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Importance of Catecholamines in Defense against Insulin Hypoglycemia in Humans
Geremia B. Bolli Department of Internal Medicine and Endocrinological and Metabolic Sciences University of Perugia 06 I26 Perugia, Italy
Importance of Catecholamines in Defense against Insulin Hypoglycemia in Humans It has been suggested that, in defense against hypoglycemia in humans, secretion of glucagon plays the dominant counterregulatory role (1).According to the view, secretion of adrenaline is usually not critical as long as the responses of glucagon are appropriate, and it becomes important only when secretion of glucagon is prevented (1)or impaired, as in type I diabetes mellitus (2). These principles have recently been reexamined. A major criticism is that they have been derived from a model of acute hypoglycemia in which the i.v. insulin bolus produces a pharmacological increase in plasma insulin with a subsequently rapid decrease in plasma insulin over a few minutes' time (1).In common clinical situations, such as insulinoma, sulphonylurea-induced hypoglycemia, and insulin-dependent diabetes mellitus, hypoglycemia develops gradually, is less severe, and is reversed more slowly than that of the acute experimental models (1). We have developed a model of prolonged hypoglycemia that mimics the clinical situation better than the acute model (1).In this model, insulin is infused to slowly increase the plasma insulin concentration to a plateau in a physiological range (two- to threefold above baseline), and the plasma glucose concentration gradually decreases into the hypoglycemic range (50-60 mg'dl). Using this model, it has already been shown (1)that mechanisms that were not found to be important for counterregulation in the acute model, such as suppression of endogenous insulin secretion and glucose utilization, do play important counterregulatory role; (2) that growth hormone and cortisol, which were not considered as counterregulatory hormones in the acute model, do indeed play an important role in defense against clinical hypoglycemia; and ( 3 ) that neither the failure of glucagon to respond to hypoglycemia nor that of growth hormone or cortisol is fully compensated by a larger secretion of epinephrine and norepinephrine. The several differences in the counterregulatory mechanisms already observed between the acute (1)and the clinical model of hypoglycemia ( 3 )suggest that, in the clinical model, the secretion of epinephrine also may play a much more important counterregulatory role as compared with that envisaged on the basis of the acute hypoglycemia model. In this chapter, the contribution of the adrenergic mechanisms to counterregulation in an experimental model of prolonged hypoglycemia closely mimickAdvances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reprodnction in a n y form reserved. 1054-3589/98 $25.00
627
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ing the clinical situation will be reviewed. For this purpose, the pancreaticadrenocortical-pituitary clamp, a technique that allows one to examine the role of a single counterregulatory hormone without confounding changes in the plasma concentrations of other hormones, was used. In an initial series of studies to determine the overall counterregulatory contribution of adrenergic response to hypoglycemia, insulin (15 mU * m-2 * min-' for 12 hr) was infused subcutaneously either alone or in combination with propranolol and phentolamine ( 3 ) .Exogenous glucose was infused whenever needed in the a-,@blockade experiments to match studies for plasma glucose concentrations. In both studies, plasma insulin slowly increased to a plateau of approximately 25 pU/ml, while plasma glucose decreased to a plateau of approximately 50 mg/dl. However, during catecholamine blockade, hepatic glucose production increased less both in an early phase and throughout. In addition, after 5 hr, glucose utilization was less suppressed than in control experiments. However, plasma C-peptide was more suppressed and plasma growth hormone less stimulated when catecholamine receptors were not blocked. These studies provided the first evidence of an important counterregulatory role of catecholamines in defense against hypoglycemia in humans. Interestingly, the effect was evident in both an early and a late phase and located both at the hepatic and muscular level. Finally, the role of catecholamines in defense against hypoglycemia was so important that other counterregulatory hormones, including glucagon, did not substitute for blockade of catecholamine effects. However, these studies were not matched for plasma C-peptide (possible overestimation of the role of catecholamines because of greater portal plasma insulin concentrations during a,P blockade) nor for plasma growth hormone (which could have underestimated the role of catecholamines in a late phase). Thus, a second series of studies were designed to assess the extrapancreatic counterregulatory role of catecholamines without the confounding changes in plasma C-peptide and growth hormone. The pancreatic-adrenocorticalpituitary clamp was preformed either alone or in combination with i.v. propran0101 and phentolamine to block the receptors of plasma and tissue catecholamines. In these experiments, studies were matched for portal plasma insulin concentrations and peripheral plasma counterregulatory concentrations, except for greater plasma adrenaline during a,p blockade. The results were similar to those observed during a,P blockade in the absence of pancreatic-adrenocorticalpituitary clamp; that is, blockade of catecholamines resulted in an early and sustained suppression of hepatic glucose production, and later in less suppression of peripheral glucose utilization. In a final series of studies to quantitate the severity of hypoglycemia that would develop in the absence of the contribution of catecholamines to counterregulation, no exogenous glucose was infused in the experiments of adrenergic blockade. However, these experiments had to be interrupted early because plasma glucose decreased below 40 mg/dl, despite greater secretion of plasma glucagon, growth hormone, and cortisol. One interesting question regarding the counterregulatory role of catecholamines is the contribution of direct versus indirect effects. For example, suppression of endogenous insulin secretion is a well-known indirect effect of catecholamines that contributes to counterregulation by limiting portal hyperinsulinemia.
Importance of CAs in Defense against Insulin Hypoglycemia in Humans
629
However, there are, potentially, additional indirect mechanisms by which catecholamines may contribute to counterregulation. For example, in response to hypoglycemia, after initial suppression, lipolysis is activated. The following rebound increase in plasma free fatty acids (FFAs),glycerol, and ketone bodies, which are largely mediated by adrenergic activation, might both stimulate gluconeogenesis and suppress peripheral glucose utilization. To test the hypothesis that catecholamine-mediatedlipolysis plays a physiological role in counterregulation to prolonged insulin-induced hypoglycemia in humans, two series of experiments were performed (4, 5). In an initial series of experiments, hypoglycemia (plasma glucose -55 mg/dl) was induced by low-dose, continuous i.v. insulin infusion (plasma insulin -30 pUIml) on one occasion (study 1).In a second study, a,P blockade was superimposed to i.v. insulin to block adrenergic-mediated lipolysis. In a third study, heparin and 10% Intralipid werer infused at rates to mimic the posthypoglycemic increase in plasma FFAs and glycerol of the control study. In the second and third studies, exogenous glucose was infused whenever needed to match the control study for plasma glucose concentration. During the adrenergic blockade of study 2, plasma FFAs and glycerol concentrations and rates of lipid oxidation were suppressed. This was associated with a decrease in hepatic glucose output and less suppression of peripheral glucose utilization. When plasma FFAs and glycerol were increased in study 3 to the values of the control study 1, lipid oxidation increased to the values of study 1, whereas hepatic glucose production increased by at least 50%, as compared with study 2, and peripheral glucose utilization was suppressed 85% more than in study 2. These studies (4) have been the first to demonstrate the critical role of lipolysis in glucose counterregulation. In particular, these studies (4)have shown that plasma FFAs contribute to 50% of stimulated hepatic glucose production and to at least 85% suppression of peripheral glucose utlization during hypoglycemia. Notably, this impressive contribution of the FFA substrate occurs during adrenergic blockade, a condition that, if anything, underestimates the role of FFAs in counterregulation. To examine the contribution of lipolyis to counterregulation in the presence of catecholamine effects, in a second series of experiments, hypoglycemia was induced during selective pharmacological blockade of lipolysis with acipimox (5). As compared with a control study in which lipolysis was not blocked, blockade of lipolysis resulted in an approximate 40% decrease in overall hepatic glucose production, an approximate 70% decrease in gluconeogenesis (from alanine) as well as an approximate 15% increase in glucose utilization. These studies (5)have confirmed that the contribution of lipolysis to glucose counterregulation is important. Notably, failure of FFAs to increase during hypoglycemia results in severe impairment of hepatic and peripheral mechanisms of defense against hypoglycemia, despite physiological activation of the adrenergic system. Thus, a large part of the counterregulatory effects of catecholamines is indirect, because they are mediated by the substrate FFAs. In conclusion, catecholamines contribute to human glucose counterregulation since an early phase of hypoglycemia throughout; they exert effects both at the liver and at the muscle level; their counterregulatory effects are not compensated by other hormones, in particular glucagon; thus, the failure of
630
M. Vaz et al.
catecholamines to exert their effects in hypoglycemia, such as during (Y,P pharmacological blockade, results in severe hypoglycemia, despite increase in other counterregulatory hormones. Finally, catecholamines largely work during counterregulation via indirect mechanisms such as stimulation of lipolysis, which enhances gluconeogenesis and hepatic glucose production, and suppress oxidation and utilization of glucose. The findings of these studies are that catecholamines are in the very first line of defense against hypoglycemia along with glucagon and that most of their effects are indirect (i.e., mediated by stimulation of lipolysis and mobilization of FFAs to the liver and muscle. References 1. Rizza, R. A., Cryer, P. E., and Gerich, J. E. (1979). Role of glucagon, catecholamines, growth hormone in human glucose counterregulation. J. Clin. Invest. 64, 64-71. 2. Gerich, J., Langlois, M., Noacco, C., Karam, J., and Forsham, P. (1973).Lack of glucagon response to hypoglycemia in diabetes: Evidence for an intrisinc pancreatic alpha-cell defect. Science 182, 171-173. 3. De Feo, P., Perriello, G., Torlone, E., et al. (1991). The contribution of adrenergic mechanisms to glucose counterregulation in humans. Am. J. Z'hysiol. 261, E725-E736. 4. Fanelli, C., De Feo, P., Porcellati, F., et al. (1992). Adrenergic mechanisms contribute to the late phase of hypoglycemic glucose counterregulation in humans by stimulating lipolysis. J. Clin. Invest. 89, 2005-2013. 5 . Fanelli, C., Calderone, S., Epifano, L., et al. (1993).Demonstration of a critical role for free fatty acids in mediating counterregulatory stimulation of gluconeogenesis and suppression of glucose utilization in humans. J. Clin. Invest. 92, 1617-1622.
M. Vaz, M.
D. Esler, H. S.
Cox, G. L. Jennings, D. M. Kaye, and A. G. Turner Baker Medical Research Institute Melbourne 3 18 I , Australia
Sympathetic Nervous Activity and the Thermic Effect of Food in Humans Studies have demonstrated that a reduced rate of energy expenditure is a risk factor for body weight gain and that the sympathetic nervous system (SNS) is a determinant of energy expenditure, although racial differences in Advances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reproduction in any form reserved. 10S4-3S89/98$25.00
Importance of Catecholamines in Defense against Insulin Hypoglycemia in Humans It has been suggested that, in defense against hypoglycemia in humans, secretion of glucagon plays the dominant counterregulatory role (1).According to the view, secretion of adrenaline is usually not critical as long as the responses of glucagon are appropriate, and it becomes important only when secretion of glucagon is prevented (1)or impaired, as in type I diabetes mellitus (2). These principles have recently been reexamined. A major criticism is that they have been derived from a model of acute hypoglycemia in which the i.v. insulin bolus produces a pharmacological increase in plasma insulin with a subsequently rapid decrease in plasma insulin over a few minutes' time (1).In common clinical situations, such as insulinoma, sulphonylurea-induced hypoglycemia, and insulin-dependent diabetes mellitus, hypoglycemia develops gradually, is less severe, and is reversed more slowly than that of the acute experimental models (1). We have developed a model of prolonged hypoglycemia that mimics the clinical situation better than the acute model (1).In this model, insulin is infused to slowly increase the plasma insulin concentration to a plateau in a physiological range (two- to threefold above baseline), and the plasma glucose concentration gradually decreases into the hypoglycemic range (50-60 mg'dl). Using this model, it has already been shown (1)that mechanisms that were not found to be important for counterregulation in the acute model, such as suppression of endogenous insulin secretion and glucose utilization, do play important counterregulatory role; (2) that growth hormone and cortisol, which were not considered as counterregulatory hormones in the acute model, do indeed play an important role in defense against clinical hypoglycemia; and ( 3 ) that neither the failure of glucagon to respond to hypoglycemia nor that of growth hormone or cortisol is fully compensated by a larger secretion of epinephrine and norepinephrine. The several differences in the counterregulatory mechanisms already observed between the acute (1)and the clinical model of hypoglycemia ( 3 )suggest that, in the clinical model, the secretion of epinephrine also may play a much more important counterregulatory role as compared with that envisaged on the basis of the acute hypoglycemia model. In this chapter, the contribution of the adrenergic mechanisms to counterregulation in an experimental model of prolonged hypoglycemia closely mimickAdvances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reprodnction in a n y form reserved. 1054-3589/98 $25.00
627
628
Geremia 6.Bolli
ing the clinical situation will be reviewed. For this purpose, the pancreaticadrenocortical-pituitary clamp, a technique that allows one to examine the role of a single counterregulatory hormone without confounding changes in the plasma concentrations of other hormones, was used. In an initial series of studies to determine the overall counterregulatory contribution of adrenergic response to hypoglycemia, insulin (15 mU * m-2 * min-' for 12 hr) was infused subcutaneously either alone or in combination with propranolol and phentolamine ( 3 ) .Exogenous glucose was infused whenever needed in the a-,@blockade experiments to match studies for plasma glucose concentrations. In both studies, plasma insulin slowly increased to a plateau of approximately 25 pU/ml, while plasma glucose decreased to a plateau of approximately 50 mg/dl. However, during catecholamine blockade, hepatic glucose production increased less both in an early phase and throughout. In addition, after 5 hr, glucose utilization was less suppressed than in control experiments. However, plasma C-peptide was more suppressed and plasma growth hormone less stimulated when catecholamine receptors were not blocked. These studies provided the first evidence of an important counterregulatory role of catecholamines in defense against hypoglycemia in humans. Interestingly, the effect was evident in both an early and a late phase and located both at the hepatic and muscular level. Finally, the role of catecholamines in defense against hypoglycemia was so important that other counterregulatory hormones, including glucagon, did not substitute for blockade of catecholamine effects. However, these studies were not matched for plasma C-peptide (possible overestimation of the role of catecholamines because of greater portal plasma insulin concentrations during a,P blockade) nor for plasma growth hormone (which could have underestimated the role of catecholamines in a late phase). Thus, a second series of studies were designed to assess the extrapancreatic counterregulatory role of catecholamines without the confounding changes in plasma C-peptide and growth hormone. The pancreatic-adrenocorticalpituitary clamp was preformed either alone or in combination with i.v. propran0101 and phentolamine to block the receptors of plasma and tissue catecholamines. In these experiments, studies were matched for portal plasma insulin concentrations and peripheral plasma counterregulatory concentrations, except for greater plasma adrenaline during a,p blockade. The results were similar to those observed during a,P blockade in the absence of pancreatic-adrenocorticalpituitary clamp; that is, blockade of catecholamines resulted in an early and sustained suppression of hepatic glucose production, and later in less suppression of peripheral glucose utilization. In a final series of studies to quantitate the severity of hypoglycemia that would develop in the absence of the contribution of catecholamines to counterregulation, no exogenous glucose was infused in the experiments of adrenergic blockade. However, these experiments had to be interrupted early because plasma glucose decreased below 40 mg/dl, despite greater secretion of plasma glucagon, growth hormone, and cortisol. One interesting question regarding the counterregulatory role of catecholamines is the contribution of direct versus indirect effects. For example, suppression of endogenous insulin secretion is a well-known indirect effect of catecholamines that contributes to counterregulation by limiting portal hyperinsulinemia.
Importance of CAs in Defense against Insulin Hypoglycemia in Humans
629
However, there are, potentially, additional indirect mechanisms by which catecholamines may contribute to counterregulation. For example, in response to hypoglycemia, after initial suppression, lipolysis is activated. The following rebound increase in plasma free fatty acids (FFAs),glycerol, and ketone bodies, which are largely mediated by adrenergic activation, might both stimulate gluconeogenesis and suppress peripheral glucose utilization. To test the hypothesis that catecholamine-mediatedlipolysis plays a physiological role in counterregulation to prolonged insulin-induced hypoglycemia in humans, two series of experiments were performed (4, 5). In an initial series of experiments, hypoglycemia (plasma glucose -55 mg/dl) was induced by low-dose, continuous i.v. insulin infusion (plasma insulin -30 pUIml) on one occasion (study 1).In a second study, a,P blockade was superimposed to i.v. insulin to block adrenergic-mediated lipolysis. In a third study, heparin and 10% Intralipid werer infused at rates to mimic the posthypoglycemic increase in plasma FFAs and glycerol of the control study. In the second and third studies, exogenous glucose was infused whenever needed to match the control study for plasma glucose concentration. During the adrenergic blockade of study 2, plasma FFAs and glycerol concentrations and rates of lipid oxidation were suppressed. This was associated with a decrease in hepatic glucose output and less suppression of peripheral glucose utilization. When plasma FFAs and glycerol were increased in study 3 to the values of the control study 1, lipid oxidation increased to the values of study 1, whereas hepatic glucose production increased by at least 50%, as compared with study 2, and peripheral glucose utilization was suppressed 85% more than in study 2. These studies (4) have been the first to demonstrate the critical role of lipolysis in glucose counterregulation. In particular, these studies (4)have shown that plasma FFAs contribute to 50% of stimulated hepatic glucose production and to at least 85% suppression of peripheral glucose utlization during hypoglycemia. Notably, this impressive contribution of the FFA substrate occurs during adrenergic blockade, a condition that, if anything, underestimates the role of FFAs in counterregulation. To examine the contribution of lipolyis to counterregulation in the presence of catecholamine effects, in a second series of experiments, hypoglycemia was induced during selective pharmacological blockade of lipolysis with acipimox (5). As compared with a control study in which lipolysis was not blocked, blockade of lipolysis resulted in an approximate 40% decrease in overall hepatic glucose production, an approximate 70% decrease in gluconeogenesis (from alanine) as well as an approximate 15% increase in glucose utilization. These studies (5)have confirmed that the contribution of lipolysis to glucose counterregulation is important. Notably, failure of FFAs to increase during hypoglycemia results in severe impairment of hepatic and peripheral mechanisms of defense against hypoglycemia, despite physiological activation of the adrenergic system. Thus, a large part of the counterregulatory effects of catecholamines is indirect, because they are mediated by the substrate FFAs. In conclusion, catecholamines contribute to human glucose counterregulation since an early phase of hypoglycemia throughout; they exert effects both at the liver and at the muscle level; their counterregulatory effects are not compensated by other hormones, in particular glucagon; thus, the failure of
630
M. Vaz et al.
catecholamines to exert their effects in hypoglycemia, such as during (Y,P pharmacological blockade, results in severe hypoglycemia, despite increase in other counterregulatory hormones. Finally, catecholamines largely work during counterregulation via indirect mechanisms such as stimulation of lipolysis, which enhances gluconeogenesis and hepatic glucose production, and suppress oxidation and utilization of glucose. The findings of these studies are that catecholamines are in the very first line of defense against hypoglycemia along with glucagon and that most of their effects are indirect (i.e., mediated by stimulation of lipolysis and mobilization of FFAs to the liver and muscle. References 1. Rizza, R. A., Cryer, P. E., and Gerich, J. E. (1979). Role of glucagon, catecholamines, growth hormone in human glucose counterregulation. J. Clin. Invest. 64, 64-71. 2. Gerich, J., Langlois, M., Noacco, C., Karam, J., and Forsham, P. (1973).Lack of glucagon response to hypoglycemia in diabetes: Evidence for an intrisinc pancreatic alpha-cell defect. Science 182, 171-173. 3. De Feo, P., Perriello, G., Torlone, E., et al. (1991). The contribution of adrenergic mechanisms to glucose counterregulation in humans. Am. J. Z'hysiol. 261, E725-E736. 4. Fanelli, C., De Feo, P., Porcellati, F., et al. (1992). Adrenergic mechanisms contribute to the late phase of hypoglycemic glucose counterregulation in humans by stimulating lipolysis. J. Clin. Invest. 89, 2005-2013. 5 . Fanelli, C., Calderone, S., Epifano, L., et al. (1993).Demonstration of a critical role for free fatty acids in mediating counterregulatory stimulation of gluconeogenesis and suppression of glucose utilization in humans. J. Clin. Invest. 92, 1617-1622.
M. Vaz, M.
D. Esler, H. S.
Cox, G. L. Jennings, D. M. Kaye, and A. G. Turner Baker Medical Research Institute Melbourne 3 18 I , Australia
Sympathetic Nervous Activity and the Thermic Effect of Food in Humans Studies have demonstrated that a reduced rate of energy expenditure is a risk factor for body weight gain and that the sympathetic nervous system (SNS) is a determinant of energy expenditure, although racial differences in Advances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reproduction in any form reserved. 10S4-3S89/98$25.00