Ralph F. Knopf

Stimulation of insulin secretion by amino acids.

In previous studies we have demonstrated that in healthy subjects the ingestion of protein meals results in large increases in plasma levels of insulin and have concluded that this is an important physiologic phenomenon (1, 3). In those experiments the magnitude of the increases in plasma insulin exceeded that expected from the small increases in plasma leucine measured (3). In addition, chlorpropamide pretreatment failed to accentuate the protein-induced changes in blood glucose and plasma free fatty acids (3), although it greatly augments such leucine-induced changes (4, 5). These findings suggested that amino acids other than leucine or amino acids in combination with leucine stimulate the release of insulin (1, 3, 6). The studies described below were performed to assess in healthy subjects the effects upon plasma levels of insulin of the administration of single amino acids and combinations of amino acids with and without leucine. Essential 1 I-amino acids were administered intravenously individually or as mixtures. Plasma levels of insulin, amino nitrogen, free fatty acids, and blood levels of glu-

Clinical characteristics of primary aldosteronism from an analysis of 145 cases

Abstract With the passage of time the clinical characteristics of primary aldosteronism have been clearly defined. The condition is an easily recognized and fascinating form of curable hypertension. A small number of cases of primary aldosteronism with severe secondary renal damage will present difficulties in diagnosis. This should not distract attention from the cases in which the diagnosis is simple and straightforward. In three quarters of these cases a dramatic cure is produced by surgical removal of a small adrenal cortical adenoma. In the remainder significant improvement can be expected following surgery. Pitfalls in diagnosis have been charted.

Insulin secretion in response to protein ingestion.

W\e have reported previously that the oral or intravenous administration of the amino acid l-leucine to healthy subjects results in increases in plasma insulin and decreases in blood glucose and plasma free fatty acids (2, 3). After the administration of leucine to healthy subjects pretreated with either chlorpropamide or tolbutamide (2, 3), and also to some patients with functioning islet cell tumors of the pancreas (4), increments in plasma leucine caused increases in plasma insulin and decreases in blood glucose that were significantly greater than those observed in healthy subjects not pretreated with sulfonylurea drugs. Wehave also shown that increased release of insulin from the pancreatic beta cells is the mechanism by which leucine increases peripheral levels of insulin (5). Wesuggested that a rising plasma level of leucine is a physiologic stimulus for the release of insulin, and that the more pronounced sensitivity to leucine hypoglycemia produced experimentally by. administration of sulfonylureas and observed in some patients with idiopathic hypoglycemia or insulin-secreting tumors of the pancreas represents a great exaggeration of a normal physiologic phenomenon (6, 7). In an effort to determine the effect of leucine on insulin release under physiologic circumstances, protein meals rich in leucine were fed to healthy subjects, and the levels of plasma insulin, leucine, amino nitrogen, free fatty acids, and blood glucose

Effect of Amino Acids and Proteins on Insulin Secretion in Man

Following the demonstration that administration of leucine accentuates the hypoglycemia of some patients with “idiopathic hypoglycaemia of childhood” (1956) and of some patients with pancreatic islet cell tumours (1959) we initiated studies to explore the mechanism of leucine-induced hypoglycaemia. Sensitivity to leucine-hypoglycaemia can be induced consistently in healthy subjects after administration of sulfonylurea compounds. Increased release of pancreatic insulin is the primary mechanism by which leucine causes a fall in blood glucose in sulfonylurea-induced as well as in naturally occurring leucine hypoglycaemia. Experimentally-induced sensitivity to leucine hypoglycaemia can be used as a model for the further study of leucine hypoglycaemia. Potentiation of insulin activity has not been demonstrated to play a role in the production of leucine-induced hypoglycaemia in man. Leucine induces release of insulin and lowers blood glucose in healthy subjects without prior administration of hypoglycaemic agents, but to a quantitatively lesser extent than in sulfonylurea-induced leucine hypoglycaemia. The more pronounced sensitivity to leucine hypoglycaemia produced experimentally by administration of sulfonylureas and that observed in some patients with “idiopathic hypoglycaemia” or functioning islet cell tumours represents a great exaggeration of what appears to be a normal physiological phenomenon. To determine the effect of leucine on insulin release under physiologic circumstances, protein meals (cooked beef or chicken liver) rich in leucine were fed to healthy subjects. The increases in plasma insulin which resulted from the ingestion of the protein meals were considerably greater than those which would have been expected to have resulted from the modest increases in plasma leucine which occurred. These findings suggested that amino acids other than leucine or amino acids in combination with leucine stimulated the release of insulin. Essential 1-amino acids, either as mixtures or individually, were administered intravenously to healthy subjects. The various mixtures — whether they contained leucine or not — and most, but not all, of the individual amino acids stimulated the release of insulin. The most effective stimulus for insulin release was either a mixture of 10 essential amino acids or arginine given alone. Histidine was ineffective. Thus, the phenomenon of amino-acid-induced release of insulin does not depend on the presence of leucine in the infusion mixture. A variety of individual amino acids, induce the release of insulin, but there are large differences among these amino acids in their capacities to stimulate its secretion. Increases in blood glucose observed during some of the amino-acid infusions cannot be the major cause of the increases in plasma insulin. Rising plasma levels of certain amino acids after protein feeding can be considered to be physiologic stimuli for the secretion of insulin. It is speculated that the purpose of the insulinogenic response to protein ingestion is to aid in the utilization of absorbed amino acids and in their synthesis to protein. Leucine and arginine themselves, rather than one of their metabolites, are the potens stimuli to insulin release when these amino acids are administered. The mechanism by which leucine induces insulin release differs from that by which the other essential amino acids induce release of insulin. The magnitude of insulin secretion induced by the administration of mixtures of amino acids or ingested proteins depends not only upon the amount administered but also on the synergism between particular amino acids and in the case of mixed meals of protein and carbohydrate upon synergism between amino acids and glucose as well. Administration of human growth hormone and adrenalcortical steroids increases the sensitivity of the pancreatic islet cells to the insulin-releasing stimulus of amino acids. In non-obese, mildly diabetic subjects, increases in plasma insulin after administration of the 10-amino-acid mixture and of arginine were smaller than in healthy subjects. The history of the study of “leucine hypoglycaemia” and related phenomena is an example of how the exploration of a seemingly uncommon metabolic aberration observed in pathologic states and its experimental reproduction may lead to the recognition of what appear to be important physiologic relationships. In this instance the participation of amino acids in control of secretion of insulin (and also of growth hormone and glucagon) have evolved.

Synergistic Effect of Essential Amino Acids and Glucose upon Insulin Secretion in Man

Arginine, leucine, or histidine, and glucose were each administered intravenously, in 30-gm. amounts on separate occasions to the same healthy subjects. Each of the amino acids and glucose in these same amounts were administered also as mixtures. Synergism in the stimulation of the release of insulin was demonstrated when the sum of the increases in plasma insulin which resulted from the separate administrations of an amino acid and glucose was exceeded by that obtained when they were administered as a mixture. A synergistic effect was exerted by each of the three amino acid-glucose combinations; it was greatest with arginine and glucose and least with histidine and glucose. Synergism probably results from the effects of these stimuli upon the pancreatic beta cell; it is not dependent upon humoral gastrointestinal factors released after the ingestion of these nutrients.

Effect of amino acids and proteins on insulin secretion in man.

Summary Following the demonstration that administration of leucine accentuates the hypoglycemia of some patients with “idiopathic hypoglycaemia of childhood” (1956) and of some patients with pancreatic islet cell tumours (1959) we initiated studies to explore the mechanism of leucine-induced hypoglycaemia. Sensitivity to leucine-hypoglycaemia can be induced consistently in healthy subjects after administration of sulfonylurea compounds. Increased release of pancreatic insulin is the primary mechanism by which leucine causes a fall in blood glucose in sulfonylurea-induced as well as in naturally occurring leucine hypoglycaemia. Experimentally-induced sensitivity to leucine hypoglycaemia can be used as a model for the further study of leucine hypoglycaemia. Potentiation of insulin activity has not been demonstrated to play a role in the production of leucine-induced hypoglycaemia in man. Leucine induces release of insulin and lowers blood glucose in healthy subjects without prior administration of hypoglycaemic agents, but to a quantitatively lesser extent than in sulfonylurea-induced leucine hypoglycaemia. The more pronounced sensitivity to leucine hypoglycaemia produced experimentally by administration of sulfonylureas and that observed in some patients with “idiopathic hypoglycaemia” or functioning islet cell tumours represents a great exaggeration of what appears to be a normal physiological phenomenon. To determine the effect of leucine on insulin release under physiologic circumstances, protein meals (cooked beef or chicken liver) rich in leucine were fed to healthy subjects. The increases in plasma insulin which resulted from the ingestion of the protein meals were considerably greater than those which would have been expected to have resulted from the modest increases in plasma leucine which occurred. These findings suggested that amino acids other than leucine or amino acids in combination with leucine stimulated the release of insulin. Essential 1-amino acids, either as mixtures or individually, were administered intravenously to healthy subjects. The various mixtures — whether they contained leucine or not — and most, but not all, of the individual amino acids stimulated the release of insulin. The most effective stimulus for insulin release was either a mixture of 10 essential amino acids or arginine given alone. Histidine was ineffective. Thus, the phenomenon of amino-acid-induced release of insulin does not depend on the presence of leucine in the infusion mixture. A variety of individual amino acids, induce the release of insulin, but there are large differences among these amino acids in their capacities to stimulate its secretion. Increases in blood glucose observed during some of the amino-acid infusions cannot be the major cause of the increases in plasma insulin. Rising plasma levels of certain amino acids after protein feeding can be considered to be physiologic stimuli for the secretion of insulin. It is speculated that the purpose of the insulinogenic response to protein ingestion is to aid in the utilization of absorbed amino acids and in their synthesis to protein. Leucine and arginine themselves, rather than one of their metabolites, are the potens stimuli to insulin release when these amino acids are administered. The mechanism by which leucine induces insulin release differs from that by which the other essential amino acids induce release of insulin. The magnitude of insulin secretion induced by the administration of mixtures of amino acids or ingested proteins depends not only upon the amount administered but also on the synergism between particular amino acids and in the case of mixed meals of protein and carbohydrate upon synergism between amino acids and glucose as well. Administration of human growth hormone and adrenalcortical steroids increases the sensitivity of the pancreatic islet cells to the insulin-releasing stimulus of amino acids. In non-obese, mildly diabetic subjects, increases in plasma insulin after administration of the 10-amino-acid mixture and of arginine were smaller than in healthy subjects. The history of the study of “leucine hypoglycaemia” and related phenomena is an example of how the exploration of a seemingly uncommon metabolic aberration observed in pathologic states and its experimental reproduction may lead to the recognition of what appear to be important physiologic relationships. In this instance the participation of amino acids in control of secretion of insulin (and also of growth hormone and glucagon) have evolved.

Failure of Sulfonylureas to Suppress Plasma Glucagon in Man

Others2 have reported that sulfonylureas suppress the release of glucagon and concluded that this is an important mechanism by which these compounds lower blood glucose. The studies reported here demonstrate the following: 1) oral administration of chlorpropamide for three days or rapid intravenous injection of tolbutamide did not induce a significant change in basal levels of immunoreactive glucagon (IRG) ; 2) intravenous infusion of tolbutamide in saline over thirty minutes did not evoke significant decreases in IRC; 3) tolbutamide added to solutions of arginine or of a mixture of ten amino acids (10-AA) failed to reduce the increases in IRG evoked by the infusion of these amino acid solutions in the absence of tolbutamide; and 4) tolbutamide injected rapidly at the midpoint of infusions of arginine or of 10-AA did not alter the steady rise in IRG induced by these amino acid solutions. In addition, tolbutamide failed to prevent hypoglycemia-induced increases in IRG. These results indicated that in man sulfonylureas do not suppress plasma levels of glucagon and that sulfonylurea-induced decreases in blood glucose are not mediated through suppression of glucagon release.

Synergistic Effect of Certain Amino Acid Pairs upon Insulin Secretion in Man

Individual amino acids were administered intravenously in amounts of 15 gm. and again together with 15 gm. of a second amino acid as 30-gm. mixtures. Synergism in the stimulation of the release of insulin was demonstrated when the sum of the increases in plasma insulin which resulted from the separate infusions of two amino acids was exceeded by that obtained when they were administered as a mixture. A synergistic effect was exerted by arginine and leucine, arginine and phenylalanine, but not by arginine and lysine, arginine and histidine, or by leucine and histidine. Synergism was greater with arginine and leucine than with arginine and phenylalanine. It appears to result from the effects of these amino acid pairs upon the pancreatic beta cell.

Effect of Changes in Plasma Levels of Free Fatty Acids on Plasma Glucagon, Insulin, and Growth Hormone in Man

FFA. These findings contrasted with the effects upon plasma levels of immunoreactive insulin (IRI) and growth hormone (IGH). During elevation of F7A levels, the mean basal level of plasma IRI increased by lOO%, and the IRI response to arginine increased by 50%. Concomitantly, basal IGH levels and the plasma IGH response to arginine were suppressed markedly by elevation of FFA levels. The results of these studies do not offer support for a significant role of variation in plasma level of FFA as a regulator of acute changes in plasma IRG in man. An influence of changing levels of FFA on insulin secretion was found, and an effect on levels of growth hormone was confirmed.

Effect of Protein Meals on Plasma Insulin in Mildly Diabetic Patients

We have reported that in mildly diabetic patients increases in plasma insulin are subnormal in response to infused amino acids while others have reported excessive increases in response to ingested protein. Increases in plasma insulin exhibited by nonobese, mildly diabetic patients in response to the ingest ion of protein meals and to the intravenous administration of a mixture of ten essential amino acids were compared to those of healthy nonobese control subjects. The plasma insulin responses to protein meals were also determined in a group of mildly obese, mildly diabetic patients. The results of these studies indicate that: (1) the mean insulin response of nonobese mildly diabetic patients to protein meals is subnormal and not excessive, (2) mildly obese, mildly diabetic patients respond excessively to protein meals as compared to control subjects of normal weight, and (3) the excessiye insulin response to protein meals reported by others to occur in diabetics is probably the result of a greater degree of adiposity in their diabetics than in their control subjects, rather than of the presence of diabetes mellitus. Unless the degree of adiposity is similar in diabetic and healthy subjects, meaningful comparison of their insulin responses to protein meals or other stimuli cannot be made.