A theoretical study of double-inhibitor-titration curves in free-energy-transducing networks

The general relationship between the double-inhibitor-titration curves and the kinetic properties of pumps in a delocalized chemiosmotic free-energy-transducing network is studied. The kinetic conditions for a delocalized system to generate the observed double-inhibitor-titration results are derived and the effectiveness of double-inhibitor experiments in discriminating between the localized and the delocalized proton-coupling mechanisms is assessed. It is found that, using simple enzymatic cycles for the kinetics of the pumps in a delocalized network, one can reproduce the experimentally measured double-inhibitor-titration curves that were widely used to argue against the delocalized mechanism. This implies that double-inhibitor-titration curves alone are not sufficient to discriminate between localized and delocalized coupling systems. Additional information concerning the kinetic responses of isolated pumps on the proton gradient across the membrane and inhibitor concentrations are required.

How enzymes can capture and transmit free energy from an oscillating electric field

Recently, it has been demonstrated that free energy from an alternating electric field can drive the active transport of Rb+ by way of the Na+, K+-ATPase. In the present work, it is shown why many transmembrane enzymes can be expected to absorb free energy from an oscillating electric field and transduce that to chemical or transport work. In the theoretical analysis it turned out to be sufficient that (i) the catalytic process be accompanied by either net or cyclic charge translocation across the membrane and (ii) the stability of the enzyme states involved be asymmetric. Calculations based on a four-state model reveal that free-energy transduction occurs with sinusoidal, square-wave, and positive-only oscillating electric fields and for cases that exhibit either linear or exponential field-dependent rate constants. The results suggest that in addition to oscillating electric field-driven transport, the proposed mechanism can also be used to explain, in part, the "missing" free energy term in the cases in which ATP synthesis has been observed with insufficient transmembrane proton electrochemical potential difference.

Effect of differences in glucose tolerance on insulin's ability to regulate carbohydrate and free fatty acid metabolism in obese individuals

The effect of variations in glucose tolerance on insulin's ability to regulate glucose uptake and plasma glucose and FFA concentrations was assessed in 22 obese individuals [8 with normal glucose tolerance, 7 with impaired glucose tolerance (IGT), and 7 with noninsulin-dependent diabetes mellitus (NIDDM)]. Patients with IGT had ambient insulin levels that were higher than normal, associated with elevated postprandial glucose levels and a marked reduction in insulin-stimulated glucose uptake. On the other hand, plasma FFA levels were relatively normal in IGT, possibly because of the hyperinsulinemia. Patients with NIDDM were also hyperinsulinemic, with insulin levels throughout the day that were approximately twice normal. Hyperinsulinemia in patients with NIDDM was associated with a significant decline in insulin-stimulated glucose uptake as well as with significant increases in both ambient plasma glucose and FFA concentrations. Thus, and in contrast to patients with IGT, plasma FFA metabolism in NIDDM was grossly abnormal, despite the concomitant hyperinsulinemia. These data indicate that insulin resistance in obese individuals varies as a function of degree of glucose tolerance, and insulin resistance in patients with NIDDM involves defects in the regulation of both plasma glucose and FFA metabolism.

Uptake of low density lipoproteins by rat tissues. Special emphasis on the luteinized ovary

The aim of this study was to determine how luteal cells of the hormone-primed (luteinized) ovary process low density lipoproteins (LDL). Ovary uptake of perfused 125I-LDL was assessed by tissue levels of radioactivity; the distribution of LDL protein in cells was assessed on autoradiograms of the fixed tissue; and the level of stimulation of steroidogenesis, as well as degradation of LDL protein, was assessed on effluent perfusion samples. Human LDL ligand used in these studies was rigorously defined biochemically and physiologically. Homologous (rat) LDL was used as a special ligand control. Other tissue controls included the use of perfused or in vivo-infused luteinized ovaries from animals pretreated to reduce circulating lipoprotein levels, perfused ovaries from a second hormone-primed model, perfused liver from estrogen-treated rats, and isolated and cultured cells from the same ovarian tissues used in the perfusion experiments. The results show that perfused LDL promptly stimulates steroidogenesis. However, the labeled protein moiety of the LDL is not interiorized by the luteal cells, nor is there evidence of LDL protein degradation in the effluent samples. In contrast, internalization of the ligand occurs when luteal cells are incubated with the ligand in vitro. We have observed also that uptake of the 125I-LDL by the ovary can be displaced equally well by excess unlabeled LDL or HDL3. Overall, these experiments suggest that in the intact luteinized ovary, LDL binds to the same sites on the cell surface where HDL "binds," and that LDL cholesterol must be obtained by these steroid hormone-producing cells by a mechanism that does not require internalization of the intact lipoprotein particle.

Is the insulin resistance of patients with noninsulin-dependent diabetes mellitus secondary to insulin deficiency?

Defects in both insulin secretion and action have been documented in patients with noninsulin-dependent diabetes mellitus (NIDDM), leading to the suggestion that both fasting hyperglycemia and insulin resistance in NIDDM are secondary to insulin deficiency. In order to test this hypothesis, insulin secretion (plasma insulin response to oral glucose) and insulin action (insulin clamp) were determined in 25 patients with NIDDM. The results documented relationships between incremental plasma insulin response to glucose and degree of fasting hyperglycemia (r = -.045, P less than 0.05) and insulin-stimulated glucose utilization (r = 0.25, P = NS). These data indicate that differences in insulin secretory response accounted for only approximately 20% of the variance in fasting plasma glucose level and 6% of the variance in insulin resistance in NIDDM. Thus, differences in insulin-secretory response contribute modestly to magnitude of glycemia, and not at all to variations in insulin resistance in NIDDM, permitting rejection of the hypothesis that insulin resistance is secondary to insulin deficiency.

Reduced plasma high density lipoprotein-cholesterol concentrations need not increase when hyperglycemia is controlled with insulin in noninsulin-dependent diabetes mellitus

The ability of intensive insulin treatment to increase plasma high density lipoprotein (HDL)-cholesterol levels was evaluated in 12 patients with noninsulin-dependent diabetes mellitus. Patients were treated for 6 weeks with one daily morning injection of ultralente insulin, in combination with administration of regular insulin before breakfast, lunch, and dinner. The mean (+/- SEM) fasting plasma glucose concentration fell from 289 +/- 21 to 122 +/- 9 mg/dl (P less than 0.001), and the mean hourly postprandial glucose concentration fell from 313 +/- 24 to 102 +/- 7 mg/dl (P less than 0.001). In addition, insulin treatment was associated with a reduction in both fasting plasma triglyceride (256 +/- 45 to 137 +/- 18 mg/dl; P less than 0.001) and cholesterol (224 +/- 25 to 199 +/- 19 mg/dl; P less than 0.05) concentrations. However, plasma HDL-cholesterol concentrations, which were low to begin with, did not rise in association with excellent glycemic control. These results demonstrate that hyperglycemia and hypertriglyceridemia can be effectively reduced by an aggressive program of insulin treatment in patients with noninsulin-dependent diabetes mellitus, but this intervention need not lead to an improvement in the abnormal HDL-cholesterol metabolism in these patients.

The effect of colchicine on cholesterol processing by the progesterone-producing cells of the luteinized ovary

Previously, we indicated that luteal cells from colchicine-treated superovulatory (luteinized) rats show decreased capacity for progesterone production. The current study investigates the possibility that colchicine exerts this effect by interfering with the mechanism by which cholesterol is processed and/or synthesized by luteal cells. To this end, animals were treated with saline or colchicine after which the luteinized ovary or isolated luteal cells were assayed for their cholesterol content, their ability to synthesize cholesterol endogenously, or their ability to utilize lipoprotein-delivered cholesterol for the production of progesterone. The results show that animals treated with colchicine show a number of changes in luteal cell cholesterol metabolism: namely a 60% decline in stored cholesterol, a 3-fold rise in the activity of the cholesterol synthesizing enzyme HMG CoA reductase (although no change occurs in other cholesterol metabolizing enzymes), and a 3-fold rise in the capacity of the cells to incorporate precursor [14C]acetate into cholesterol. On the other hand, cells of animals treated with colchicine or cells treated with colchicine under in vitro circumstances are unable to fully utilize cholesterol provided by high density lipoproteins (HDL): this occurs despite the fact that the binding of HDL particles to luteal cells is quite normal after colchicine treatment. These findings are consistent with the view that a primary effect of colchicine on luteal cell progesterone production is in preventing the normal uptake of HDL-cholesterol.

Ambient plasma free fatty acid concentrations in noninsulin-dependent diabetes mellitus: evidence for insulin resistance

Plasma glucose, insulin, and FFA concentrations were determined in 15 normal subjects and 15 patients with noninsulin-dependent diabetes mellitus (NIDDM) from 0800 to 1600 h. Breakfast and lunch were consumed at 0800 and 1200 h, respectively, and plasma concentrations were measured at hourly intervals from 0800-1600 h. Plasma glucose concentrations between 0800 and 1600 h were significantly elevated in patients with NIDDM, and the higher the fasting glucose level, the greater the postprandial hyperglycemia. Hyperglycemia in patients with NIDDM was associated with plasma insulin levels that were significantly higher (P less than 0.001) than those in normal subjects, and substantial hyperinsulinemia occurred between 0800 and 1600 h in patients with mild NIDDM (fasting plasma glucose concentrations, less than 140 mg/dl). Both fasting and postprandial FFA levels were also increased in patients with NIDDM (P less than 0.001), and the greater the plasma glucose response, the higher the FFA response (r = 0.70; P less than 0.001). However, there was no significant correlation between plasma insulin and FFA concentrations. More specifically, hyperinsulinemic patients with mild diabetes (fasting plasma glucose, less than 140 mg/dl) maintained normal ambient FFA levels, while FFA concentrations were significantly elevated in patients with severe NIDDM (fasting plasma glucose, greater than 250 mg/dl), with insulin concentrations comparable to those in normal subjects. These results demonstrate that patients with NIDDM are not capable of maintaining normal plasma FFA concentrations. This defect in FFA metabolism is proportionate to the magnitude of hyperglycemia and occurs despite the presence of elevated levels of plasma insulin. These results are consistent with the view that insulin resistance in NIDDM also involves the ability of insulin to regulate FFA metabolism.

Quantitation of insulin-stimulated glucose disposal in patients with non-insulin-dependent diabetes mellitus

Glucose disposal rates (Rd) during an insulin clamp study reflect both basal and insulin-stimulated Rd. To quantify the amount of glucose taken up in response to a known increase in insulin concentration, two consecutive studies were performed on 10 patients with mild to moderate NIDDM (mean fasting glucose = 146 mg/dl) and 10 normal subjects. Endogenous insulin secretion was inhibited by somatostatin and plasma glucose level maintained at 180 mg/dl for 5. Rd (mg/m2/min) was determined isotopically for 2.5 h at insulin concentrations approximately 6 microU/ml and during 2.5 h of physiologic hyperinsulinemia at approximately 60 microU/ml (total glucose disposal), with the increase in Rd resulting from the approximate 10-fold elevation of plasma insulin concentration defined as insulin-stimulated glucose disposal. Results showed that the increment in Rd resulting from the elevation of plasma insulin concentration was relatively minor in NIDDM (38 +/- 6), increasing from a mean (+/- SEM) value of 83 +/- 8 to 121 +/- 12. Similar values in normal subjects were 90 +/- 7 and 274 +/- 26 with an increment of 183 +/- 21. Thus, insulin-stimulated glucose uptake in patients with NIDDM was only one-fifth of that in normals, and accounted for only 31% (38 divided by 121) of total glucose disposal during the clamp study. These data indicate that the majority of previous insulin clamp studies of in vivo insulin action in patients with NIDDM, in which total glucose disposal and insulin-stimulated glucose disposal have been equated, have underestimated the magnitude of insulin resistance present in NIDDM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of noninsulin-dependent diabetes mellitus on the uptake of very low density lipoproteins by thioglycolate-elicited mouse peritoneal macrophages

The lipid-laden foam cells from atherosclerotic lesions appear to be derived from macrophages which have accumulated lipids from plasma lipoproteins. When examined in vitro, thioglycolate-elicited mouse peritoneal macrophages do not accumulate lipids when exposed to normal low density lipoproteins (LDL), but take up very low density lipoproteins (VLDL) or chemically modified LDL with resultant lipid accumulation. Patients with noninsulin-dependent diabetes mellitus (NIDDM) have an increased incidence of atherosclerosis, due in part to disturbances in lipoprotein metabolism. We investigated the possibility that VLDL isolated from patients with NIDDM are taken up by mouse peritoneal macrophages more avidly than normal. Two groups of patients with NIDDM were studied; one group was normotriglyceridemic and the other group was hypertriglyceridemic. The VLDL from both normotriglyceridemic and hypertriglyceridemic patients were enriched in cholesterol and triglyceride compared to that from normal subjects. Thioglycolate-elicited mouse peritoneal macrophages bound and degraded greater amounts of VLDL isolated from patients with NIDDM (both normo- and hypertriglyceridemic) than of VLDL from normal subjects. While normal VLDL caused a marked increase in cellular triglyceride and cholesteryl ester contents in macrophages, VLDL isolated from patients with NIDDM resulted in an even greater cellular accumulation of lipids. These results suggest that the VLDL of patients with NIDDM have alterations in their composition and metabolic behavior. The increased uptake of VLDL by macrophages may contribute to the enhanced atherosclerosis present in NIDDM.

Estimates of in vivo insulin action in humans: comparison of the insulin clamp and the minimal model techniques

The insulin clamp technique, which is often assumed to measure the ability of insulin to stimulate glucose uptake, actually measures both insulin-independent and insulin-dependent glucose uptake. In contrast, the minimal model technique, recently introduced by Bergman, Philips and Cobelli (1981), attempts to directly estimate insulin sensitivity (insulin-dependent glucose uptake = S1) by measurement of plasma glucose and insulin values during a 3 hour intravenous glucose tolerance test (IVGTT). In the present study estimates of insulin action derived from the insulin clamp and the minimal model technique were compared in 20 humans with varying degrees of glucose tolerance. The insulin response during the IVGTT was too low to permit calculation of S1 in 5 subjects - 4 with Type II diabetes and 1 with normal glucose tolerance. Although the correlation coefficient between the two tests in the other 15 patients was statistically significant (r = 0.53, P less than 0.05), this statement is somewhat misleading. Thus, S1 in the 4/7 patients with Type II diabetes in whom it could be measured was zero, and the correlation between estimates of insulin action with the two techniques in the 11 non-diabetic patients was not statistically significant (r = 0.41, P = NS) when these 4 patients were removed from the analysis. In conclusion, these data indicate that there was only a weak correlation between estimates of insulin action assessed with the insulin clamp and the minimal model techniques. One explanation for this observation is that the insulin-independent component of total glucose disposal both varies widely among patients and contributes significantly to glucose uptake as assessed by the insulin clamp technique.(ABSTRACT TRUNCATED AT 250 WORDS)

How insulin resistant are patients with noninsulin-dependent diabetes mellitus?

The study was carried out to quantify the ability of physiological increases in the plasma insulin concentration to stimulate glucose disposal above basal levels in 25 normal subjects and 25 patients with noninsulin-dependent diabetes mellitus (NIDDM). Patients were sex, age, and weight matched, and glucose disposal was determined under basal conditions (plasma insulin, approximately 10 microU/ml) and after plasma insulin levels had been increased to approximately 90 microU/ml. The mean (+/- SEM) glucose disposal rate was significantly greater (P less than 0.001) under basal conditions in patients with NIDDM (110 +/- 5 mg/m2 X min) than in individuals with normal glucose tolerance (77 +/- 4 mg/m2 X min). Glucose disposal rates increased in both normal subjects and NIDDM patients when plasma insulin concentrations were increased to about 90 microU/ml; however, the increment was much greater in normal subjects. Thus, glucose disposal only rose to a mean (+/- SEM) value of 145 +/- 7 mg/m2 X min in patients with NIDDM, representing an approximate 30% increase due to insulin. In contrast, a similar elevation of plasma insulin in normal subjects resulted in an increase in glucose disposal of approximately 300%, reaching a mean (+/- SEM) value of 310 +/- 24 mg/m2 X min. These results indicate that the defect in insulin-stimulated glucose uptake is significantly greater in patients with NIDDM than has previously been found.

Glucose and fructose feeding lead to alterations in structure and function of very low density lipoproteins

Young male rats were fed regular lab chow, or a diet containing 66% of total calories as either glucose or fructose. Both experimental diets led to hypertriglyceridemia, with fasting TG concentrations after one week of 195 +/- 20 and 296 +/- 44 mg/dl for rats fed glucose and fructose, respectively, compared to 94 +/- 10 mg/dl in the control rats. Moderate changes in VLDL composition were observed with both test diets, characterized by slight increases in TG: protein ratio, and increased total cholesterol and phospholipid content. In addition, VLDL isolated from rats fed high carbohydrate diets were increased in size, with a mean VLDL particle diameter of 666 A and 720 A in glucose-fed and fructose-fed rats, as compared to 536 A in control rats. The changes in lipid composition and size of VLDL particles isolated from glucose and fructose-fed donor rats were associated with an increase in their rate of removal from the circulation following their injection into normal recipient rats (half-life time 2.4 +/- 0.2 and 3.2 +/- 0.3 min respectively) as compared to VLDL-TG derived from chow fed donors (4.1 +/- 0.2 min). These data indicate that diets high in either glucose or fructose can lead to both structural and functional changes in VLDL, and provide additional evidence that the ability of fructose to induce profound hypertriglyceridemia is not secondary to a defect in VLDL-TG catabolism.

Presentation of a new method for specific measurement of in vivo insulin-stimulated glucose disposal in humans: comparison of this approach with the insulin clamp and minimal model techniques

This study was initiated to compare the abilities of two alternative approaches to the measurement of insulin-dependent glucose disposal in normal humans. The ability of insulin to stimulate glucose disposal was measured in 12 normal subjects by determining glucose disposal rates during insulin clamp studies carried out at both basal insulin concentrations (approximately 6 microU/ml) and during a period of sustained hyperinsulinemia (approximately 60 microU/ml). The increment in glucose disposal was defined as insulin-dependent disposal and compared to estimates of insulin action generated by both the conventional insulin clamp approach and the minimal model technique. The results documented an extremely close correlation (r = 0.99; P less than 0.001) between the direct determination of insulin-dependent glucose disposal and insulin-stimulated glucose disposal as estimated by the insulin clamp technique. In contrast, there was a poor correlation (r = 0.44; P = NS) between insulin sensitivity as estimated by the minimal model technique and insulin-dependent glucose disposal. These results indicate that the value of glucose disposal determined by the insulin clamp approach, which includes both insulin-independent and insulin-dependent glucose disposal, provides an excellent estimate of insulin-dependent glucose disposal in subjects with normal glucose tolerance. Unfortunately, this does not appear to be true of the minimal model technique. However, it must be emphasized that these conclusions are only applicable to normal humans, and may not apply to normal subjects of other species or to humans under different physiological or pathological situations.

Regulation of luteal cell 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by estradiol

Recent studies have suggested that estradiol or androgen precursor may stimulate steroidogenesis in the luteal cell by modulating intracellular sterol availability and metabolism. This investigation was performed to examine the effect of estradiol on de novo synthesis of cholesterol. Pregnant rats hypophysectomized and hysterectomized on Day 12 were treated for 72 h with either estradiol or testosterone. De novo cholesterol synthesis was determined by measurement of the specific activity of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, the rate limiting enzyme in cholesterol biosynthesis, in microsome-enriched preparations of luteal tissue and incorporation of [14C] acetate into cholesterol by corpora lutea incubated in vitro. Estradiol or testosterone treatment caused a 4- to 5-fold stimulation of luteal cholesterol biosynthesis, as measured by these techniques. NaF, an inhibitor of phosphatase which blocks the conversion of the inactive enzyme to the active form, reduced the HMG CoA reductase activity to 30% in corpora lutea obtained from either steroid or vehicle-treated rats. However, an increase in enzyme activity of comparable magnitude by steroids was observed whether microsomes were isolated with or without NaF. The effect of estradiol appears to be enzyme-specific, since it failed to affect the microsomal marker, NADPH-cytochrome c reductase. Since the cholesteryl ester content of corpora lutea falls in response to steroid treatment, rats were treated with 4-aminopyrazolo-[3,4d]pyrimidine (4-APP) to deplete cellular cholesterol content.(ABSTRACT TRUNCATED AT 250 WORDS)

Monte Carlo study of the GTP cap in a five-start helix model of a microtubule

Earlier Monte Carlo studies on a single-helix model of the GTP cap at the end of a microtubule are extended here to a more realistic five-start helix model of the microtubule end. As in the earlier work, phase changes occur at the microtubule end: the end is either capped with GTP and growing slowly or uncapped and shortening rapidly, and these two regimes alternate (at a given tubulin concentration) at steady state. Macroscopic rate constants for the two-phase model are deduced from the Monte Carlo results. The macroscopic rate constants lead to properties that are in semiquantitative agreement with related experiments of Mitchison and Kirschner.

Influence of high-density lipoprotein on estradiol stimulation of luteal steroidogenesis

The aim of this investigation was to determine whether luteal cells utilize cholesterol derived from high-density lipoprotein (HDL) for steroidogenesis and whether estrogen enhances luteal utilization of exogenous sterol. Incubation of Day 15 corpora lutea (CL) with different doses of human HDL resulted in a dose-dependent increase in progesterone production. HDL in vitro enhanced the overall steroidogenic capacity. However, the percentage of increases in 17 alpha-hydroxyprogesterone, testosterone and estradiol were significantly less than that of progesterone. Day 12 hypophysectomized and hysterectomized pregnant rats were treated with either estradiol, testosterone or vehicle for 72 h. Serum pregnenolone and progesterone were markedly increased by the steroid treatment, yet in vitro production of progesterone by CL in all the groups was similar. However, in the presence of HDL in the media, only luteal tissues from steroid-treated rats increased their progesterone output. The reduced production of progesterone by luteal cells of vehicle-treated rats was not due to an accumulation of pregnenolone but to an overall reduction in exogenous sterol utilization. In summary, results of this investigation suggest 1) luteal cells of pregnant rats effectively utilize cholesterol from HDL for maximal steroidogenesis, and 2) estradiol may stimulate luteal steroidogenesis, at least in part, by affecting the incorporation or utilization of cholesterol from HDL into the cell.

Morphological evidence that high density lipoproteins are not internalized by steroid-producing cells during in situ organ perfusion

Although it is clear that high density lipoproteins (HDL) can support steroidogenesis in several rat cell systems, questions still arise as to how HDL are processed by cells. In particular, it is not yet clear whether HDL are internalized by a pathway similar to that used for low density lipoproteins. This issue was examined in the present study using the luteinized ovaries of hormone-primed immature rats in an in situ perfusion system. Ovaries were perfused for 2-120 min with 125I-labeled human or rat HDL and processed for autoradiographic studies at the light and electron microscopic level, or homogenized and used for isolation of subcellular membranes. The results show that the luteal cells of this tissue bind both human and rat HDL with great specificity. Moreover, the intact HDL particle does not appear to be internalized by the luteal cell during the period of perfusion: i.e., the protein moiety of the labeled HDL remains associated with the plasma membrane at all times. Evidence from the autoradiographs suggest, however, that with time, an increasing proportion of the plasma membrane-bound protein is associated with inverted microvilli, which are embedded within the cytoplasm and make close contact with structures of the interior of the cell. We speculate that HDL-cholesterol may be transferred at such sites.

Gonadotropin modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in desensitized luteinized rat ovary

These studies were done to examine the effect of gonadotropin on rat luteal 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity (the rate-limiting step in cholesterol biosynthesis) in ovaries of pregnant mare's serum gonadotropin (PMSG)-human chorionic gonadotropin (hCG) primed rats. Administration of hCG stimulated HMG CoA reductase activity in a time- and dose-dependent manner: significant increases were noted within 4 h, with maximum effects (30-40-fold increases) seen 24 h after hCG (25 IU) administration. This effect was specific in that only LH, of several hormones tested, was as effective as hCG in stimulating HMG CoA reductase activity, and no change in the activity of either liver microsomal HMG CoA reductase or luteal microsomal NADPH-cytochrome c reductase was seen after hCG. The gonadotropin-induced increase in HMG CoA reductase activity seemed to be due to a net increase in enzyme activity, not to a change in the phosphorylated/dephosphorylated state of the enzyme. Pretreatment of animals with aminoglutethimide, an inhibitor of the conversion of cholesterol to steroid (pregnenolone), prevented the hCG-induced rise in HMG CoA reductase activity, whereas treatment with 4-aminopyrazolo[3,4-d]pyrimidine (4-APP), which depletes cellular cholesterol content, led to striking increases in enzyme activity. However, the combined effects of 4-APP and hCG were additive, suggesting that the stimulating effect of hCG on HMG CoA reductase activity is not entirely due to a depletion of cellular sterol content of luteinized ovaries. Similarly, cholesteryl ester and cholesterol syntheses as measured by [14C]acetate conversion were also increased by hCG and 4-APP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

How do enzyme activities control metabolite concentrations? An additional theorem in the theory of metabolic control

A simple theorem is derived relating the extent to which enzymes in a metabolic pathway control the steady-state concentration of metabolites to the kinetic properties of those enzymes. The theorem gives insight into the mechanism by which the concentration of a second messenger is controlled by the enzymes that form and degrade it, and provides an alternative to the 'cross-over theorem'.

A comparison of the relative effects of obesity and non-insulin-dependent diabetes mellitus on in vivo insulin-stimulated glucose utilization

Insulin clamp studies were carried out in 50 individuals, 25 with normal glucose tolerance and 25 with non-insulin-dependent diabetes mellitus (NIDDM). Both diagnostic groups were further subdivided on the basis of body mass index (BMI) into an obese (BMI greater than 28 kg/m2) or nonobese group (BMI less than 28 kg/m2). The obese and nonobese subjects in each diagnostic category had similar plasma glucose levels in response to an oral glucose challenge. In addition, insulin-stimulated glucose utilization, as assessed by determination of glucose metabolic clearance rate (MCR), was not different as a function of obesity. Glucose MCR (mean +/- SEM) in obese subjects (mean BMI = 32.1) with normal glucose tolerance was 162 ml/min/m2, as compared with a value of 205 ml/min/m2 in nonobese individuals (mean BMI = 23.8). This difference was not statistically significant. Similarly, glucose MCR in obese patients (mean BMI = 34.7) with NIDDM was 55 ml/min/m2, as compared with a value of 80 ml/min/m2 in nonobese subjects (mean BMI = 24.9) with NIDDM. However, as can be seen from the above data, glucose MCR in patients with NIDDM, either nonobese or obese, was markedly reduced (P less than 0.001) when compared with that of normal subjects. These data emphasize the profound effect of NIDDM on reducing in vivo insulin action, and point out that the impact of obesity on insulin resistance is minor in comparison.

Insulin-stimulated glucose disposal increases with time in patients with non-insulin-dependent diabetes mellitus

The relative effects of time versus ambient glucose concentration on insulin-stimulated glucose uptake was estimated by performing 5-h insulin clamp studies in patients with NIDDM. Each experimental subject was studied three times, at steady-state plasma insulin levels approximately 2000 microU/ml, but at different steady-state plasma glucose concentrations (studies A, B, and C). Study A consisted of a 5-h clamp, with plasma glucose level maintained at the basal level of fasting hyperglycemia; study B differed in that the basal level of fasting hyperglycemia was reduced during the first hour to approximately 80 mg/dl, and maintained there for the next 4 h; and study C was carried out by clamping the patient at the basal glucose level for 2 h, lowering the glucose concentration to approximately 80 mg/dl during the third hour, and then clamping at this level for the last 2 h. The glucose metabolic clearance rate (MCR) was calculated from 60 to 120 min and from 240 to 300 min during each study, and the results indicated that values for glucose MCR were time dependent, being significantly greater (20-60%) in the fifth than in the second hour in two (studies A and B) of the three studies. In contrast, glucose MCR was independent of plasma glucose concentration, and relatively constant in each subject, as long as it was measured during the same time period. The time-dependent increase in glucose MCR was associated with an approximate 30% increase in steady-state plasma insulin concentrations when comparing the second and fifth hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between the plasma insulin response to oral glucose and insulin-stimulated glucose utilization in normal subjects

The relationship between in vivo insulin-stimulated glucose utilization (euglycemic clamp technique) and various estimates of the plasma insulin response to oral glucose was defined in 62 subjects with normal glucose tolerance. Both the incremental insulin increase above fasting (r = 0.61) and the total integrated insulin response (r = 0.65) were highly correlated (P less than 0.001) with in vivo insulin action, and the relationship between total insulin response and insulin action remained significant (r = 0.61, P less than 0.001) when corrected for variations in total glucose response, age, and obesity. In a subset of these subjects (N = 27) we were also able to assess state of habitual physical activity by estimating maximal oxygen consumption during bicycle ergometry. A significant correlation also existed between insulin action and response in these subjects (r = 0.67, P less than 0.001), which remained significant (r = 0.65) when differences in total glucose response, obesity, age, and maximal oxygen consumption were taken into account. These data demonstrate that there is a significant correlation between insulin response and insulin action in normal individuals that can account for approximately one-third of the total variance in insulin action seen in subjects with normal glucose tolerance. Thus, determination of plasma insulin levels after an oral glucose challenge can only provide a qualitative estimate of insulin-stimulated glucose utilization.

Why does experimental insulin deficiency lead to a decrease in removal of very low density-triglyceride from plasma?

We have previously suggested that mechanisms other than reduced lipoprotein lipase (LPL) activity might contribute to the defect in plasma removal of very low density lipoprotein (VLDL)-triglyceride (TG) observed in insulin-deficient rats. To further evaluate this phenomenon, removal rates of TG in nonfractionated plasma, as well as in isolated lipoprotein fractions obtained from insulin-deficient and control rats, were compared in a new, sensitive in vivo bioassay system (estradiol-treated male rats with a consistently low endogenous VLDL-TG pool). Removal of TG in nonfractionated plasma from insulin-deficient rats was slower than that of control rats: 3.0 +/- 0.3 vs 1.6 +/- 0.2 min (P less than 0.001). No difference was found in removal rate of isolated VLDL-TG (2.5 +/- 0.3 vs 2.6 +/- 0.4 min), or in removal rates of TG carried in other lipoprotein fractions. We next determined the effect of injection into normal rats of aliquots of dialyzed lipoprotein-free (D greater than 1.215) plasma from insulin-deficient and control rats on the removal rate of normal VLDL-TG, and found that lipoprotein-free plasma from insulin-deficient rats significantly (P less than 0.01) prolonged removal of normal VLDL-TG (4.3 +/- 0.4 to 6.8 +/- 0.7 min). This same fraction did not interfere with the in vitro hydrolysis of normal VLDL-TG by post-heparin LPL. Thus, a factor in the D greater than 1.215 plasma fraction of insulin-deficient rats is present which interferes with the rate of removal of TG from plasma, unrelated to inhibition of LPL activity.

Regulation of luteal cell lipoprotein receptors, sterol contents, and steroidogenesis by estradiol in the pregnant rat

Although estradiol has been found to possess receptors in the luteal cell and to stimulate progesterone synthesis, its mechanism of action in the corpus luteum remains completely unknown. To determine whether estradiol modulates cellular uptake of lipoprotein substrate and intracellular cholesterol utilization, pregnant rats were hypophysectomized and hysterectomized on day 12 to reduce the luteal content of estradiol. They were treated with either 100 micrograms estradiol daily or with a 1-cm capsule filled with testosterone, which maintained luteal estradiol at levels found in intact pregnant rats. Blood was obtained 24, 48, and 72 h later for progesterone and cholesterol measurement. At 72 h, rats were killed, and corpora lutea (CL) were isolated for measurement of cholesteryl ester, free cholesterol, and [125I]iodo high density lipoprotein [( 125I]iodo-HDL)- and [125I]iodo-hCG-binding activities. In vivo treatment with estradiol or testosterone increased serum progesterone concentrations from 35 +/- 7 ng/ml in vehicle-treated rats to 128 +/- 21 and 118 +/- 16, respectively, and luteal weight from 2.1 +/- 0.2 mg/CL to 3.9 +/- 0.3 and 4.0 +/- 0.3 within 72 h. However, steroid treatment did not induce a change in luteal cell number, since the content of DNA per CL remained similar in all groups. It also did not modify levels of serum cholesterol. [125I]Iodo-HDL binding in luteal cells increased from 3.2 +/- 0.3 pg/cell in vehicle-treated rats to 9.9 +/- 0.8 and 7.9 +/- 0.6 after estradiol or testosterone treatment, while the luteal cell content of cholesteryl ester declined from 12.5 +/- 2.0 to 7.7 +/- 0.5 and 8.4 +/- 0.9 micrograms/CL, respectively. Thus, estradiol or testosterone increases luteal cell size but not cell number, depletes cholesteryl ester, and enhances HDL receptor content and progesterone synthesis. These results suggest that one possible mechanism by which estradiol and testosterone stimulate luteal cell steroidogenesis is by increasing the delivery of cholesterol substrate through a receptor-mediated process and by enhancing cholesterol utilization.

Characterization of the binding site on thioglycolate-stimulated mouse peritoneal macrophages that mediates the uptake of very low density lipoproteins

Isolated mouse peritoneal macrophages that had been stimulated with thioglycolate were shown to take up and degrade normal human 125I-very low density lipoproteins (VLDL). Uptake occurred via a specific cell surface receptor which was shown to be 1) temperature-dependent, 2) calcium-dependent, and 3) susceptible to proteolytic digestion. The receptor-mediated uptake and degradation of VLDL markedly stimulated the synthesis and accumulation of triglyceride and cholesteryl ester within macrophages. The degradation of the protein and lipid portions of VLDL occurred within lysosomes. Competition studies showed that the binding site for VLDL was different from the receptor for normal low density lipoproteins or for acetylated low density lipoproteins but that there was cross competition with beta-VLDL. In addition, positive charges appeared to play an important role in the recognition of VLDL by their receptors since polyamines were able to markedly inhibit VLDL binding, degradation, and lipid accumulation while negatively charged compounds were without effects. These studies indicate that 1) stimulated mouse peritoneal macrophages possess specific receptors which recognize normal human VLDL and 2) the receptor-mediated uptake of VLDL results in the accumulation of triglyceride and cholesteryl ester within macrophages.

Does day-long absolute hypoinsulinemia characterize the patient with non-insulin-dependent diabetes mellitus?

Plasma glucose and insulin responses to a standard oral glucose tolerance test (75 g of glucose) and to mixed meals were compared in 15 normal subjects and 15 patients with non-insulin-dependent diabetes mellitus (NIDDM). Fasting plasma glucose levels were above 140 mg/dL in all patients with NIDDM, and the two groups were weight matched. Plasma glucose levels were significantly higher in patients with NIDDM throughout the glucose tolerance test, and this was associated with a marked reduction in plasma insulin response. Plasma glucose levels were also higher in patients with NIDDM when measured hourly from 8 AM to 5 PM (mixed meals were consumed at 8 AM and 12 PM), but the plasma insulin concentration of the two groups were similar. Thus, the day-long circulating insulin levels of patients with NIDDM are not reduced. Consequently, these patients cannot be considered to be absolutely insulin deficient.

Synthesis of the 3'-half molecule of yeast alanine tRNA

This paper deals with the synthesis of the 3'-half molecule of yeast alanine transfer RNA (tRNAAlay) by ligation with T4 RNA ligase of three component oligonucleotide fragments corresponding to nucleotides 36-45(I), 46-57(II) and 58-76(III) in succession extending from the 3'-end to the 5'-end. First, in a ratio of acceptor to donor at 1.5 to 1, we adopted a method of three successive reactions, namely, the 5'-phosphorylation of the nonadecamer (III), ligation with the dodecamer (II) and the 5'-phosphorylation of the ligation product formed; with one isolation step and obtained the 5'-phosphorylated 31mer(46-76) (IV) in an overall yield of 70%. Then the 31mer(IV) as a donor was ligated with 3 times of decamer (I) to form the 41mer(36-76) (V), the 3'-half molecule of tRNAAlay. The yield was 67%. After 5'-phosphorylation, (V) was ligated with the natural 5'-half molecule to form the semi-synthetic tRNAAlay, which was biologically active, i.e. accepting and transferring (3H)-alanine into proteins.

Studies of the mechanism of fructose-induced hypertriglyceridemia in the rat

Carbohydrate-induced hypertriglyceridemia is easily produced in the rat, and fructose has been shown to be particularly potent in this regard. In this study we have compared the effects of feeding rats diets high (66% of total calories) in fructose or glucose on various aspects of carbohydrate and lipid metabolism. The results confirmed previous observations that fructose (456 +/- 276 mg/dl) was more potent (p less than 0.001) in raising plasma TG concentration than was glucose (242 +/- 13 mg/dl), and indicated that the difference in magnitude of hypertriglyceridemia produced by the two carbohydrates was closely related to the ability of the test diets to increase VLDL-TG secretion (r = 0.85, p less than 0.001). Both glucose and fructose feeding led to comparable degrees of hyperinsulinemia, and plasma TG concentrations increased before hyperinsulinemia evolved in fructose-fed rats. Therefore, it was concluded that fructose can act directly on the liver to increase VLDL-TG secretion, and that fructose-induced hypertriglyceridemia can occur in the absence of hyperinsulinemia. On the other hand, the rise in plasma TG concentration produced by fructose was reduced dramatically in exercise-trained rats, and this was associated with a decrease in plasma insulin concentration. Based upon these observations, we suggest that fructose feeding produces hypertriglyceridemia by directly stimulating hepatic VLDL-TG secretion, as well as by producing insulin resistance and hyperinsulinemia, and that it is the combined effect of these two separate actions which accounts for the magnitude of fructose-induced hypertriglyceridemia.

Are the binding and degradation of low density lipoprotein altered in Type 2 (non-insulin-dependent) diabetes mellitus?

Studies in vitro have shown that glycosylation of low density lipoprotein (LDL) will decrease its ability to bind to its receptor. We have evaluated the possibility that such an event might occur in vivo in diabetes by comparing the binding and degradation by normal fibroblasts and mouse peritoneal macrophages of LDL obtained from normal control subjects and patients with Type 2 (non-insulin-dependent) diabetes mellitus. When compared with control subjects, Type 2 diabetic patients had elevated fasting glucose (increased by 160%), haemoglobin AIc (increased by 75%), triglyceride (increased by 550%), and cholesterol (increased by 48%) levels. LDL from Type 2 diabetic patients displayed populations of particles with more heterogeneous hydrated densities than LDL from control subjects, with enrichment in the triglyceride content of the lighter population. 125I-LDL from normal and Type 2 diabetic subjects bound to fibroblasts with similar binding affinities and binding capacities. The kinetics of degradation of LDL from normal and Type 2 diabetic subjects by fibroblasts were also similar. Furthermore, all populations of LDL particles from Type 2 diabetic patients were bound and degraded by normal fibroblasts in identical fashions. In addition, 125I-LDL from normal and Type 2 diabetic subjects were not bound or degraded by mouse peritoneal macrophages. It is concluded that the LDL of patients with Type 2 diabetes with moderate hyperglycaemia are not modified sufficiently to alter their normal binding and degradation by human fibroblasts or to cause their uptake by mouse peritoneal macrophages.

Hypertriglyceridemia and lipoprotein lipase activity in experimental uremia

The relationship between changes in tissue lipoprotein lipase (LPL) activity and triglyceride (TG) concentration has been studied in chronically uremic rats. Uremic rats had a 6-fold increase in BUN and 75% rise in TG levels associated with a 50% reduction in adipose tissue heparin-releasable LPL activity. However, total LPL activity of adipose tissue and muscle was not significantly decreased. Furthermore, there was no significant correlation between the rise in plasma TG concentration seen in uremic rats and the reduction in heparin-releasable adipose tissue LPL activity. These data suggest that the TG removal defect associated with chronic uremia is not a simple function of a decrease in LPL activity.

Serum lipoproteins increase testosterone production in hCG-desensitized Leydig cells

The effects of high density and low density lipoproteins (HDL and LDL) on testosterone synthesis by Leydig cells from rats treated three days earlier with 50 IU human chorionic gonadotropin (hCG) or from saline-injected control rats were investigated. HDL caused a significant, dose-dependent increase in both basal and hCG-stimulated testosterone production by Leydig cells from hCG-treated rats, but had no marked effect on Leydig cells from control rats. The serum testosterone concentration of hCG-treated rats was three times higher than in controls. Basal testosterone production by Leydig cells from hCG-treated rats was elevated two-fold in comparison to controls, while hCG-stimulated testosterone production was significantly lower than in controls. Both basal and hCG-stimulated testosterone production by Leydig cells from hCG-treated rats were significantly increased by the addition of HDL or LDL. Furthermore, HDL restored the hCG-stimulated testosterone production to the maximal amount produced by hCG-stimulated Leydig cells from control animals. When equivalent amounts of HDL or LDL cholesterol (140 microgram/ml) were added to the incubation, HDL caused a greater increase in testosterone production than did LDL. These results suggest that hCG-treatment, in vivo, caused a prolonged stimulation of the Leydig cells which resulted in depletion of the cholesterol available for steroidogenesis, since the addition of cholesterol in the form of HDL or LDL caused an increase in the testosterone production of these Leydig cells.

Evidence for a new cause of defective plasma removal of very low density lipoproteins in insulin-deficient rats

These studies were initiated to see if factors other than reduced lipoprotein lipase activity might contribute to the defect in plasma removal of very low density lipoprotein (VLDL) that is observed in insulin-deficient rats. VLDL-triglyceride (TG) was labeled in vivo with 3H-glycerol in control and diabetic rats, and aliquots of plasma containing 3H-VLDL were injected into normal recipient rats. The half-time (t 1/2) of removal was almost twice as long when plasma from diabetic rats was injected, and this was true when the diabetic rats were fed either sucrose or regular chow. A comparable increase in t 1/2 was observed when 3H-VLDL isolated from normal rats was recombined with VLDL-free plasma from control and diabetic rats and injected into normal recipients. As before, the changes observed were not dependent upon antecedent diet. However, no significant difference in t 1/2 was observed when 3H-VLDL was isolated from control and diabetic rats and injected into normal recipients. Thus, there appears to be a factor present in VLDL-free plasma obtained from diabetic rats that interferes with removal of VLDL from the vascular compartment. Whether this factor is found in diabetic plasma in vivo, or is transferred from diabetic VLDL to diabetic plasma in the isolation procedure, remains to be clarified. In either event, there appears to be a factor, other than reduced LPL activity, that may play a role in the defect of VLDL-TG removal seen in insulin deficiency.

Relationship between plasma triglyceride concentration and adipose tissue lipoprotein lipase activity in rats of different ages

Plasma triglyceride (TG) concentration and adipose tissue lipoprotein lipase (LPL) activity was determined in male and female rats aged 40 days, 100 days and 1 year. Plasma TG levels increased with age in both sexes, but the rate of rise was greater in female as compared to male rats. As a consequence, plasma TG levels, which were lower in 40 and 100 day old female rats, were similar in both sexes at 1 year. Adipose tissue LPL activity, measured at the same time of day as were plasma TG levels, did not vary as a function of either age or sex. On the other hand, when adipose tissue LPL activity was measured in the early morning, just after rats had finished eating, there was a marked increase in the activity of 40 day old rats of either sex. This dietary-induced rise in adipose tissue LPL activity following eating was not seen in 100 day and 1-year-old rats. Thus, changes in adipose tissue LPL activity cannot account for previously described effects of age and sex on plasma TG concentration.

Identification of specific high density lipoprotein-binding sites in rat testis and regulation of binding by human chorionic gonadotropin

Freshly prepared rat testicular membranes bind iodinated rat high density lipoprotein (HDL) with high affinity (Kd = 32 microgram of HDL protein/ml). This high density lipoprotein binding differs from low density lipoprotein binding by cultured human fibroblast cells in two ways; it is not affected by Ca2+ or ethylenediaminetetraacetic acid, and it is not sensitive to pronase and trypsin. Testicular binding activity is primarily found in interstitial tissue containing Leydig cells and can be modulated by human chorionic gonadotropin administration in vivo (2-fold increase of binding capacity, with no affinity change) with 250 units/kg of human chorionic gonadotropin injection daily for 4 days. The interstitial high density lipoprotein binding site also recognizes rat very low density lipoprotein, but not rat low density lipoprotein, as shown by displacement experiments. When membrane preparations of various other tissues were assessed for their high density lipoprotein binding, we found that the adrenal density lipoprotein binding, we found that the adrenal gland binds rat high density lipoprotein with similar affinity and capacity, while spleen, kidney, and heart showed no high affinity binding. In addition, when iodinated rat low density lipoprotein was tested for its ability to bind to testicular membranes, no high affinity saturable binding was observed. We conclude that there are specific high density lipoprotein-binding sites present in steroidogenic tissues; these binding sites are not found in the nonsteroidogenic tissues tested. Furthermore, no high affinity low density lipoprotein-binding sites can be demonstrated in the testis; thus it appears that high density lipoprotein, rather than low density lipoprotein, is the major cholesterol-carrying lipoprotein recognized by the rat testis.

Dissociation between plasma triglyceride concentration and tissue lipoprotein lipase deficiency in insulin-deficient rats

Insulin deficiency was produced by streptozotocin in young (5-6 wk old) male rats, and measurements were made of plasma triglyceride and glucose concentrations and of lipoprotein lipase (LPL) activity of adipose tissue (epididymal) and muscle (gastrocnemius and soleus). Rats with streptozotocin-induced diabetes underwent a significant reduction in adipose tissue LPL activity (both total and heparin releasable), but the fall in LPL activity in these rats bore little relationship to their rise in plasma triglyceride concentration. Furthermore, muscle LPL activity was essentially unchanged in diabetic rats. Qualitatively similar changes were observed when measurements were made at either 8 a.m. (after the normal evening access to food) or 2 p.m. (6 h after food withdrawal). It is concluded that the hypertriglyceridemia that occurs secondary to insulin deficiency is not a simple function of decreased tissue LPL activity.

Hyperinsulinemia in fructose-induced hypertriglyceridemia in the rat

Rats were fed diets containing (as percent of calories) 66% glucose or fructose, 22% casein, and 12% lard, for 1 wk. The effects of these diets on plasma triglyceride, glucose, and insulin concentrations were compared to those of control rats eating regular rat chow. Plasma triglyceride levels increased from a mean (+/- SE) control level of 85 +/- 7 to 142 +/- 9 (p less than 0.001) and 380 +/- 38 (p less than 0.001) mg/dl in dextrose- and fructose-fed rats, respectively. Plasma insulin concentrations demonstrated a similar increase, rising from a mean (+/- SE) control value of 29 +/- 4 microunits/ml to 55 +/- 10 microunits/ml in dextrose-fed rats and 85 +/- 12 microunits/ml in rats eating the fructose diet. Plasma glucose concentrations of the three groups were comparable. These results indicate that fructose-induced hypertriglyceridemia is associated with significant hyperinsulinemia.

Is the hypertriglyceridemia associated with insulin deficiency caused by decreased lipoprotein lipase activity?

Questions have been raised as to whether reduced lipoprotein lipase (LPL) activity can account for the defect in plasma removal of very low density lipoprotein (VLDL) associated with insulin deficiency. In order to study this issue, streptozotocin-induced insulin deficiency was produced in 2-mo-old rats. Three groups of rats were studied: control, moderate diabetes (glucose > 200 < 350 mg/dl), and severe diabetes (glucose > 350 mg/dl). One week later food was withdrawn at 8 a.m., and the following measurements made at 2 p.m. (6 h after removal of food): (1) plasma glucose, insulin, and triglyceride (TG) levels; (2) VLDL-TG secretion rate; and (3) adipose tissue and skeletal muscle LPL activity. Rats with moderate diabetes had higher glucose and lower insulin levels than did control rats, and these differences were accentuated in rats with severe diabetes. Plasma TG levels were elevated (P < 0.001) in rats with moderate (↑99%) and severe diabetes (↑126%), and this was associated with a fall (P < 0.001) in VLDL-TG secretion. An increase in plasma TG levels, despite a decrease in VLDL-TG secretion, indicates the presence of a profound defect in VLDL-TG removal from plasma. However, muscle LPL of rats with severe diabetes and moderate diabetes was equal to that of control rats, and adipose tissue LPL was only moderately (P < 0.05) reduced (MD = ↓25%, SD = ↓32%). These results indicate that plasma TG levels rise in young, insulin-deficient rats as a result of a defect in removal of VLDL-TG from plasma. This removal defect is associated with normal muscle and only moderately reduced adipose tissue LPL activity, suggesting that the VLDL-TG removal defect associated with insulin deficiency may not be a simple function of a decrease in total (muscle + adipose tissue) LPL activity.

Characterization of a model of dietary-induced hypertriglyceridemia in young, nonobese rats

Healthy, nonobese, young rats developed hypertriglyceridemia (mean triglyceride levels of 250 mg/dl) following consumption of a sucrose-lard diet. The hypertriglyceridemia was apparent three days after start of the diet and persisted throughout the 4-week experimental period. Body weight, liver weight, and serum glucose levels were similar in animals eating either the sucrose-lard diet or standard rat chow. On the other hand, serum free fatty acid levels were slightly increased and serum insulin levels were substantially increased in animals eating the sucrose-lard diet. Determination of very low density lipoprotein turnover revealed that total triglyceride secretion in rats eating the sucrose-lard diet was significantly (P < 0.01) increased over that of rats eating standard chow. Direct measurement of hepatic and intestinal very low density lipoprotein secretion indicated that the observed rise in total triglyceride secretion was secondary to increased secretion of very low density lipoproteins by the liver. Finally, lipoprotein lipase activity of adipose tissue from rats eating the sucrose-lard diet was equal to, or greater than (depending upon sampling time), the activity of the enzyme from adipose tissue of rats eating the control diet. These data indicate that young, nonobese, rats develop hypertriglyceridemia when they ingest a sucrose-lard diet, and that the rise in plasma triglyceride levels results from an increase in hepatic very low density lipoprotein secretion. The dietary-induced hypertriglyceridemia is associated with elevated serum insulin levels, and, as such, may provide a useful animal model to use in studies aimed at defining the pathogenesis of endogenous hypertriglyceridemia in man.-Reaven, G. M., T. R. Risser, Y-D. I. Chen, and E. P. Reaven. Characterization of a model of dietary-induced hypertriglyceridemia in young, nonobese rats.

Interacting enzyme systems at steady state: further Monte Carlo calculations on two-state molecules

In this work, Monte Carlo calculations were made on a 10 x 10 lattice of two-state, steady-state enzyme molecules in two special cases for which the Bragg-Williams (mean field) approximation had earlier produced some very interesting phase-transition properties. The Monte Carlo results proved to be similar to Bragg-Williams in some respects but not in others. The discrepancies are attributed primarily to; (i) inadequate treatment by Bragg-Williams of strong negative cooperativity; and (ii) the finite size of the 10 x 10 lattice used in the exact calculations.

Interacting enzyme systems at steady state: location of the phase transition in approximations of the mean field type

We consider a phase transition "loop," obtained from a mean field type of approximate treatment of a closed steady-state Ising system. Where is the cut (stable path) across the loop located? The general procedure, in answering this question, is to pass to an open version of the same system and use the cut that appears automatically in this case (no loop is possible in an open system). This is equivalent to finding the point at which the two phases have equal total probability in the open system. It is shown here that this procedure, when applied to a system of two-state enzyme molecules, is formally equivalent to well-known thermodynamic methods (Maxwell's theorem, etc.). These can be applied directly to the closed system without considering the open system explicitly. However, for enzyme molecules with more than two states, the "thermodynamic" method generally fails and one must fall back on the open system procedure mentioned above. Practical implementation of this procedure is not easy.