Hypoglycemia per se stimulates sympathetic neural as well as adrenomedullary activity, but, unlike the adrenomedullary response, the forearm sympathetic neural response is not reduced after recent hypoglycemia

We tested the hypotheses that 1) hypoglycemia per se stimulates the sympathetic neural as well as the adrenomedullary component of the sympathochromaffin system, and 2) sympathetic neural responses to hypoglycemia, like adrenomedullary responses, are reduced after recent hypoglycemia. To this end, we studied 10 healthy young adults on 2 consecutive days on two separate occasions, on one occasion with euglycemia (5.0 mmol/l) and on the other occasion with hypoglycemia (2.8 mmol/l) from 1000 to 1200 and 1400 to 1600 on day 1 of each occasion. On day 2 of each occasion, plasma epinephrine and norepinephrine (NE) concentrations and rates of systemic NE spillover (SNESO) and forearm NE spillover (FNESO) were measured during hyperinsulinemic (12.0 pmol x kg(-1) x min(-1)) euglycemia (5.0 mmol/l) and hypoglycemia (2.8 mmol/l). Compared with values during euglycemia, plasma epinephrine and NE and rates of SNESO and FNESO all increased during hypoglycemia (P < 0.01). After day 1 hypoglycemia, there were reductions during hypoglycemia on day 2 in plasma epinephrine (2,050 +/- 500 vs. 2,960 +/- 400 pmol/l; P < 0.02), plasma NE (1.35 +/- 0.16 vs. 1.92 +/- 0.20 nmol/l; P < 0.01), and SNESO rates (5.13 +/- 0.84 vs. 6.87 +/- 0.81 nmol/min; P < 0.02). However, FNESO rates were unaltered (1.16 +/- 0.25 vs. 1.27 +/- 0.17 pmol x min(-1) x 100 ml tissue(-1). Thus we conclude that 1) hypoglycemia per se stimulates both the sympathetic neural and adrenomedullary components of the sympathochromaffin system and 2) adrenomedullary, but not forearm sympathetic neural, responses to hypoglycemia are reduced after recent hypoglycemia. The extent to which the lower plasma NE levels and reduced SNESO responses to hypoglycemia after day 1 hypoglycemia reflect reduced NE release from the adrenal medullae, sympathetic nerves other than those in the forearm, or both cannot be determined from these data.

Norepinephrine response to exercise at the same relative intensity before and after endurance exercise training

It is well documented that endurance exercise training results in a blunted norepinephrine (NE) response to exercise of a given absolute exercise intensity. However, it is not clear what effect training has on the catecholamine response to exercise of the same relative intensity because previous studies have provided conflicting results. The purpose of the present study was, therefore, to determine the catecholamine response to exercise of the same relative exercise intensity before and after endurance exercise training. Six women and three men [age 28 +/- 8 (SD) yr] performed 10 wk of training. Maximal O2 uptake (VO2 max) was determined during treadmill exercise. Fifteen-minute treadmill exercise bouts were performed at 60, 65, 70, 75, 80, and 85% of VO2 max before and after training. VO2 max was increased by 20% (from 39.2 +/- 7.7 to 46.9 +/- 8.1 ml. kg-1. min-1; P < 0.05) in response to training. Plasma NE concentrations were higher (P < 0.05) during exercise at the same relative intensity after, compared with before, training at 65-85% of VO2 max. Differences between heart rates and plasma epinephrine concentrations after, compared with before, training were not statistically significant. These results provide evidence that the NE response to exercise is dependent on the absolute as well as the relative intensity of the exercise.

Forearm norepinephrine spillover during standing, hyperinsulinemia, and hypoglycemia

Plasma norepinephrine (NE) concentrations are a fallible index of sympathetic neural activity because circulating NE can be derived from sympathetic nerves, the adrenal medullas, or both and because of regional differences in sympathetic neural activity. We used isotope dilution measurements of systemic and forearm NE spillover rates (SNESO and FNESO, respectively) to study the sympathochromaffin system during prolonged standing, hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia in healthy humans. Prolonged standing led to decrements in blood pressure without increments in heart rate, the pattern of incipient vasodepressor syncope. FNESO was not increased (0.58 +/- 0.20 to 0. 50 +/- 0.21 pmol. min-1. 100 ml tissue-1), suggesting that the approximately twofold increments in plasma NE and SNESO were derived from sympathetic nerves other than those in the forearm (with a possible contribution from the adrenal medullas). Hyperinsulinemia per se (euglycemia maintained) stimulated sympathetic neural activity, as evidenced by increments in FNESO (0.57 +/- 0.11 to 1.25 +/- 0.25 pmol. min-1. 100 ml tissue-1, P < 0.05), but not adrenomedullary activity. Hypoglycemia per se stimulated adrenomedullary activity (plasma epinephrine from 190 +/- 70 to 1720 +/- 320, pmol/l, P < 0.01). Although SNESO (P < 0.05) and perhaps plasma NE (P < 0.06) were raised to a greater extent during hyperinsulinemic hypoglycemia than during hyperinsulinemic euglycemia, FNESO was not. Thus these data do not provide direct support for the concept that hypoglycemia per se also stimulates sympathetic neural activity.

Absence of the dawn phenomenon and abnormal lipolysis in type 1 (insulin-dependent) diabetic patients with chronic growth hormone deficiency

To determine the role of growth hormone in overnight insulin requirements and lipolysis, five patients with chronic growth hormone deficiency and Type 1 (insulin-dependent) diabetes mellitus and six control patients with diabetes were each studied on two separate nights. Insulin was infused at a variable rate throughout one night to maintain euglycaemia and fixed at 04.00 hours on another. During the variable infusion, euglycaemia was maintained in control patients by a 36% increase in insulin infusion rate between 03.00 and 08.00 hours while a 46% decrease in the rate was required in growth hormone deficient patients (p less than 0.02). Despite this difference, mean free insulin values were equivalent. This finding is suggestive of increased insulin clearance in growth hormone sufficient patients. Glucose levels rose in control and fell in growth hormone deficient patients when insulin infusion rates were fixed at 04.00 hours. Glycerol production and non-esterified fatty acid concentrations were significantly lower in the growth hormone deficient diabetic patients, p less than 0.001, and when normalized with a heparin infusion, had no effect on insulin requirements. We conclude that: (1) growth hormone contributes to the development of the "dawn phenomenon," possibly by increasing insulin clearance (2) growth hormone helps sustain nocturnal lipolysis in Type 1 diabetes and (3) non-esterified fatty acids are not involved in the dawn phenomenon.

Plasma chromogranin A as a marker of sympathochromaffin activity in humans

The extent to which the sympathochromaffin system compared with other endocrine/neuroendocrine tissues contributes to the plasma chromogranin A pool has not been defined. To test the hypothesis that the sympathochromaffin system is the major source of circulating chromogranin A only when that system is activated markedly, we measured chromogranin A concentrations in 200 human plasma samples known to have a broad range of norepinephrine and epinephrine concentrations, reflecting therefore a broad range of sympathochromaffin activity at the time of sampling. Plasma chromogranin A and norepinephrine concentrations were highly correlated when the sympathochromaffin system was activated markedly (cardiac arrest samples, n = 13, r = 0.8392, P less than 0.0005) and when there was release of large amounts of norepinephrine from tumors (pheochromocytoma samples, n = 17, r = 0.8132, P less than 0.001). However, when the sympathochromaffin system was activated less markedly, resulting in plasma catecholamine concentrations that spanned the physiological and lower pathophysiological range (nonpheochromocytoma noncardiac arrest samples, n = 170), correlations between plasma chromogranin A and norepinephrine (r = 0.2877, P less than 0.0001) and epinephrine (r = 0.3814, P less than 0.0001) levels were relatively weak, although still statistically significant. Thus, at basal through moderate stress levels, norepinephrine and epinephrine concentrations accounted for only approximately 10-15% of the variance in plasma chromogranin A levels. We conclude that, although plasma chromogranin A concentrations are a valid marker of sympathochromaffin activity in humans, they are not a sensitive marker under physiological conditions.

Failure of nocturnal hypoglycemia to cause daytime hyperglycemia in patients with IDDM

To test the hypothesis that nocturnal hypoglycemia causes postprandial hyperglycemia the next day (the Somogyi phenomenon) in patients with insulin-dependent diabetes mellitus (IDDM), we studied 10 moderately well controlled patients, who were on their usual therapeutic regimens, from 2000 to 2000 on three occasions. On a control day, samples were obtained without intervention. On another day, nocturnal hypoglycemia was prevented (by intravenous infusion of glucose, if necessary, from 2200 to 0400 to keep plasma glucose levels at greater than 5.6 mM). On another day, nocturnal hypoglycemia was induced (by stepped intravenous insulin infusions between 2200 and 0200 to reduce plasma glucose levels to less than 2.8 mM). After nocturnal hypoglycemia (1.9 +/- 0.2 mM), fasting (0800), morning (0800-1100), afternoon (1200-1500), evening (1600-2000), and entire-day (0800-2000) plasma glucose concentrations were no higher than those after prevention of nocturnal hypoglycemia or sampling only. On the control day, fasting and daytime plasma glucose levels were directly related to the preceding 2200 (r = 0.723, P less than 0.02, and r = 0.762, P = 0.01, respectively) and nocturnal nadir (r = 0.714, P less than 0.02, and r = 0.728, P less than 0.02) plasma glucose concentrations. Daytime plasma glucose levels were unrelated to peak nocturnal plasma glucagon, epinephrine, norepinephrine, growth hormone, or cortisol concentrations. We conclude that nocturnal hypoglycemia does not appear to cause clinically important daytime hyperglycemia in patients representative of most patients with IDDM.

Human tissue adrenergic receptors are not predictive of responses to epinephrine in vivo

To test the hypotheses that adrenergic receptor and adenylate cyclase characteristics of easily accessible circulating cells reflect those of relatively inaccessible extravascular catecholamine target tissues in a subtype-specific fashion and that these characteristics predict responses to catecholamines in vivo, we studied 22 normal humans. Adrenergic receptors and their linked adenylate cyclase systems were measured in mononuclear leukocytes (MNL; beta 2), platelets (alpha 2), skeletal muscle membranes (beta 2), and fat cells (B1 and alpha 2) and compared with the responses to stepped, intravenous epinephrine infusions in vivo. MNL beta 2-adrenergic receptor densities (but not antagonist affinities) were correlated (r = 0.627; P less than 0.01) with skeletal muscle beta 2-adrenergic densities. However, other adrenergic receptor characteristics and basal and maximally stimulated adenosine 3',5'-cyclic monophosphate (cAMP) contents of MNL and all adrenergic receptor characteristics and cAMP contents of platelets were unrelated to the corresponding measurements in skeletal muscle and fat. Furthermore, there were no consistent relationships between tissue adrenergic receptor-adenylate cyclase characteristics and the chronotropic, diastolic depressor, lipolytic, ketogenic, glycemic, or glycogenolytic-glycolytic responses to epinephrine in vivo. Thus the data support the hypothesis that adrenergic receptor densities on circulating cells reflect those of extravascular target tissues in a subtype-specific fashion. On the other hand, the data do not support the hypothesis that physiological interindividual variation of adrenergic receptor characteristics is of sufficient magnitude to alter sensitivity to epinephrine in vivo.

Insulin, glucagon, and catecholamines in prevention of hypoglycemia during fasting

To dissect the mechanisms of the prevention of hypoglycemia during fasting, eight normal humans were studied after overnight and 3-day fasts. Prolonged fasting resulted in the expected decrements in base-line glucose production and plasma glucose, insulin, and C-peptide and increments in plasma glucagon, epinephrine, norepinephrine, growth hormone, and cortisol. After the overnight and 3-day fasts, insulin restoration (0.2 mU.kg-1.min-1) alone resulted in transient decrements in glucose production and only 15 and 19% decrements in plasma glucose, respectively. Selective glucagon deficiency (somatostatin infusion with insulin and growth hormone replacement) resulted in transient decrements in glucose production and additional 24 and 29% decrements in plasma glucose, respectively. Notably, plasma glucose plateaued under both fasting conditions in both instances. Combined alpha- and beta-adrenergic blockade (phentolamine and propranolol infusions) alone had no effect on glycemia under either fasting condition. However, progressive hypoglycemia developed during adrenergic blockade coupled with glucagon deficiency after the overnight fast (85 +/- 2 to 48 +/- 4 mg/dl, P less than 0.001) and after the 3-day fast (65 +/- 2 to 33 +/- 1 mg/dl, P less than 0.001). These were the result of both decrements in glucose production and increments in glucose clearance. Thus we conclude that during fasting 1) the prevention of hypoglycemia is not due solely to decreased insulin secretion. 2) Glucagon plays a primary counterregulatory role. Sympathochromaffin catecholamines are not normally critical but compensate and become critical when glucagon is deficient. Adrenomedullary epinephrine is probably the relevant catecholamine. 3) Other hormones, neurotransmitters, or substrate effects may, or may not, be involved; if they are, they appear to stand low in the hierarchy of glucoregulatory factors.

Increased fat and skeletal muscle beta-adrenergic receptors but unaltered metabolic and hemodynamic sensitivity to epinephrine in vivo in experimental human thyrotoxicosis

Based largely on evidence of increased target tissue beta-adrenergic receptor densities and responsiveness in animal and, to a lesser extent, human tissues, it is often assumed that thyroid hormone excess results in increased sensitivity to catecholamines in vivo, thus explaining several clinical manifestations of thyrotoxicosis. To test the hypothesis that thyrotoxicosis results in increased target tissue beta-adrenergic receptor densities and correspondingly increased metabolic and hemodynamic sensitivity to epinephrine in vivo, we measured these in 10 normal humans before and after administration of triiodothyronine (100 micrograms daily) for 10 d. Thyrotoxicosis increased beta-adrenergic receptor densities in fat (approximately 60%) and skeletal muscle (approximately 30%). Despite increments in beta-adrenergic receptor densities in these and probably other target tissues, metabolic and hemodynamic sensitivity to epinephrine in vivo was unaltered. An apparently adaptive increase in insulin secretion plausibly explains normal glycemic, glycogenolytic/glycolytic, lipolytic, and ketogenic sensitivity to epinephrine in the thyrotoxic state. In view of this striking homeostatic efficiency of the intact individual, the finding of altered adrenergic receptors, even in relevant target tissues, should not be extrapolated to altered sensitivity to catecholamines in vivo in the absence of direct testing of that hypothesis. With respect to the clinical issue, these data suggest that increased sensitivity to catecholamines does not explain clinical manifestations of thyrotoxicosis in humans.

Direct muscarinic cholinergic inhibition of hepatic glucose production in humans

To explore the potential role of the parasympathetic nervous system in human glucoregulatory physiology, responses to the muscarinic cholinergic agonist bethanechol (5.0 mg s.c.) and antagonist atropine (1.0 mg i.v.) were measured in normal humans. There were no changes in the plasma glucose concentration or rates of glucose production or utilization following atropine administration. After bethanechol administration there were no changes in the plasma glucose concentration or fluxes despite increments in plasma glucagon (75 +/- 7 to 103 +/- 10 pg/ml, P less than 0.02). There were no changes in insulin or C-peptide levels. To test the hypothesis that direct muscarinic inhibition of glucose production was offset by an indirect action of the agonist, specifically increased glucagon secretion with consequent stimulation of glucose production, bethanechol was administered while glucagon levels were held constant with the islet clamp technique (somatostatin infusion with insulin, glucagon and growth hormone replacement at fixed rates). Under that condition the muscarinic agonist induced a 25% decrement in the plasma glucose concentration (101 +/- 8 to 75 +/- 8 mg/dl, P less than 0.05). When compared with separate clamp control studies (with placebo rather than bethanechol injection) both the rate of glucose production and the glucose concentration were reduced (P less than 0.05) following bethanechol injection; the rate of glucose utilization was unaltered. Thus, we conclude: Withdrawal of parasympathetic tone does not appear to be an important glucoregulatory process in humans. Direct muscarinic cholinergic inhibition of hepatic glucose production occurs in humans but during generalized muscarinic activation this is offset by an indirect muscarinic action, increased glucagon secretion with consequent stimulation of glucose production. Thus, particularly if regional neuronal firing occurs, the parasympathetic nervous system may play an important role in human glucoregulatory physiology.

Direct relationship between mononuclear leukocyte and lung beta-adrenergic receptors and apparent reciprocal regulation of extravascular, but not intravascular, alpha- and beta-adrenergic receptors by the sympathochromaffin system in humans

To examine putative relationships between adrenergic receptors on accessible circulating cells and relatively inaccessible extravascular catecholamine target tissues, we measured mononuclear leukocyte (MNL) and lung beta-adrenergic receptors and platelet and lung alpha-adrenergic receptors in tissues obtained from 15 patients undergoing pulmonary resection. Plasma catecholamine concentrations were measured concurrently to explore potential regulatory relationships between the activity of the sympathochromaffin system and both intravascular and extravascular adrenergic receptors. MNL and lung membrane beta-adrenergic receptor densities were correlated highly (r = 0.845, P less than 0.001). Platelet alpha 2-adrenergic receptor and lung alpha 1-adrenergic receptor densities were not. Lung alpha 1-adrenergic receptor densities were positively related to plasma norepinephrine (r = 0.840, P less than 0.01) and epinephrine (r = 0.860, P less than 0.01) concentrations; in contrast, lung beta-adrenergic receptor densities were not positively related to plasma catecholamine concentrations (they tended to be inversely related to plasma norepinephrine and epinephrine [r = -0.698, P less than 0.05] levels). This apparent reciprocal regulation of alpha- and beta-adrenergic receptors by the sympathochromaffin system was only demonstrable with adrenergic receptor measurements in the extravascular catecholamine target tissue. Neither MNL beta-adrenergic receptor nor platelet alpha-adrenergic receptor densities were correlated with plasma catecholamine levels. Thus, although measurements of beta-adrenergic receptors on circulating mononuclear leukocytes can be used as indices of extravascular target tissue beta-adrenergic receptor densities (at least in lung and heart), it would appear that extravascular tissues should be used to study adrenergic receptor regulation by endogenous catecholamines in humans. These data provide further support for the concept of up regulation, as well as down regulation, of some adrenergic receptor populations during short-term activation of the sympathochromaffin system in humans.

Plasma glucose concentrations at the onset of hypoglycemic symptoms in patients with poorly controlled diabetes and in nondiabetics

We tested the hypothesis that during decrements in plasma glucose concentration, symptoms of hypoglycemia may occur at higher glucose concentrations in patients with poorly controlled insulin-dependent diabetes mellitus than in persons without diabetes. Symptoms of hypoglycemia and counterregulatory neuroendocrine responses were quantified during hypoglycemic and euglycemic clamp studies in eight patients with insulin-dependent diabetes mellitus selected because their hemoglobin A1 levels were above 10 percent. These data were compared with similar observations in 10 nondiabetic subjects studied previously. Glycemic thresholds--the plasma glucose concentrations during each hypoglycemic clamp study at which a given symptom or biochemical measurement first exceeded its 95 percent confidence interval determined in the euglycemic clamp studies--were calculated for each variable. The mean (+/- SE) glycemic threshold for the symptoms of hypoglycemia was 4.3 +/- 0.3 mmol per liter (78 +/- 5 mg per deciliter) in patients with poorly controlled diabetes--significantly higher (P less than 0.001) than the value of 2.9 +/- 0.1 mmol per liter (53 +/- 2 mg per deciliter) in subjects without diabetes. The mean glycemic thresholds for growth hormone, epinephrine, and cortisol secretions were not significantly different in the two groups. Thus, during decreases in the plasma glucose concentration, patients with poorly controlled insulin-dependent diabetes mellitus may experience symptoms of hypoglycemia at higher plasma glucose concentrations than persons without diabetes. The mechanism underlying this observation remains to be defined.

Glucagon, not insulin, may play a secondary role in defense against hypoglycemia during exercise

The sympathochromaffin system, probably sympathetic neural norepinephrine, plays a primary role in the prevention of hypoglycemia during exercise in humans. Our previous data indicated that changes in pancreatic islet hormones are not normally critical but decrements in insulin, increments in glucagon, or both become critical when catecholamine actions are blocked pharmacologically. To distinguish between the role of insulin and that of glucagon in this secondary line of defense against hypoglycemia during exercise in humans, glucoregulation during moderate exercise (approximately 55% of maximum O2 consumption over 60 min) was studied in people who could not decrease insulin but could increase glucagon, i.e., patients with insulin-dependent diabetes mellitus (IDDM). While receiving constant intravenous infusions of regular insulin, in individualized doses shown to result in stable plasma glucose concentrations of approximately 95 mg/dl before exercise, patients with IDDM were studied under two conditions: 1) a control study (n = 13) and 2) an adrenergic blockade study (propranolol infusion, n = 8). In the control study, mean plasma glucose concentrations did not change (from 95 +/- 2 to 100 +/- 11 mg/dl) during exercise despite constant plasma free insulin levels. In the adrenergic blockade study plasma glucose declined (from 96 +/- 2 to 74 +/- 7 mg/dl, P less than 0.01) but stabilized; hypoglycemia did not occur. Exercise-associated increments in plasma glucagon were comparable in the two studies. These data confirm that decrements in insulin are not critical to the prevention of hypoglycemia during moderate exercise in humans and indicate that compensation for deficient catecholamine action does not require decrements in insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of beta-adrenergic receptors of human skeletal muscle obtained by needle biopsy

Human skeletal muscle beta-adrenergic receptors were characterized by 125I-iodopindolol radioligand-binding studies of homogenates prepared from small muscle samples obtained by percutaneous needle biopsy from the gastrocnemius of six normal subjects. Binding was saturable, reversible, and stereospecific, with typical kinetics and a rank-order potency characteristic of a beta-adrenergic receptor. In saturation-binding studies, the receptor density was 9.7 +/- 1.9 fmol/mg protein, with a dissociation constant of 24 +/- 2.2 pM. Competition studies with selective antagonists revealed a population of receptors exclusively of the beta 2-subtype. Basal and isoproterenol-stimulated adenylate cyclase activities were 79 +/- 22 and 150 +/- 60 pmol adenosine 3',5'-cyclic monophosphate.min-1.mg protein-1, respectively. These results support pharmacological observations of beta-adrenergic receptor-mediated cellular responses in mammalian skeletal muscle. By use of these methods, small quantities of skeletal muscle obtained in this manner can be used to study in vivo beta-adrenergic receptor regulatory phenomena in humans.

Delayed extra-adrenal epinephrine secretion after bilateral adrenalectomy in rats

Regulated systemic extra-adrenal epinephrine secretion has been demonstrated in long-term bilaterally adrenalectomized humans. To determine whether this is demonstrable immediately after adrenalectomy and therefore presumably ongoing when the adrenal medullas are intact or if it develops over time after the adrenal medullas are removed, we measured plasma catecholamine concentrations before and serially after bilateral adrenalectomy with cortical reimplantation in rats. We found plasma epinephrine concentrations to decrease from 244 +/- 41 pg/ml to levels that were not convincingly detectable, using a single-isotope derivative assay with a detection limit of 10 pg/ml, for up to 1 wk after bilateral adrenalectomy with cortical reimplantation. Plasma epinephrine concentrations increased thereafter, becoming detectable in all animals and averaging 31 +/- 6 pg/ml 4 wk after adrenalectomy. Thus extra-adrenal epinephrine secretion appears to be a delayed response to removal of the adrenal medullas and cannot be assumed to be ongoing when the adrenal medullas are intact.

Glycemic thresholds for activation of glucose counterregulatory systems are higher than the threshold for symptoms

To define glycemic thresholds for activation of glucose counterregulatory systems and for symptoms of hypoglycemia, we measured these during stepped reductions in the plasma glucose concentration (in six 10-mg/dl hourly steps) from 90 to 40 mg/dl under hyperinsulinemic clamp conditions, and compared these with the same measurements during euglycemia (90 mg/dl) under the same conditions over 6 h in 10 normal humans. Arterialized venous plasma glucose concentrations were used to calculate glycemic thresholds of 69 +/- 2 mg/dl for epinephrine secretion, 68 +/- 2 mg/dl for glucagon secretion, 66 +/- 2 mg/dl for growth hormone secretion, and 58 +/- 3 mg/dl for cortisol secretion. In contrast, the glycemic threshold for symptoms was 53 +/- 2 mg/dl, significantly lower than the thresholds for epinephrine (P less than 0.001), glucagon (P less than 0.001), and growth hormone (P less than 0.01) secretion. Thus, the glycemic thresholds for activation of glucose counterregulatory systems during decrements in plasma glucose lie within or just below the physiologic plasma glucose concentration range, and are substantially higher than the threshold for hypoglycemic symptoms in normal humans. These findings provide further support for the concept that glucose counterregulatory systems are involved in the prevention, as well as the correction, of hypoglycemia.

Epinephrine is not critical to prevention of hypoglycemia during exercise in humans

We documented stability of plasma glucose concentrations and glucose production and utilization rates, and levels of other metabolic substrates and regulatory factors, during the islet clamp (somatostatin infusion with glucagon and insulin replacement) in the absence of an intervention in five normal humans and further applied this technique to the study of glucoregulation during moderate exercise. Based on previous evidence that sympathochromaffin activation plays a primary role in the prevention of hypoglycemia during exercise, the role of adrenomedullary catecholamines was assessed by exercise (60% of maximum oxygen consumption for 60 min) studies in four bilaterally adrenalectomized, epinephrine-deficient humans under two conditions: control (saline infusion) and islet clamp. Increased glucose utilization and production rates were matched and plasma glucose was unchanged during exercise under both conditions. Thus adrenomedullary catecholamines including epinephrine are not critical to glucoregulation during moderate exercise in humans even when changes in insulin and glucagon are prevented. These findings provide further support for the suggestion that sympathetic neural norepinephrine is the operative catecholamine in the prevention of hypoglycemia during exercise in humans.

Glucoregulation during exercise: hypoglycemia is prevented by redundant glucoregulatory systems, sympathochromaffin activation, and changes in islet hormone secretion

During mild or moderate nonexhausting exercise, glucose utilization increases sharply but is normally matched by increased glucose production such that hypoglycemia does not occur. To test the hypothesis that redundant glucoregulatory systems including sympathochromaffin activation and changes in pancreatic islet hormone secretion underlie this precise matching, eight young adults exercised at 55-60% of maximal oxygen consumption for 60 min on separate occasions under four conditions: (a) control study (saline infusion); (b) islet clamp study (insulin and glucagon held constant by somatostatin infusion with glucagon and insulin replacement at fixed rates before, during and after exercise with insulin doses determined individually and shown to produce normal and stable plasma glucose concentrations prior to each study); (c) adrenergic blockage study (infusions of the alpha- and beta-adrenergic antagonists phentolamine and propranolol); (d) adrenergic blockade plus islet clamp study. Glucose production matched increased glucose utilization during exercise in the control study and plasma glucose did not fall (92 +/- 1 mg/dl at base line, 90 +/- 2 mg/dl at the end of exercise). Plasma glucose also did not fall during exercise when changes in insulin and glucagon were prevented in the islet clamp study. In the adrenergic blockade study, plasma glucose declined initially during exercise because of a greater initial increase in glucose utilization, then plateaued with an end-exercise value of 74 +/- 3 mg/dl (P less than 0.01 vs. control). In contrast, in the adrenergic blockade plus islet clamp study, exercise was associated with glucose production substantially lower than control and plasma glucose fell progressively to 58 +/- 7 mg/dl (P less than 0.001); end-exercise plasma glucose concentrations ranged from 34 to 72 mg/dl. Thus, we conclude that: (a) redundant glucoregulatory systems are involved in the precise matching of increased glucose utilization and glucose production that normally prevents hypoglycemia during moderate exercise in humans. (b) Sympathochromaffin activation, perhaps sympathetic neural norepinephrine release, plays a primary glucoregulatory role by limiting glucose utilization as well as stimulating glucose production. (c) Changes in pancreatic islet hormone secretion (decrements in insulin, increments in glucagon, or both) are not normally critical but become critical when catecholamine action is deficient. (d) Glucoregulation fails, and hypoglycemia can develop, both when catecholamine action is deficient and when changes in islet hormones do not occur during exercise in humans.

External and internal standards in the single-isotope derivative (radioenzymatic) measurement of plasma norepinephrine and epinephrine

In plasma from normal humans (n = 9, 35 samples) and from patients with diabetes mellitus (n = 12, 24 samples) single-isotope derivative (radioenzymatic) plasma norepinephrine and epinephrine concentrations calculated from external standard curves constructed in a normal plasma pool were identical to those calculated from internal standards added to an aliquot of each plasma sample. In plasma from patients with end-stage renal failure receiving long-term dialysis (n = 34, 109 samples), competitive catechol-O-methyltransferase (COMT) inhibitory activity resulted in a systematic error when external standards in a normal plasma pool were used, as reported previously; values so calculated averaged 21% (+/- 12%, SD) lower than those calculated from internal standards. However, when external standard curves were constructed in plasma from a given patient with renal failure and used to calculate that patient's values, or in a renal failure plasma pool and used to calculate all renal failure values, norepinephrine and epinephrine concentrations were not significantly different from those calculated from internal standards. We conclude: (1) External standard curves constructed in plasma from a given patient with renal failure can be used to measure norepinephrine and epinephrine in plasma from that patient; further, external standards in a renal failure plasma pool can be used for assays in patients with end-stage renal failure receiving long-term dialysis. (2) Major COMT inhibitory activity is not present commonly if samples from patients with renal failure are excluded. Thus, it would appear that external standard curves constructed in normal plasma can be used to measure norepinephrine and epinephrine precisely in samples from persons who do not have renal failure.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The human sympathochromaffin system

Hypoglycemia stimulates adrenomedullary epinephrine secretion; standing stimulates sympathetic neural norepinephrine release. In five bilaterally adrenalectomized persons plasma epinephrine, measured with a sensitive single-isotope derivative assay, rose from 15 +/- 2 to 35 +/- 7 pg/ml (P less than 0.02) during hypoglycemia but did not increase during standing. In contrast, plasma norepinephrine rose during standing but not during hypoglycemia. Thus, in humans 1) extra-adrenal epinephrine secretion is regulated and derived from innervated cells other than sympathetic postganglionic neurons; 2) because the plasma levels of epinephrine in adrenalectomized individuals even in response to the potent stimulus of hypoglycemia are below physiological thresholds, any biological actions of extra-adrenal epinephrine in adults must be paracrine rather than endocrine in nature; 3) hypoglycemia does not appear to stimulate the sympathetic nervous system. In view of these findings, we propose that extra-CNS catecholamine-producing tissues be termed the sympathochromaffin system consisting of two components: 1) the sympathetic nervous system that releases the neurotransmitter norepinephrine from its postganglionic neurons, and 2) the chromaffin tissues, including the adrenal medullae, that contain cells that secrete epinephrine, norepinephrine, or dopamine. The plasma epinephrine concentration is a valid measure of its chromaffin tissue (predominantly adrenomedullary) secretion, whereas the plasma norepinephrine concentration is an index of sympathetic neuronal activity under some but not all conditions.

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.

beta 2- and alpha 2-adrenergic receptors and receptor coupling to adenylate cyclase in human mononuclear leukocytes and platelets in relation to physiological variations of sex steroids

In view of evidence, largely in animals, indicating effects of sex steroids on adrenergic receptors, we measured mononuclear leukocyte (MNL) beta 2-adrenergic receptors and adenylate cyclase sensitivity to stimulation by isoproterenol as well as platelet alpha 2-adrenergic receptors and sensitivity of sodium fluoride-stimulated adenylate cyclase to inhibition by epinephrine in 3 groups of normal humans with physiologically disparate levels of testosterone, estradiol, and progesterone (10 normal men and 10 normal women, the latter sampled in both the follicular and luteal phases of their menstrual cycles). Differences in testosterone, estradiol, and progesterone were as expected; testosterone levels were 10-fold higher in men, and progesterone levels were 20-fold higher in luteal phase women. T4, cortisol , and norepinephrine levels did not differ. Basal plasma epinephrine concentrations were slightly but significantly higher in luteal phase women [34 +/- 5 (+/-SE) pg/ml] than in follicular phase women (16 +/- 3 pg/ml; P less than 0.01) or men (20 +/- 3 pg/ml; P less than 0.05). There were no significant differences among these 3 groups in the densities or affinities of MNL beta 2-adrenergic or platelet alpha 2-adrenergic receptors or in the corresponding MNL and platelet adenylate cyclase sensitivities. Thus, there is not a generalized effect of physiological variations of testosterone, estradiol, and progesterone on adrenergic receptors or adenylate cyclase. To the extent that the adrenergic receptors and adenylate cyclase activities of circulating cells reflect those of extravascular catecholamine target cells, these data provide no support for a role of physiological variations of testosterone, estradiol, or progesterone in the regulation of catecholamine action in humans.

Oral propranolol and metoprolol both impair glucose recovery from insulin-induced hypoglycemia in insulin-dependent diabetes mellitus

To the extent that they have deficient glucagon secretory responses to plasma glucose decrements, as they commonly do, patients with insulin-dependent diabetes mellitus (IDDM) are dependent on epinephrine-mediated beta-adrenergic mechanisms to promote recovery from hypoglycemia. Thus, they are at increased risk for prolonged hypoglycemia if treated with a nonselective beta-adrenergic antagonist such as propranolol. If the hyperglycemic actions of epinephrine are mediated through beta 2-adrenergic mechanisms, therapeutic efficacy (e.g., for hypertension or ischemic heart disease) could be accomplished without increased risk of hypoglycemia by selective beta 1-adrenergic blockade in such patients. However, oral administration of the relatively selective beta 1-adrenergic antagonist metoprolol (100 mg) and of the nonselective beta-adrenergic antagonist propranolol (80 mg) both impaired recovery from insulin-induced hypoglycemia in patients with IDDM. Thus, at a dose of 100 mg, oral metoprolol is not safer than oral propranolol with respect to recovery from hypoglycemia in patients with IDDM.

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.