William E. Clutter

Epinephrine plasma metabolic clearance rates and physiologic thresholds for metabolic and hemodynamic actions in man.

To determine the plasma epinephrine thresholds for its metabolic and hemodynamic actions and plasma epinephrine metabolic clearance rates, 60-min intravenous epinephrine infusions at nominal rates of 0.1, 0.5, 1.0, 2.5, and 5.0 microgram/min were performed in each of six normal human subjects. These 30 infusions resulted in steady-state plasma epinephrine concentrations ranging from 24 to 1,020 pg/ml. Plasma epinephrine thresholds were 50-100 pg/ml for increments in heart rate, 75-125 pg/ml for increments in blood glycerol and systolic blood pressure, 150-200 pg/ml for increments in plasma glucose (the resultant of increments in glucose production and decrements in glucose clearance), blood lactate, blood beta-hydroxybutyrate, and diastolic blood pressure, and greater than 400 pg/ml for early decrements in plasma insulin. Changes in blood alanine, plasma glucagon, plasma growth hormone, and plasma cortisol were not detected. At steady-state plasma epinephrine concentrations of 24-74 pg/ml, values overlapping the basal normal range, the mean (+/-SE) plasma metabolic clearance rate of epinephrine was 52 +/- 4 ml x min-1 x kg-1; this value rose to 89 +/- 6 ml x min-1 x kg-1 (P less than 0.01) at steady-state epinephrine concentrations of 90-1,020 pg/ml. We conclude that in human subjects: (a) the plasma epinephrine thresholds for its hemodynamic and metabolic actions lie within the physiologic range, (b) epinephrine and norepinephrine accelerate their own metabolic clearance, and (c) epinephrine is 10 times more potent than norepinephrine.

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.

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.

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

To determine the plasma epinephrine thresholds for its lipolytic effect, 60-min epinephrine infusions at nominal rates of 0.1, 0.5, 1.0, 2.5, and 5.0 micrograms/min were performed in each of four normal young adult men while they also received a simultaneous infusion of [1-13C]palmitic acid to estimate inflow transport of plasma free fatty acids. These 20 infusions resulted in steady-state plasma epinephrine concentrations ranging from 12 to 870 pg/ml. Plasma epinephrine thresholds for changes in blood glucose, lactate, and beta-hydroxybutyrate were in the 150--200-pg/ml range reported by us previously (Clutter, W. E., D. M. Bier, S. D. Shah, and P. E. Cryer. 1980. J. Clin. Invest. 66: 94--101.). Increments in plasma glycerol and free fatty acids and in the inflow and outflow transport of palmitate, however, occurred at lower plasma epinephrine thresholds in the range of 75 to 125 pg/ml. Palmitate clearance was unaffected at any steady-state epinephrine level produced. These data indicate that (a) the lipolytic effects of epinephrine occur at plasma levels approximately threefold basal values and (b) lipolysis is more sensitive than glycogenolysis to increments in plasma epinephrine.

Triiodothyronine-induced Thyrotoxicosis Increases Mononuclear Leukocyte β-Adrenergic Receptor Density in Man

beta-Adrenergic receptors are increased in some tissues of experimentally thyrotoxic animals but are reported to be unchanged in mononuclear leukocytes of spontaneously thyrotoxic humans. We examined the effects of triiodothyronine (100 mug/d for 7 d) and placebo on high-affinity mononuclear leukocyte beta-adrenergic receptors in 24 normal human subjects, using a double-blind design. beta-Adrenergic receptors were assessed by specific binding of the antagonist (-)[(3)H]dihydroalprenolol. Triiodothyronine administration resulted in objective evidence of moderate thyrotoxicosis and an increase in mean (-)[(3)H]dihydroalprenolol binding from 25+/-3 to 57+/-9 fmol/mg protein (P < 0.001). The latter was attributable, by Scatchard analysis, to an increase in beta-adrenergic receptor density (967 +/- 134 to 2250 +/- 387 sites per cell, P < 0.01); apparent dissociation constants did not change. Placebo administration had no effects. Marked inter- and intraindividual variation in mononuclear leukocyte beta-adrenergic receptor density was also noted. Because this was approximately threefold greater than analytical variation, it is largely attributable to biologic variation. Thus, we conclude: (a) The finding of a triiodothyronine-induced increase in mononuclear leukocyte beta-adrenergic receptor density in human mononuclear leukocytes, coupled with similar findings in tissues of experimentally thyrotoxic animals, provides support for the use of mononuclear leukocytes to assess receptor status in man. (b) There is considerable biologic variation in beta-adrenergic receptor density in man. (c) The findings of thyroid hormone-induced increments in beta-adrenergic receptor density provide a plausible mechanism for the putative enhanced responsiveness to endogenous catecholamines of patients with thyrotoxicosis.

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.

Attenuated Glucose Recovery from Hypoglycemia in the Elderly

Advanced age is a risk factor for hypoglycemia caused by sulfonylureas (and insulin) used to treat diabetes mellitus. Therefore, we hypothesized that there is an age-associated impairment of glucose counterregulation and further that this is the result of a sedentary life-style. To test these hypotheses, glycemic and neuroendocrine responses to hypoglycemia, produced by 0.05 U/kg body wt insulin i.v. were measured in nondiabetic elderly subjects (age 65.1 ± 0.9 yr n = 23)–and in a subset (n = 11) again after 1 yr of physical training (which increased VO2 max by 5.2 ± 0.9 ml · kg−1 · min−1 P < 0.05)–and compared with these responses in nondiabetic young subjects (23.8 ± 0.6 yr, n = 18). Recovery from hypoglycemia was attenuated (analysis of variance P < 0.001) in the elderly (plasma glucose recovery rate 29.4 ± 2.2 vs. 42.7 ± 5.0 μM/min, P < 0.02). This attenuation was the result of a smaller counterregulatory increment in glucose production (maximum increment 13.3 ± 1.1 vs. 17.2 ± 1.1 μmol · kg−1 · min−1; P < 0.05) rather than a greater increment in glucose utilization in the elderly. The attenuated glucose recovery was associated with higher plasma insulin concentrations (maximum increment 1385 ± 122 vs. 940 ± 72 pM, P < 0.01) and reduced glucagon responses to hypoglycemia (maximum increment 43 ± 6 vs. 66 ± 12 ng/L). The epinephrine, norepinephrine, cortisol, and growth hormone responses were similar, although the epinephrine response was slightly delayed and the growth hormone response appeared smaller in the elderly. Training had no effect on glucose recovery from or neuroendocrine responses to hypoglycemia in the elderly. Thus, we conclude that there is an age-associated impairment of glucose counterregulation best attributed to decreased insulin clearance, reduced glucagon secretion, or both. Delayed epinephrine secretion may also contribute. These are not the result of a sedentary life-style.

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.

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.

Mechanisms of postprandial glucose counterregulation in man. Physiologic roles of glucagon and epinephrine vis-a-vis insulin in the prevention of hypoglycemia late after glucose ingestion.

The transition from exogenous glucose delivery to endogenous glucose production late after glucose ingestion is not solely attributable to dissipation of insulin and, therefore, must also involve factors that actively raise the plasma glucose concentration--glucose counterregulatory factors. We have shown that the secretion of two of these, glucagon and epinephrine, is specific for glucose ingestion and temporally related to the glucose counterregulatory process. To determine the physiologic roles of glucagon and epinephrine in postprandial glucose counterregulation, we produced pharmacologic interventions that resulted in endogenous glucagon deficiency with and without exogenous glucagon replacement, adrenergic blockade, and adrenergic blockade coupled with glucagon deficiency starting 225 min after the ingestion of 75 g of glucose in normal subjects. Also, we assessed the effect of endogenous epinephrine deficiency alone and in combination with glucagon deficiency late after glucose ingestion in bilaterally adrenalectomized subjects. Glucagon deficiency resulted in nadir plasma glucose concentrations that were approximately 30% lower (P less than 0.01) than control values, but did not cause hypoglycemia late after glucose ingestion. This effect was prevented by glucagon replacement. Neither adrenergic blockade nor epinephrine deficiency alone impaired the glucose counterregulatory process. However, combined glucagon and epinephrine deficiencies resulted in a progressive fall in mean plasma glucose to a hypoglycemic level late after glucose ingestion; the final glucose concentration was 40% lower (P less than 0.02) than the control (epinephrine deficient) value in these patients, and was nearly 50% lower (P less than 0.001) than the control value and approximately 30% lower (P less than 0.05) than the glucagon-deficient value in normal subjects. We conclude (a) the transition from exogenous glucose delivery to endogenous glucose production late after glucose ingestion is the result of the coordinated diminution of insulin secretion and the resumption of glucagon secretion. (b) Epinephrine does not normally play a critical role in this process, but enhanced epinephrine secretion compensates largely and prevents hypoglycemia when glucagon secretion is deficient.