John C. Floyd

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.

Levels of Plasma Insulin During Cortisone Glucose Tolerance Tests in “Nondiabetic” Relatives of Diabetic Patients: Implications of Diminished Insulin Secretory Reserve in Subclinical Diabetes

Plasma levels of insulin were determined during standard oral glucose tolerance tests and during cortisone glucose tolerance tests performed upon “nondiabetic” subjects (normal standard GTT) who were first degree relatives of diabetic patients. The standard test was also performed upon mildly diabetic patients and healthy control subjects. The results of the standard glucose tolerance test (levels of plasma insulin and blood glucose) did not differentiate the relatives with positive cortisone glucose tolerance tests (subclinical diabetes) from those with negative cortisone glucose tolerance tests although in both groups levels of plasma insulin were lower than in healthy subjects. However, the results of the cortisone glucose tolerance test did distinguish the group with subclinical diabetes from the relatives with negative tests since the secretion of insulin was delayed and insufficient in the subjects with positive testsl During the standard test patients with mild “maturityonset” diabetes showed delayed and insufficient secretion of insulin as compared to the healthy subjects. It is concluded that during the cortisone glucose tolerance test, “nondiabetic” relatives of diabetic patients who have positive tests (subclinical diabetes) exhibit a defect in the secretion of insulin which distinguishes them from subjects with negative tests. This defect is similar in pattern to that observed in mildly diabetic patients during the standard test. This deficiency in the secretion of insulin represents an important part of the mechanism which induces a positive cortisone glucose tolerance test. This demonstration that, in some relatives of diabetics, a grossly normal standard glucose tolerance can exist at a time when decreased reserve insulin secretory capacity can be measured, justifies a careful evaluation of whether some form of therapy is indicated at this stage of the disease (subclinical diabetes) and, if so, what form it should be. The meaning of the statistically significant subnormal response of plasma insulin to a standard glucose load in relatives with negative cortisone glucose tests is not yet apparent. Since in them the reserve insulin secretory capacity, as measured by the CGTT, remains intact, it is premature, pending additional data, to raise the question of possible treatment of this group.

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.

Glucose tolerance in two unacculturated Indian tribes of Brazil

SummaryPlasma levels of glucose, insulin, growth hormone, and pancreatic polypeptide in response to a standard oral glucose load were studied in the Yanomama and the Marubo, two relatively unacculturated Amerindian tribes of the Brazilian Amazon. The findings in the two tribes differed significantly from each other and in the degree of deviation from control subjects. The average responses in both tribes differed significantly from those of age- and sex-matched Caucasoid control subjects studied in Ann Arbor, Michigan; however, of the two tribes, the Marubo, the more acculturated group, resembled the controls more closely. Plasma concentrations of glucose and the hormones at three time points (fasting, 1 h, 2 h) were compared by means of a multivariate analysis. When the Marubo were compared with the control subjects, the only highly significant difference was in the plasma glucose concentrations (all three points were higher in the Marubo); however, the Yanomama differed significantly from the control subjects with respect to all four plasma indicators (p < 0.05). Unlike the Marubo, the Yanomama showed no significant rise in plasma glucose at 1 h and no decrease at 2 h. Neither tribe exhibited the bimodality of the 2 h glucose value characteristic of acculturated Amerindians, such as the Pima, but the samples studied were small.

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.

Studies on the Natural History of Asymptomatic Diabetes in Young People

Abstract Forty-five young patients with chemical diabetes have been followed for 1–16 yr. Among 21 of them aged 9–17 yr, 4 have developed insulin-dependent diabetes mellitus. None of the 24 aged 18–25 yr have decompensated. Delayed and subnormal insulin response to oral glucose was noted in those with chemical diabetes compared to normals, except for one nonobese patient who had hyperinsulinemia. Considerable fluctuation in glucose and/or insulin responses and their relationship was noted in repeated testing. These observations emphasize that validation of diagnostic criteria for glucose tolerance test interpretation need not depend on a high rate of progression to more severe hyperglycemia, and that the slow progression of latent diabetes permits consideration of prophylactic measures.