Effect of insulin deficiency on hepatic ribosomal aggregation

Ribosomes were isolated from liver homogenates of normal and alloxan diabetic rats. Sedimentation of these ribosomes through a continuous sucrose gradient indicated that insulin deficiency was associated with an increase in the proportion of ribosomes existing as the 80 S monomer. In a second series of experiments ribosomes from diabetic animals were also shown to bind more avidly to 3H-polyuridylic acid (polyU). When hepatic ribosomes from normal and diabetic rats were incubated for forty-five minutes at 37° C. in a complete amino acid-incorporating system they underwent essentially total disaggregation into single ribosomes, and the previously noted difference between normal and diabetic ribosome binding to polyU was abolished. The sedimentation characteristics of ribosomes from normal and diabetic rats indicate that insulin deficiency results in an increase in the proportion of ribosomes free of mRNA, and, since they are free of mRNA, a greater than normal proportion of ribosomes from diabetic animals are capable of complexing with polyU. Obviously, producing equal numbers of monosomes by prior incubation in an amino acid-incorporating system would be expected to obliterate the difference in polyU binding between ribosomes from normal and diabetic animals. Thus, both lines of experimental evidence are consistent with the hypothesis that insulin deficiency produces a disaggregation of hepatic poly-somes, leading to an increase in the proportion of ribosomes that are not bound to mRNA.

Prevalence of plasma lipoprotein abnormalities in a free-living population of the Central Valley, California

In an epidemiologic study of 1,118 free-living volunteers, aged 25 to 79 (552 male, 566 female) drawn from eight counties of the Central Valley, California, the following determinations were included: a medical history, blood pressure, resting electrocardiogram, plasma total cholesterol and glycerides (following a light, fat-free breakfast), and (when lipids were considered to be elevated) plasma lipoproteins determined by electrophoresis. A subgroup of the entire study population (494 male, 503 female) free of overt diabetes and electrocardiographic abnormalities, and with diastolic blood pressure not above 100 mm Hg, that was considered to be clinically "normal," forms the basis of this report.

Significant sex differences for mean levels of total cholesterol were absent or of small magnitude at all age decades up to and including the seventh. In contrast, glyceride levels for men were significantly higher than for women aged 25-59, and differences were substantial. Beyond the seventh decade, females had higher mean levels of both plasma total cholesterol and glycerides.

The type IV lipoprotein pattern was the most common abnormality (8.6%) within the entire normal population and was 2.7 times as common in men (13%) as in women (4.8%). Type II pattern was less common overall (3.7%) and was more frequent in women than in men. Types III (0.2 to 0.4%) and V (0.2%) were both very uncommon, and type I was not encountered. Results were similar when the entire population (normal and abnormal) was considered.

These findings suggest that sex differences in plasma total-cholesterol level cannot account for the known male preponderance in coronary heart disease within the population studied. Prolonged elevation of plasma glycerides (usually expressed as the type IV pattern) in males during the years when atherogenesis is occurring might account for the sex difference.

Evidence that glucose load is an important determinant of plasma insulin response in normal subjects

Two consecutive oral glucose tolerance tests were performed on twelve healthy, malemedical students, in which each subject received 20 and 40 gm. of glucose per square meter body surface area. The tests were conducted one week apart, and the sequence in which the two glucose loads was given was randomized. The larger glucose load elicited anincrease in both plasma glucose and immunoreactive insulin response. However, the increase in the plasma insulin response was much greater than the increase in plasma glucoseresponse. Thus, the insulinogenic index, denned as the plasma insulin level divided bythe plasma glucose level, significantly increased as the glucose load increased. These results indicate that glucose load is an important determinant of plasma insulin response, and that in any individual the plasma insulin response is not a simple linear function of coexisting plasma glucose concentration.

Effect of experimental diabetes mellitus on kidney ribosomal protein synthesis

Rats were made diabetic by the administration of alloxan, streptozotocin or anti-insulin serum, and the capacity of kidney ribosome preparations to incorporate labeled amino acid into protein in vitro was compared for diabetic and normal rats. Kidney ribosomes from diabetic rats always incorporated markedly more amino acid into protein than normal controls. These results are in contrast to the decrease in hepatic and muscle ribosomal protein synthesis that has previously been shown to accompany experimental diabetes in the rat. When polysomes were preincubated to “strip off” endogenous messenger RNA, and artificial messenger was added, the increased protein synthetic activity of diabetic ribosomes persisted. Consequently, it is concluded that the effect of insulin deficiency to increase the incorporation of amino acid into protein by kidney ribosomes is most likely due to a post-transcriptional change in protein synthesis.

Is there a delay in the plasma insulin response of patients with chemical diabetes mellitus?

We have compared the plasma glucose and immunoreactive insulin responses to oral glucose, intravenous glucose and intravenous tolbutamide of twenty normal subjects with those of eleven patients with chemical diabetes. The two groups were similar in terms of age, sex and degree of adiposity. The insulin response of patients with chemical diabetes was greater at every time interval during all these tests. Thus, we could document no defect in the rapidity of insulin response to these stimuli in patients with chemical diabetes. These results do not support the view that the commonest metabolic abnormality in patients with diabetes is a delay in the insulin response.

Effect of experimental diabetes mellitus on protein synthesis by liver ribosomes

Rats were made diabetic by the administration of either alloxan or anti-insulin serum (AIS), and the capacity of hepatic polysome preparations from these animals to incorporate aniino acid into protein in an in vitro incubating system was compared to that of similar preparations from normal animals. Hepatic polysomes from rats with either form of experimental diabetes were characterized by a marked reduction in their ability to carry out in vitro protein synthesis. Prior washing with 0.5 M NH4CI accentuated the abtivity difference between normal and diabetic ribosomes, suggesting that the decrease in amino acid incorporation of diabetic ribosomes was not related to loss of “initiation” factors. In contrast, normal monosomes (polysomes stripped of natural messenger RNA) were no more active than monosomes from diabetic animals when protein synthesis was directed by polyuridylic acid, and this relationship was independent of the concentration of either polyuridylic acid or MgCI2 in the incubation medium. These results are consistent with the thesis that the decrease in hepatic ribosomal protein synthesis that results from experimental diabetes mellitus is related to a concomitant decrease in the amount or availability of messenger RNA.

Comparison of impedance to insulin-mediated glucose uptake in normal subjects and in subjects with latent diabetes

A technique was devised for a more accurate measurement than has been heretofore possible of one of the factors responsible for hyperglycemia in the complex syndrome of diabetes. This factor is termed impedance and represents the tissues' insensitivity or resistance to insulin-mediated glucose uptake. It was measured by use of steady-state exogenous insulin and glucose infusions during a period of pharmacological suppression of endogenous insulin secretion. Endogenous new glucose production was also inhibited. Impedance as calculated is a direct function of steady-state glucose concentrations, since exogenous insulin concentrations were similar in all studies. Two groups of normal weight subjects were studied. One had maturity onset latent diabetes, and the other (matched for age, weight, and per cent adiposity) was normal. Impedance was closely reproducible in the same individual and remained relatively constant during prolonged infusions. The diabetics had average infusion glucose concentrations (and thus impedance) 68% higher than the normal group, and it is of note that their previously measured glucose intolerance differed by a similar degree; that is, the diabetic's intolerance (as defined by mean weighted plasma glucose response after oral glucose) was 52% greater than that of the normal individuals.

Study of the relationship between plasma insulin concentration and efficiency of glucose uptake in normal and mildly diabetic subjects

Removal of glucose-U-C-14 from plasma was studied in normal subjects and in patients with maturity-onset diabetes mellitus. All measurements were made after an overnight fast, during a period in which plasma glucose and immunoreactive insulin concentrations were not changing. The mean irreversible loss rate (ILR) of glucose from the plasma of normal subjects was not different from that of patients with diabetes: mean ILR (± the coefficient of variation) of all patients was 1.36 (± 14 per cent) mg. glucose/kg, body weight per min. This narrow range attests to the similarity of ILR in the patients studied. There was no correlation between glucose ILR and plasma glucose concentration, plasma immunoreactive insulin concentration, or ponderal index. In contrast, fractional glucose irreversible loss rate varied considerably from patient to patient, with a coefficient of variation of 35 per cent. Furthermore, there was a strong negative correlation between plasma glucose concentration and fractional glucose irreversible loss rate (r = −0.82, p < .01). This decrease in efficiency of glucose removal from plasma of hyperglycemic patients was not related to an absolute lack of insulin. It is concluded that net glucose utilization in the fasting state is similar in normal and mildly diabetic patients, and that fasting hyperglycemia is due to a decrease in efficiency of glucose removal which cannot be attributed to absolute insulin deficiency.

Derivation of a three compartment model describing disappearance of plasma insulin-131-I in man

Insulin-(131)I was administered intravenously to normal subjects, to patients with maturity-onset diabetes and normal renal function, and to nondiabetic patients with renal failure. The ensuing plasma disappearance curves of immunoprecipitable radioactive insulin were determined, and these data were analyzed in a variety of ways. Firstly, fractional irreversible loss rates of insulin from plasma were calculated and found to be greatly diminished in patients with renal failure (t((1/2)) = 39 min), as compared with normal (t((1/2)) = 15 min) and diabetic subjects (t((1/2)) = 12 min). Secondly, plasma insulin-(131)I disappearance curves were resolved into sums of three exponentials by the method of "peeling," and values for the resultant three slopes and half-lives were determined. Patients with normal renal function had similar values for all parameters, while those patients with renal failure were differentiated on the basis of the slope of the last component, with a prolongation of its half-life to 275 min (approximately twice normal). Finally, a three pool model was formulated to describe the kinetics of plasma insulin disappearance in man, representing plasma (pool 1), interstitial fluid (pool 2), and all tissues in which insulin is utilized and degraded (pool 3). The proposed model adequately describing the disappearance curves of insulin-(131)I observed in all patients indicated that volumes (per cent body weight) of pool 1 (4.04) and pool 2 (10.11), calculated on the basis of the model and the experimental data, corresponded closely to estimates of plasma and interstitial fluid volumes obtained by independent means. It also demonstrated that patients with renal failure were characterized by a decreased removal rate of insulin from pool 3 and an increased recycling rate of insulin from pool 3 to pool 2. It is concluded that the proposed model represents a reasonable description of the kinetics of insulin distribution and degradation, and that its use provides quantitative insights into the physiology of insulin metabolism.

Steady state plasma insulin response to continuous glucose infusion in normal and diabetic subjects

Normal subjects and diabetic patients with various degrees of hyperglycemia were given continuous intravenous glucose infusions for three hours in order to achieve constant plasma levels of glucose and immunoreactive insulin. During the last ninety minutes of this infusion period, at which time steady state conditions were observed, blood was obtained for measurement of plasma glucose and immunoreactive insulin concentration. Absolute levels of plasma insulin produced by the stimulus of the glucose infusions were extremely variable from patient to patient and were relatively independent of the degree of hyperglycemia. Patients with diabetes did not have lower steady state plasma insulin concentrations as a group, and this was true whether the insulin response was considered in absolute or relative terms. It is concluded that significant differences do exist in individual sensitivity to endogenous insulin, and that hyperglycemia in patients with maturity onset diabetes is not simply a function of lack of insulin.

Insulin delivery rate into plasma in normal and diabetic subjects

Removal of insulin-(131)I from plasma was studied in normal and diabetic subjects with both single injection and continuous infusion of isotope techniques. Patients were studied either in the fasting state or during steady-state hyperglycemia produced by a continuous intravenous glucose infusion. Steady-state plasma insulin concentration during these studies ranged from 10 to 264 muU/ml. Labeled insulin specific activity time curves consisted of more than one exponential, indicating that a multicompartmental system for insulin metabolism exists. A mathematical technique which is applicable to non-first order processes was used to calculate the rate at which insulin was lost irreversibly from the plasma insulin pool. A direct, linear relationship was found between insulin irreversible loss rate and plasma insulin concentration over the range of concentrations studied. This linearity implies lack of saturability of the insulin removal mechanism. Since the plasma insulin pool was in a steady state during these studies, insulin irreversible loss rate was equal to the rate at which newly secreted insulin was being delivered to the general circulation. Therefore, these results indicate that changes in plasma insulin concentration result from parallel changes in the rate of insulin delivery and not from changes in the opposite direction of the rate of insulin removal. A wide range of insulin delivery rates was found among patients with similar plasma glucose concentrations, suggesting that there exists considerable variability in responsiveness to endogenous insulin among these patients.

Role of insulin in endogenous hypertriglyceridemia

Dietary carbohydrate accentuation of endogenous triglyceride production has been studied in 33 patients. A broad and relatively continuous spectrum of steady-state plasma triglyceride concentrations was produced in 31 of the 33 subjects during 3 wk of a high carbohydrate (fat-free) liquid formula diet. Two patients developed plasma triglyceride concentrations in excess of 2000 mg/100 ml, and these were the only patients we have studied in which carbohydrate induction of hypertriglyceridemia seemed to be associated with a defect in endogenous plasma triglyceride removal mechanisms. In the remaining 31 patients the degree of hypertriglyceridemia was highly correlated with the insulin response elicited by the ingestion of the high carbohydrate formula (P < 0.005). No significant correlation existed between fasting plasma triglyceride concentration and either plasma glucose or free fatty acid concentrations after the high carbohydrate diet, nor was the degree of hypertriglyceridemia related to degree of obesity. It is suggested that hypertriglyceridemia in most subjects results from an increase in hepatic triglyceride secretion rate secondary to exaggerated postprandial increases in plasma insulin concentration.