Adrenergic mechanisms in the metabolic adaptation to fasting and feeding: effects of phlorizin on diet-induced changes in sympathoadrenal activity in the rat

Core temperature: a forgotten variable in energy expenditure and obesity?

A substantial proportion of energy expenditure is utilized for maintenance of the 'warm-blooded' or homoeothermic state. In normally active humans, this compartment of energy output approximates 40% of total energy expenditure. Many mammalian species utilize regulated decreases in temperature, such as hibernation or shallow torpor, as a means of energy conservation. Inherited forms of rodent obesity (ob/ob mouse, fa/fa rat) have lower core temperatures and withstand cold poorly. Obese humans, however, have normal core temperatures. This review addresses the role of core temperature in the metabolic economy of the obese state and raises the possibility that (i) lower temperatures may contribute to the increase in metabolic efficiency that accompanies weight loss in the obese; and (ii) that lower core temperatures may have initiated weight gain in the pre-obese state and that the normal temperatures in the obese may represent metabolic compensation to restore energy balance and limit further weight gain.

A teleological view of obesity, diabetes and hypertension

1. The current worldwide epidemic of obesity and its major complications, namely type 2 diabetes and hypertension, is well documented. The present mini-review develops the thesis that 'thrifty' metabolic traits, evolved in the setting of intermittent famine, contribute to the obesity pandemic. 2. These thrifty traits, namely a decreased capacity for dietary thermogenesis and an increased resistance to insulin-mediated glucose uptake in skeletal muscle, would prolong survival during famine but predispose to obesity and diabetes in the face of abundance. The regulation of dietary thermogenesis by the sympathetic nervous system also explains the well-established association between obesity and high blood pressure. 3. These observations provide a deep-seated rationale for the efficacy of lifestyle interventions in the treatment of obesity and its complications and may also provide a predicate for the development of new therapeutic strategies aimed at neutralizing the impact of these thrifty traits. Such strategies may entail, for example, therapeutic agents that enhance metabolic rate during low-energy diets, thereby reversing the physiological impediment imposed by suppression of the sympathetic nervous system.

Feast or famine: the sympathetic nervous system response to nutrient intake

: 1. The use of tritiated norepinephrine (NE) to measure the turnover rate of NE in sympathetically innervated organs was pioneered in the laboratory of Julius Axelrod. This technique provides an organ specific assessment of sympathetic activity, integrated over a 24 h period, in free living laboratory animals. As such it has proved useful in estimating changes in sympathetic outflow in different physiologic and patho-physiologic states. 2. Studies employing NE turnover techniques in laboratory rodents have demonstrated conclusively that fasting suppresses and overfeeding stimulates the sympathetic nervous system (SNS). These changes in sympathetic activity also occur in humans. 3. Diet-induced changes in SNS activity are regulated by insulin-mediated glucose uptake and metabolism in central neurons sensitive to insulin and located anatomically in the ventro-medial hypothalamus. The regulation is imposed by descending inhibition of tonically active sympathetic brainstem centers. 4. Diet-induced changes in SNS activity mediate changes in energy production known as dietary thermogenesis. The capacity for dietary thermogenesis serves as a potential buffer against weight gain.5. Insulin stimulated SNS activity contributes to obesity-related hypertension. The insulin resistance of obesity, and consequent hyperinsulinemia, drives sympathetically mediated thermogenesis, restoring energy balance at the expense of SNS over activity. The association of obesity and hypertension, therefore, may be the unintended consequence of mechanisms recruited in the obese to limit further weight gain.

Tyrosine hydroxylase activity in the endocrine pancreas: changes induced by short-term dietary manipulation

BACKGROUND: Tyrosine hydroxylase (TH) activity and its possible participation in the control of insulin secretion were studied in pancreatic islets of adult Wistar rats fed a standard commercial diet (SD) or carbohydrates alone (CHD) for one week. TH activity, norepinephrine (NE) content, and glucose-induced insulin secretion were assessed. Blood glucose and insulin levels were measured at the time of sacrifice. RESULTS: CHD rats had significantly higher blood glucose and lower insulin levels than SD rats (114.5 PlusMinus; 6.7 vs 80.7 PlusMinus; 7.25 mg/dl, p < 0.001; 20.25 PlusMinus; 2.45 vs 42.5 PlusMinus; 4.99 &mgr;U/ml, p < 0.01, respectively). Whereas TH activity was significantly higher in CHD isolated islets (600 PlusMinus; 60 vs 330 PlusMinus; 40 pmol/mg protein/h; p < 0.001), NE content was significantly lower (18 PlusMinus; 1 vs 31 PlusMinus; 5 pmol/mg protein), suggesting that TH activity would be inhibited by the end-products of catecholamines (CAs) biosynthetic pathway. A similar TH activity was found in control and solarectomized rats (330 PlusMinus; 40 vs 300 PlusMinus; 80 pmol/mg protein/h), suggesting an endogenous rather than a neural origin of TH activity. CHD islets released significantly less insulin in response to glucose than SD islets (7.4 PlusMinus; 0.9 vs 11.4 PlusMinus; 1.1 ng/islet/h; p < 0.02). CONCLUSIONS: TH activity is present in islet cells; dietary manipulation simultaneously induces an increase in this activity together with a decrease in glucose-induced insulin secretion in rat islets. TH activity - and the consequent endogenous CAs turnover - would participate in the paracrine control of insulin secretion.

Insulin-mediated sympathetic stimulation: role in the pathogenesis of obesity-related hypertension (or, how insulin affects blood pressure, and why)

Thus, the evidence summarized here supports an important role for insulin and the sympathetic nervous system in the pathogenesis of obesity-related hypertension. Is it possible that insulin-mediated sympathetic stimulation contributes a pro-hypertensive effect in non-obese as well? It seems possible in young borderline hypertensives where sympathetically mediated thermogenic mechanisms are potent enough to compensate for the increased caloric intake, thereby enabling these young hypertensives to avoid obesity. This is consistent with an observation made in the original Framingham cohort that not only did obesity predict the eventual development of hypertension, but hypertension, as well, predicted the eventual development of obesity. A reasonable interpretation of these data suggests that as subjects age and the effectiveness of thermogenic mechanisms wanes, obesity might develop as a consequence of increased caloric intake no longer effectively buffered by the increased SNS activity. It is important to note that the mechanisms described here exert a pro-hypertensive effect and cannot properly be considered to 'cause' hypertension. Hypertension is rarely the consequence of a single mechanism. It is also true, as pointed out convincingly by Julius and his colleagues, that enhanced sympathetic activity, as a primary factor, can be associated with both hypertension, insulin resistance and, possibly, obesity [39]. And, finally, it should be noted that the mechanism described here is not the only mechanism linking obesity and hypertension. A rapidly emerging body of evidence indicates that leptin, the polypeptide product of the ob/ob gene secreted from adipose tissue, exerts potent central neural effects on both appetite and sympathetic activity. Leptin levels, elevated in obese humans, have the potential to increase both sympathetic activity and blood pressure [40-43]. A more comprehensive summary of the relationships between hypertension and obesity may, therefore, involve insulin and leptin, as well as the SNS, as represented in the schema presented in Figure 7. Both leptin and insulin may, therefore, be considered as compensatory mechanisms recruited to restore energy balance, with the SNS as one of the effector arms. Viewed in this way, obesity-related hypertension is inextricably linked to the metabolic economy of the obese.

Role of the sympathetic adrenal system in the pathogenesis of the insulin resistance syndrome

The pathophysiology of the various manifestations of Syndrome X has been poorly understood. A possible mechanism involves stimulation of the sympathetic nervous system (SNS). Insulin plays an important role in the relationship between dietary intake and SNS activity. Because insulin-mediated glucose uptake in central hypothalamic neurons regulates SNS activity in response to dietary intake, a hypothesis was developed that links the hyperinsulinemia of obesity to sympathetic stimulation, the latter exerting a prohypertensive effect mediated by the kidney, the heart, and the vasculature. Evidence in support of this hypothesis has been obtained from the Normative Aging Study (NAS) in which a relationship between insulin (and glucose) and the SNS, and between insulin and SNS activity and blood pressure was demonstrated. The characteristic dyslipidemia in NAS subjects, moreover, was related to insulin and epinephrine. As reported in other studies, insulin level was directly associated with low HDL and high triglyceride levels. An independent inverse association was also noted between urinary epinephrine excretion and lipid levels: high epinephrine excretion rates were associated with high HDL and low triglyceride levels and, conversely, low epinephrine excretion was associated with low HDL and high triglycerides. In the NAS, therefore, increased SNS activity contributes to hypertension while diminished adrenal medullary activity contributes to the low HDL and high triglyceride levels commonly seen in association with hypertension.

Hypertension and tachycardia during adrenal manipulation

The purpose of this report is to draw attention to haemodynamic changes during intraoperative adrenal gland manipulation. Severe hypertension, ventricular tachycardia and subendocardial ischaemia occurred during the manipulation of adrenal gland in a patient who underwent live related donor nephrectomy. The patient responded well to intravenous lidocaine. Plasma norepinephrine concentration was elevated at the time of event. Further investigations after surgery excluded the possibility of phaeochromocytoma. In two years follow-up patient remains well. Suspicion for the cause of the event remains the excessive release of catecholamines with manipulation of a normal adrenal gland. The presence of halothane might have contributed to the arrythmia.

Involvement of the autonomic nervous system in the in vivo memory to glucose of pancreatic beta cell in rats

The fact that the potentiating effect of prolonged hyperglycemia on the subsequent insulin secretion is observed in vivo but not in vitro suggests the involvement of extrapancreatic factors in the in vivo memory of pancreatic beta cells to glucose. We have investigated the possible role of the autonomic nervous system. Rats were made hyperglycemic by a 48-h infusion with glucose (HG rats). At the end of glucose infusion as well as 6 h postinfusion, both parasympathetic and sympathetic nerve activities were profoundly altered: parasympathetic and sympathetic activities, assessed by the firing rate either of the thoracic vagus nerve or the superior cervical ganglion, were dramatically increased and decreased, respectively. Moreover, 6 h after the end of glucose infusion, insulin secretion in response to a glucose load was dramatically increased in HG rats compared to controls. To determine whether these changes could be responsible for the increased sensitivity of the beta cell to glucose, insulin release in response to glucose was measured in HG and control rats, either under subdiaphragmatic vagotomy or after administration of the alpha 2A-adrenergic agonist oxymetazoline. Both treatments partially abolished the hyperresponsiveness of the beta cell to glucose in HG rats. Therefore chronic hyperglycemia brings about changes in the activity of the autonomic nervous system, which in turn are responsible, at least in part, for the generation of enhanced beta cell responsiveness to glucose in vivo.

Nutritional effects on neurotransmitter metabolism in the broiler chicken

Two experiments were conducted with broiler chickens to determine various nutritional effects on neurotransmitter metabolism. In Experiment 1, 21-day old chickens were fasted for 24 hr, fed on an ad libitum basis, fed a diet containing 450 g crude protein/kg (high-protein) or fed a diet containing 80 g crude protein/kg (high carbohydrate) to examine nutritional regimens that may alter neural factors regulating growth. Chickens were injected (250 mg/kg BWt) with a tyrosine hydroxylase inhibitor, alpha-methyl-DL-p-tyrosine (AMPT), to inhibit catecholamine synthesis and to estimate turnover constants as functions of these treatments. In Experiment 2, 7-day old chickens were fed diets containing 120, 180, 240, and 300 g crude protein and 1 mg T3/kg diet for 21 days to determine the effects of both dietary protein and thyroid status on catecholamine concentrations. Norepinephrine (NE) and dopamine (DA) in the brains and NE and DA in the hearts and pancreases were separated by HPLC and determined by electrochemical detection. Fractional turnover of DA in the brains of both fed and fasted chickens was equal but was over twice as great as that of NE. Fractional NE turnover in hearts of both fed and fasted chickens was 12.3%/hr although fractional NE turnover in pancreas was greater (P < 0.05) in fasted than in fed chickens (9.0%/hr vs 5.1%/hr). These same rate constants were also seen in brains of chickens fed high-carbohydrate or high-protein diets. In contrast, a protein diet increase pancreatic and cardiac NE turnover compare to a high-carbohydrate diet.

Obesity, blood pressure, and the sympathetic nervous system

Obesity has long been recognized as a major risk factor for the development of hypertension. Recently, insulin level has been shown to correlate with blood pressure in clinical and population-based studies. Since insulin is a major signal in the relationship between dietary intake and sympathetic nervous system activity, the possibility that insulin-mediated sympathetic stimulation is involved in the pathogenesis of hypertension in the obese has been raised. This hypothesis, developed on the basis of studies in laboratory rodents and normal human subjects, is currently being tested in the Normative Aging Study in Boston. Utilizing epidemiologic techniques applied to this defined population, evidence in support of this hypothesis has been accumulated. The preliminary results indicate that in this population, the abdominal form of obesity is associated with higher insulin levels and increased 24-hour urinary norepinephrine excretion (an index of sympathetic activity).

Sympathoadrenal activity and hypoglycemia in the hibernating garden dormouse

The hibernating garden dormouse is spontaneously hypophagic during the prehibernating period at which time we found a low peripheral sympathetic activity (S.A.). The aim of this work was to investigate the link between dietary intake and S.A. The S.A. was evaluated by measurement of catecholamines in both plasma and adrenal glands by HPLC. Food intake, body weight, energy expenditure and plasma glucose were measured during the reentry phase of the hibernating period. The following results were obtained: the energy intake in pretorpid animals (55 to 83 kJ/24 h/100 g body weight) was less than energy expenditure which was between 145 and 97 kJ/24 h/100 g. The energy deficit induces marked hypoglycemia immediately before the onset of hypothermia (117 mg/dl vs. 76 mg/dl) and leads to a drastic drop in the peripheral sympathetic system. This, in turn, reduced energy production, causing the hypothermia.

The sympathoadrenal system, obesity and hypertension: an overview

Obesity-related hypertension is a clinical problem of major significance. The nature of the relationship between blood pressure and body weight has not been elucidated. Recent studies suggest that insulin (and/or insulin resistance) may be involved. An hypothesis is developed, based on the relationship between dietary intake and sympathetic activity, that attributes obesity-related hypertension to sympathetic stimulation.

Insulin resistance, energy balance and sympathetic nervous system activity

Insulin resistance and hyperinsulinemia are commonly associated with hypertension in the obese. The nature of this association is obscure. An hypothesis is developed that attributes obesity-related hypertension to sympathetic stimulation. The relationship between insulin and the sympathetic nervous system (SNS) has its origins in the mediation of dietary thermogenesis. Fasting suppresses while carbohydrate and fat feeding stimulate sympathetic activity. Insulin-mediated glucose metabolism within critical central neurons links dietary intake and central sympathetic outflow. The sympathetic nervous system, in turn, contributes to changes in metabolic rate that accompany alterations in dietary intake. It is hypothesized that insulin resistance is a mechanism recruited in the obese to limit further weight gain and stabilize body mass. Insulin-mediated sympathetic stimulation is one mechanism that may restore energy balance in the obese since the obese are not resistant to the stimulatory effect of insulin on the SNS. Sympathetically mediated stimulation of the heart, vasculature and kidney contributes, in genetically predisposed individuals, to the development of hypertension. Viewed in this light, obesity-related hypertension is the unfortunate by-product of an adaptive mechanism (insulin resistance) recruited to restore energy balance in the obese. Possible implications of this formulation are discussed.

Implications of interscapular brown adipose tissue removal and sucrose overfeeding on the sympatho-adrenal activity and metabolic responses

1. The influence of sucrose overfeeding on the sympatho-adrenal (SA) and metabolic responses was studied in sham-operated (SHAM) rats and in those with interscapular brown adipose tissue (IBAT) removed. 2. Sucrose feeding significantly increased the SA activity, mobilized the free fatty acids (FFA), but did not change glucose homeostasis in sham-operated rats. 3. IBAT removal in control rats fed a stock diet modified the SA activity whereas the levels of both blood glucose and serum FFA were unchanged. 4. However, sucrose in rats void of IBAT potentiated the activity of sympathetic nervous system only and prevented the FFA rise, which is seen in sham-operated sucrose fed rats indicating that the enhanced level of serum FFA in these animals principally originated from the IBAT.

Influence of fasting and dopamine on the tissue catecholamines diurnal variations

To define the role of catecholamines (CA) in the metabolic adaptation to fasting we examined the effect of exogenous dopamine(DA) on heat production(HP) and CA content in the interscapular brown adipose tissue(IBAT) and adrenals of control-fed and 2-day fasted rats in the morning(M) and in the evening(E). DA stimulates HP in fed rats in the M by 45% but the thermogenic effect of this CA is markedly higher in the E. However, DA had no thermogenic effect in fasted rats. The tissue CA in fed rats fluctuates diurnally: in the IBAT noradrenaline(NA) was much higher in the E while adrenaline(A) in adrenals was lower. DA in fed rats did not change the adrenal A but reduced NA content both in the adrenals and in the IBAT all over the day. Fasting depleted A from adrenals but increased NA content both in the M and in the E. Unlike the adrenals in the IBAT fasting did not affect NA content. In the adrenal gland of fasted rats DA significantly increased the A content to the equal degree during the day, while this CA had no effect on NA content of the IBAT.

Sympatho-adrenal activity and metabolic responses in fasted rats--the role of interscapular brown adipose tissue

1. Sympatho-adrenal (SA) and metabolic responses to fasting were studied in sham-operated (SHAM) rats and in those with interscapular brown adipose tissue (IBAT) removed. 2. Fasting significantly increased adrenaline (A) excretion and serum free fatty acids (FFA), but decreased noradrenaline (NA) excretion and blood glucose level in SHAM rats. 3. IBAT removal did not change metabolic responses while it markedly altered the SA activity. Fasting in animals void of IBAT potentiated the activity of adrenal medulla, inhibited the FFA rise and prevented glucose reduction, which is normally observed in SHAM-fasted rats. 4. Results suggest the significance of IBAT in the regulation of the blood level of energy substrates in fasted rats and in maintaining the basal level of NA excretion.

Effect of insulin excess and deficiency on norepinephrine turnover in rats

To examine whether insulin enhances norepinephrine (NE) turnover, an index of sympathetic nerve activity, the effects of excess insulin and streptozotocin (STZ) induced insulin deficiency were examined in Sprague-Dawley rats. Exogenous insulin caused hyperphagia and elevated (approximately 300%) urinary epinephrine excretion, but did not alter cardiac NE content or turnover. STZ-induced insulin deficiency caused hyperglycemia and hyperphagia, but also did not alter cardiac NE content or turnover. Insulin deficiency reduced hepatic NE content 18%, but did not affect NE turnover or content of kidney or spleen. These data do not support the hypothesis that insulin influences cardiac sympathetic nerve activity in rats.

Differential effects of amphetamine and related compounds on locomotor activity and metabolic rate in mice

Locomotor activity was measured by photobeam interruptions, and metabolic rate by the production of CO2 (as minute volume expired CO2, or VECO2) in mice. d-Amphetamine (0.3 to 10 mg/kg IP) increased locomotor activity in a dose-dependent manner while suppressing VECO2 over the same 72-min test period, compared to saline-injected controls. This phenomenon of divergent effects on locomotor activity and metabolic rate required central stimulation, as neither ammonium sulfate nor p-hydroxyamphetamine suppressed VECO2. Oxygen consumption was also suppressed by d-amphetamine, indicating that the suppression of VECO2 involved more than a change in respiratory quotient. When baseline activity rates were increased with running wheels, VECO2 and activity were both suppressed by d-amphetamine; VECO2 was suppressed by d-amphetamine more in exercising mice than in sedentary mice. Anorexigenic agents phenmetrazine, aminoxaphen, and fenfluramine, when administered in doses equimolar to maximally effective doses of d-amphetamine, did not consistently affect activity or VECO2. Evidence for mediation of the VECO2 response by corticosterone and endogenous opioid peptides was negative. Further work, with other mediators of the stress response, or with more complete dose-effect studies with anorexigenic compounds, may be necessary to explicate the mechanism of this counter-intuitive divergence of two measures of activity in mice.

A mechanism by which primary or secondary hypothalamic involvement results in the development of insulin-dependent diabetes mellitus (IDDM)

A literature survey and hypothesis is presented in which it is concluded that an intracellular ventromedial hypothalamic (VMH) glucopenia results in a bibrachial response consisting of adenohypophysial release of growth hormone and ACTH as well as sympathetic discharge, both of which act to elevate plasma glucose and remove the VMH glucopenia. This glucopenia may occur as a result of either a deficiency of circulating insulin or alterations in the kinetic properties of the VMH cellular insulin receptor. Two mechanisms for the development of insulin dependent diabetes mellitus (IDDM) are presented: (1) a defect in VMH glucose transport and/or metabolism such that a VMH glucopenia occurs with a subsequent bibrachial response. The result of this is glucose overproduction and a chronic excess glucose stimulus will eventually cause B-cell exhaustion (primary hypothalamic involvement). (2) reduction of the B-cell population by chemical, genetic and/or viral interactions with a consequential insulopenia results in a VMH glucopenia (secondary to a reduced glucose transport into the VMH cells) and causes a bibrachial response. This VMH response may temporarily restore plasma glucose balance but a chronically enhanced counter-regulatory response to maintain this balance will eventually stress the remaining B-cell population and cause further reductions in B-cell numbers (secondary hypothalmic involvement).

Roles of glucagon and epinephrine in hypoglycemic and nonhypoglycemic glucose counterregulation in humans

Studies of two models of human glucose counterregulation, glucose recovery from insulin-induced hypoglycemia and the transition from exogenous glucose delivery to endogenous glucose production late after glucose ingestion, indicate that the principles of rapid hypoglycemic and nonhypoglycemic glucose counterregulation in these models are the same. 1) Neither is solely explicable on the basis of dissipation of insulin; 2) glucagon plays a primary counterregulatory role in both; 3) epinephrine compensates largely for deficient glucagon secretion in both; and 4) counterregulation fails to occur only in the absence of both glucagon and epinephrine in both. Thus, prevention as well as correction of hypoglycemia is effectively accomplished by redundant glucose counterregulatory systems, primarily glucagon and secondarily epinephrine, coupled with dissipation of insulin in humans. Other hormones, neural mechanisms, or autoregulation may be involved but need not be invoked and are not sufficiently potent to prevent or correct hypoglycemia when both of the key glucose counterregulatory hormones, glucagon and epinephrine, are deficient. Although confirmed in that they predict the impact of disease-related deficiencies of glucagon, epinephrine, or both, the extent to which these principles can be generalized to additional models of glucose counterregulation remains to be established. However, they provide a basis for plausible, testable hypotheses concerning the physiology and pathophysiology of glucose counterregulation.

Dissociation of sympathetic nervous system and adrenal medullary responses

The relative importance of sympathetic nerve (SNS) activity and adrenal medullary secretion in various physiological situations has generally been inferred from measurements of norepinephrine (NE) and epinephrine (E), respectively, in urine or plasma. Increasing evidence, however, indicates that under certain conditions the adrenal medulla may release substantial amounts of NE as well as E. In several of these circumstances, estimates of SNS activity based on the measurement of NE turnover in peripheral tissues of experimental animals indicate diminished SNS function, a reduction that is independent of adrenal medullary secretion. These reciprocal alterations in SNS and adrenal medullary activity fall into two patterns. First, when SNS activity is suppressed by fasting, adrenal medullary responses to various stimuli are enhanced. Second, for certain stimuli the SNS response is biphasic, with an initial suppression followed by subsequent stimulation; during the first phase adrenal medullary secretion is markedly increased. The physiological contribution of the adrenal medulla, therefore, would be particularly important under conditions of SNS suppression.

Epinephrine supports the postabsorptive plasma glucose concentration and prevents hypoglycemia when glucagon secretion is deficient in man

We hypothesized that adrenergic mechanisms support the postabsorptive plasma glucose concentration, and prevent hypoglycemia when glucagon secretion is deficient. Accordingly, we assessed the impact of glucagon deficiency, produced by infusion of somatostatin with insulin, without and with pharmacologic alpha- and beta-adrenergic blockade on the postabsorptive plasma glucose concentration and glucose kinetics in normal human subjects. During somatostatin with insulin alone mean glucose production fell from 1.5 +/- 0.05 to 0.7 +/- 0.2 mg/kg per min and mean plasma glucose declined from 93 +/- 3 to 67 +/- 4 mg/dl over 1 h; glucose production then increased to base-line rates and plasma glucose plateaued at 64-67 mg/dl over 2 h. This plateau was associated with, and is best attributed to, an eightfold increase in mean plasma epinephrine. It did not occur when adrenergic blockade was added; glucose production remained low and mean plasma glucose declined progressively to a hypoglycemic level of 45 +/- 4 mg/dl, significantly (P less than 0.001) lower than the final value during somatostatin with insulin alone. These data provide further support for the concept that maintenance of the postabsorptive plasma glucose concentration is a function of insulin and glucagon, not of insulin alone, and that adrenergic mechanisms do not normally play a critical role. They indicate, however, that an endogenous adrenergic agonist, likely adrenomedullary epinephrine, compensates for deficient glucagon secretion and prevents hypoglycemia in the postabsorptive state in humans. Thus, postabsorptive hypoglycemia occurs when both glucagon and epinephrine are deficient, but not when either glucagon or epinephrine alone is deficient, and insulin is present.

Sympathetic nervous system and adrenal medullary responses to ischemic injury in mice

Acute, severe injury is frequently attended by hypotension, hypothermia, and decreased metabolic rate despite elevated urine and plasma catecholamine levels. Because the combination of sympathetic nervous system (SNS) suppression and adrenal medullary stimulation documented in several other situations could account for these observations, SNS and adrenal medullary function were examined independently in mice in the hindlimb ischemia model of acute injury. SNS activity was assessed by the measurement of [3H]norepinephrine (NE) turnover in heart and adrenal medullary secretion by depletion of adrenal catecholamine content. In nine separate experiments during the first 10 h after termination of a 2.5-h period of hindlimb ischemia, cardiac NE turnover was reduced an average of 23% (P less than 0.05) in injured mice. At the same time, adrenal catecholamine content fell 37% (P less than 0.05) in injured animals but not in controls. In contrast to the acute reaction, SNS activity in mice surviving 3 days was 59% greater than in controls. Thus, the reduction in NE turnover and depletion of adrenal catecholamine content suggest that SNS suppression and adrenal medullary stimulation constitute the acute sympathoadrenal response in this model of severe injury. Because survival within the first 24 h after injury was decreased in adrenalectomized mice despite glucocorticoid treatment, adrenal medullary catecholamines may contribute to survival in severely injured animals. Furthermore, because the SNS plays an important role in the regulation of blood pressure and heat production, the diminution in SNS activity in the hours after injury may contribute to posttraumatic hypotension and hypometabolism.

Effect of dietary fat on sympathetic nervous system activity in the rat

Previous studies from our laboratory have demonstrated that dietary intake affects the sympathetic nervous system (SNS); carbohydrate intake, in particular, has been shown to stimulate sympathetic activity. The present studies were undertaken to characterize the effect of dietary fat on SNS activity in the rat. Sympathetic activity was assessed by measurement of norepinephrine (NE) turnover in heart, interscapular brown adipose tissue (IBAT), and pancreas and by excretion of NE in the urine. When fed a fat-enriched diet (50% chow, 50% lard), fractional NE turnover in heart (k) increased from 6.3 +/- 0.6% h in ad lib. fed controls to 14.7 +/- 1.3% h in the high-fat group (P less than 0.001); calculated NE turnover rate increased from 24.5 +/- 2.4 ng/heart per h to 36.8 +/- 3.5 (P less than 0.05). Urinary NE excretion more than doubled after 6 d of the same high fat diet (P less than 0.001). Ganglionic blockade produced a greater effect on NE turnover in fat-fed, as compared with chow-fed animals, consistent with increased sympathetic activity in the fat-fed group. When fat absorption was blocked with a bile acid binding resin (cholestyramine), the same high-fat diet did not increase cardiac NE turnover, indicating that fat absorption is required for the stimulatory effect on sympathetic activity. In another series of experiments, in which chow (and hence protein) intake was held constant, the effect of fat and isocaloric sucrose supplements on NE turnover was assessed in heart, IBAT, and pancreas. The caloric value of the supplements was 50, 100, and 335% of the chow in the different experiments. An effect of fat on NE turnover in heart and IBAT was demonstrable at the lowest level of fat supplement. Fat increased pancreatic NE turnover when added in amounts sufficient to double the caloric intake. The stimulatory effect of sucrose and fat on NE turnover in heart and IBAT was similar. These experiments demonstrate that fat increases SNS activity in the rat and that the magnitude of the effect is similar to that of sucrose. The results imply that fat may contribute to dietary thermogenesis in this species.

Insulin release after acute hydrocortisone treatment in mice

This investigation examined a potential mechanism for the inhibition of insulin release following an acute dose of hydrocortisone sodium succinate (HC). Hydrocortisone (300 mg/kg intraperitoneally) elevated plasma glucose levels (P less than or equal to 0.05) when administered to male Swiss-Webster mice, without altering plasma insulin levels. This results in a significantly lower insulinogenic index (P less than or equal to 0.05). Hydrocortisone suppressed the glucose-stimulated insulin levels (P less than or equal to 0.05) following an intravenous glucose challenge (2 g/kg) in both fed and fasted mice. Pretreatment with chlorisondamine (EC) and phentolamine (PT) did not alter the HC-induced hyperglycemia but did result in higher plasma insulin levels in response to the higher glucose levels. Adrenalectomy did not prevent the HC-depressed insulin response to hyperglycemia. This is consistent with the hypothesis that an acute dose of HC may be indirectly suppress insulin release by central activation of the sympathetic nerves at the pancreatic islets.

Coordinated responses of glucogenic hormones to central glucopenia: the role of the sympathoadrenal system

In normal humans glucagon plays a primary role in promoting glucose recovery from hypoglycemia, glucagon deficiency is largely compensated for by enhanced epinephrine secretion, and recovery from hypoglycemia fails to occur only in the absence of both glucagon and epinephrine. Defective glucose counterregulation is exemplified by patients with insulin-dependent diabetes. Although most such patients have deficient glucagon secretory responses to hypoglycemia, they counterregulate adequately because of intact epinephrine secretion. Some patients, however, become defenseless against hypoglycemia because of combined deficiencies of glucagon and epinephrine.

Adaptation to hypocaloric feeding: physiologic significance of the fall in serum T3 as measured by the pulse wave arrival time (QKd)

We have investigated the physiologic significance of the decline in serum triiodothyronine (T3) occurring during hypocaloric feeding by measurement of changes in cardiovascular function. The QKd interval, the interval between the Q wave of the electrocardiogram and the onset of Korotkoff sounds at diastolic pressure at the brachial artery, is the sum of the preejection period and pulsetransmission time, and has proven to be a sensitive and effective measure of the effect of thyroid hormones on the cardiovascular system. Fifteen euthyroid obese volunteers underwent successive 2 wk periods of hypocaloric feeding (200-400 calories per day) interspersed with periods of at least 2 wk of re-feeding on a weight-maintaining diet (1500 calories). In a later phase subjects received oral supplementation of triiodothyronine (T3) in addition to the diet to prevent the fall in serum T3. In the last study phase, subjects on the diet received supplementation with oral thyroxine (T4), which prevented the fall in serum T3 and resulted in a slight increase in serum T4. During the first 2 wk period of hypocaloric feeding, there was a statistically significant increase in QKd, and a decrease in pulse rate, compatible with a hypothyroid state relative to initial measurements. When oral T3 supplementation was given, the rise in QKd and fall in pulse rate were prevented. Likewise, with oral T4 supplementation, the changes in QKd and pulse were prevented. Thus, the fall in serum T3 occurring during hypocaloric feeding is associated with changes in the cardiovascular system which are qualitatively similar to those observed during hypothyroidism. The present data, taken with other data in the literature, suggest that the decline in serum T3 during hypocaloric feeding may be an adaptive mechanism to conserve energy during caloric deprivation.

Conjugated HVA increase in rat urine after insulin-induced hypoglycemia: involvement of central dopaminergic structures but not of adrenal medulla

To examine the possible contribution of Rat adrenal medulla to urinary DOPAC and HVA, we have studied these compounds in adrenals and urine under insulin-induced (5 IU/kg) hypoglycemic stimulation. In the urine samples collected over 16-hour-period following the intravenous insulin injection, there was a great increase in E, NE and their methoxylated derivatives MN and NMN, without change in DA, DOPAC, MHPG and free HVA excretion. In addition, there was a pronounced increase in urinary HVA conjugates (glucuronide and sulfate). Only very low amounts of DOPAC (10 +/- 2 ng/gland; 0.05% of catechols) and no detectable amounts of HVA (less than 3 ng/gland) were found in adrenal glands, without no significant change two hours after insulin, thus suggesting that Rat adrenal glands are not meaningful sources for urinary HVA and DOPAC. Since free HVA and total MHPG excretion remained unchanged, HVA contribution from sympathetic neurons seems unlikely in our study. In contrast, highly increased levels of conjugated HVA--and at a lesser extent of conjugated DOPAC--have been found in the striatum, which appears to be the most likely source of urinary HVA increment. The dopaminergic activation following insulin affected too the hypothalamus but not the nucleus accumbens. The role of such central dopaminergic activation has been discussed in terms of feeding behavior.

Initiation, duration and dissipation of diet-induced changes in sympathetic nervous system activity in the rat

Previous studies in this laboratory have demonstrated that 48 hr of fasting suppresses and 72 hr of sucrose feeding (a model of voluntary overfeeding) stimulates sympathetic nervous system activity in rats. The experiments described here were undertaken to establish whether these diet-induced change in sympathetic activity occur in the early phases of a fast and whether they persist beyond a 3 day period of overfeeding. The results indicate that changes in cardiac norepinephrine turnover can be detected during the first 24 hr of fasting or overfeeding, and that the increase in cardiac norepinephrine turnover induced by sucrose overfeeding is sustained over 8 days of sucrose administration.

Effect of concomitant fasting and cold exposure on sympathoadrenal activity in rats

Fasting decreases and cold exposure increases sympathetic nervous system activity. The present studies examine the effect of fasting and cold together on sympathoadrenal function. At 4 degrees C animals fasted for 2 days excreted 29% less norepinephrine (NE) than fed animals, averaging 458 +/- 32ng NE/mg creatinine and 646 +/- 68, respectively (P less than 0.005), but 122% more epinephrine (E) 77.9 +/- 11.7 ng E/mg creatinine and 35.1 +/- 6.7, respectively (P less than .01). Fasting for 2 days reduced cardiac NE turnover, a direct measure of sympathetic neuronal activity, 33% in animals at 22 degrees C from 28.2 +/- 3.6 ng NE . heart-1 . h-1 to 18.9 +/- 4.8 (95% confidence intervals) and 25% in animals acutely exposed to 4 degrees C from 60.7 +/- 8.0 to 45.6 +/- 9.5. Similar reductions in urinary NE excretion and cardiac NE turnover were observed in adrenal-demedullated rats. Thus fasting at 4 degrees C lowers sympathetic activity and enhances adrenal medullary secretion. This pattern of decreased sympathetic and increased adrenal medullary activity, previously seen with fasting hypoglycemia, suggests an important role for the adrenal medulla in internal homeostasis at times when the sympathetic nervous system is suppressed.