate that the glucose counterregulatory process late after glucose ingestion is not solely due to the dissipation of insulin and that sympathetic neural norepinephrine, growth hormone, and cortisol do not play critical roles. They are consistent with, but do not establish, physiologic roles for the counterregulatory hormones-glucagon, epinephrine, or both-in that process.

Nervous disorders of renal function

Renal nerves contribute to the genesis of at least four disease processes. 1. Excess renal nerve activity contributes significantly to salt and water retention by patients with congestive circulatory failure. 2. Circumstantial evidence suggests that dopamine production may be deficient in a group of patients with idiopathic edema. Aldosterone secretion is high in this group and it has been shown that dopamine exerts a tonic inhibitory effect on angiotensin-stimulated aldosterone secretion. 3. Excess renal nerve activity probably plays a crucial role in the transition from hypotension and pre-renal failure to ischemic acute tubular necrosis. 4. Without doubt hyperactivity of renal nerves causes systemic hypertension in a variety of animal disease models. There is also good reason to believe that this occurs in some forms of human hypertension. The effects of the sympathetic nervous system on renal vascular resistance, renin release, tubular electrolyte reabsorption and aldosterone secretion are discussed in the context of these four diseases.

Lipid transport in the human newborn. Palmitate and glycerol turnover and the contribution of glycerol to neonatal hepatic glucose output

Free fatty acid (FFA) transport was measured in 11 and glycerol turnover in 5 newborns with continuous tracer infusion of [1-(13)C]palmitate or [2-(13)C]glycerol, respectively. In addition, simultaneous determination of glucose production in the latter group with [6,6-(2)H(2)]glucose tracer and measurement of the appearance rate of [(13)C]glucose derived from [(13)C]glycerol allowed calculation of gluconeogenesis from glycerol.The average FFA inflow rate was 11.5+/-1.7 mumol kg(-1)min(-1), 2.5-4.5 h after the last feeding, and 16.7+/-2.8 mumol kg(-1)min(-1), 5-12 h after the last meal. These rates are comparable to those found in adults only after 8-16 h and approximately 72 h of fasting, respectively. FFA inflow in the newborn was directly correlated with time of fasting, plasma FFA level, and plasma glycerol level. Palmitate clearance and fractional removal were inversely related to palmitate level. Glycerol flux averaged 4.4+/-0.5 mumol kg(-1)min(-1), a value three- to fourfold that of the postabsorptive adult. Approximately 75% of transported glycerol was converted to glucose and represented 5.0+/-0.6% of hepatic glucose production. Furthermore, there was a direct relationship between glycerol turnover and the fraction of glucose coming from glycerol. Despite the absolutely elevated neonatal FFA and glycerol transport rates, these were quantitatively similar to values found in adults with comparable elevated substrate levels. Furthermore, other similarities with the adult in the relationships between inflow transport and substrate values, and between transport and fractional removal suggest that the regulatory aspects of lipid transport in man are already well developed by the first day of life.