Euglycemic progression: worsening of diabetic retinopathy in poorly controlled type 2 diabetes in minorities

AIMS

In type 2 diabetes, early effects of strict near-normalization of glucose control on macrovascular and microvascular disease are still uncertain. We evaluated the effects of early dramatic improvement in glycemia on retinal disease in poorly controlled diabetes.

METHODS

A retrospective, case-control study in public hospital patients with type 2 diabetes, who had annual retinal imaging as part of a case management program or standard diabetes care. Patients included had ≥2 two retinal images ≥1 one year apart, and at least 3 HbA1C measurements. Retinal images were graded using a modified Scottish Diabetic Retinopathy grading scheme. An 'intensive' group (n=34) with HbA1C decrease >1.5% was compared with randomly chosen patients (n=34) with minimal HbA1C changes.

RESULTS

Mean HbA1C (±SEM) over two years was similar in intensive (8.5 ± 0.21%) and control groups (8.1 ± 0.28%, p=NS). However, the intensive group had higher baseline HbA1C and a mean maximal decrease of 4.0 ± 0.41% in contrast to the control group (0.2 ± 0.11%). Retinopathy grade progressed +0.7 ± 0.25 units from baseline in the intensive group (p=0.015), a 22.6% worsening. The control group changed minimally from baseline (0.03 ± 0.14 units, p=NS). Change in retinopathy grade was significantly different between groups (p=0.02). More eyes worsened by ≥ 1 retinal grade (p=0.0025) and developed sight-threatening retinopathy (p=0.003) in the intensive group. Visual acuity was unchanged.

CONCLUSIONS

Diabetic retinopathy significantly worsened in poorly controlled type 2 diabetes after early intensification of glycemic control and dramatic HbA1C change. Retinal status should be part of risk-factor evaluation in patients likely to experience marked reductions in HbA1C in poorly controlled diabetes.

Pathophysiology of ketoacidosis in Type 2 diabetes mellitus

AIMS

Despite an increasing number of reports of ketoacidosis in populations with Type 2 diabetes mellitus, the pathophysiology of the ketoacidosis in these patients is unclear. We therefore tested the roles of three possible mechanisms: elevated stress hormones, increased free fatty acids (FFA), and suppressed insulin secretion.

METHODS

Forty-six patients who presented to the Emergency Department with decompensated diabetes (serum glucose > 22.2 mmol/l and/or ketoacid concentrations > or = 5 mmol/l), had blood sampled prior to insulin therapy. Three groups of subjects were studied: ketosis-prone Type 2 diabetes (KPDM2, n = 13) with ketoacidosis, non-ketosis-prone subjects with Type 2 diabetes (DM2, n = 15), and ketotic Type 1 diabetes (n = 18).

RESULTS

All three groups had similar mean plasma glucose concentrations. The degree of ketoacidosis (plasma ketoacids, bicarbonate and anion gap) in Type 1 and 2 subjects was similar. Mean levels of counterregulatory hormones (glucagon, growth hormone, cortisol, epinephrine, norepinephrine), and FFA were not significantly different in DM2 and KPDM2 patients. In contrast, plasma C-peptide concentrations were approximately three-fold lower in KPDM2 vs. non-ketotic DM2 subjects (P = 0.0001). Type 1 ketotic subjects had significantly higher growth hormone (P = 0.024) and FFA (P < 0.002) and lower glucagon levels (P < 0.02) than DM2.

CONCLUSIONS

At the time of hospital presentation, the predominant mechanism for ketosis in KPDM2 is likely to be greater insulinopenia.

Genome scan for blood pressure in Dutch dyslipidemic families reveals linkage to a locus on chromosome 4p

Genes contributing to common forms of hypertension are largely unknown. A number of studies in humans and in animal models have revealed associations between insulin resistance, dyslipidemia, and elevated hypertension. To identify genes contributing to blood pressure (BP) variation associated with insulin-resistant dyslipidemia, we conducted a genome-wide scan for BP in a set of 18 Dutch families exhibiting the common lipid disorder familial combined hyperlipidemia. Our results reveal a locus on chromosome 4 that exhibits a significant lod score of 3.9 with systolic BP. In addition, this locus also appears to influence plasma free fatty acid levels (lod=2.4). After adjustment for age and gender, the lod score for systolic BP increased to 4.6, whereas the lod score for free fatty acid levels did not change. The chromosome 4 locus contains an attractive candidate gene, alpha-adducin, which has been associated with altered BP in animal studies and in some human populations. However, we found no evidence for an association between 2 intragenic alpha-adducin polymorphisms and systolic BP in this sample. We also observed suggestive evidence for linkage (lod=1.8) of diastolic BP to the lipoprotein lipase gene locus on chromosome 8p, supporting a finding previously observed in a separate insulin-resistant population. In addition, we also obtained suggestive evidence for linkage of systolic BP (lod=2.4) and plasma apolipoprotein B levels (lod=2.0) to a locus on proximal chromosome 19p. In conclusion, our genome scan results support the existence of multiple genetic factors that can influence both BP and plasma lipid parameters.

Glucose entrainment of high-frequency plasma insulin oscillations in control and type 2 diabetic subjects

Regular high-frequency oscillations of insulin secretion are characteristic of normal beta-cell function. These oscillations are easily entrainable to an exogenous rhythm by small changes in glucose concentration in vitro. We tested whether high-frequency insulin oscillations in vivo would also be entrainable by glucose and whether a lack of entrainment would characterize the diabetic beta-cell. We tested 13 control subjects and 11 patients with type 2 diabetes. Subjects underwent serial blood sampling at 1-min intervals for 60-120 min in the basal state or with small (15 mg/kg) boluses of glucose injected intravenously at exact 29-min intervals. Time series analysis was carried out using spectral analysis. Oscillations of basal plasma glucose concentrations were observed in both control and type 2 diabetic subjects, with a mean period of 11.3 +/- 3.1 and 11.6 +/- 2.0 min, respectively. These oscillations were entrained to mean periods of 15.0 +/- 0.6 and 14.2 +/- 0.9 min, respectively, by exogenous glucose. Regular high-frequency insulin oscillations were observed in control subjects; the mean period of basal plasma insulin oscillations was 10.7 +/- 1.2 min and was entrained to exogenously injected glucose, with a period of 15.2 +/- 0.1 min. In contrast, in the type 2 diabetic subjects, spontaneous insulin oscillations were unchanged by the glucose rhythm; the mean periods were 10.0 +/- 1.0 min during the basal period, and 10.1 +/- 0.0 min during glucose injections. These results demonstrate that spontaneous high-frequency insulin oscillations can be successfully entrained by glucose in control subjects. However, these oscillations in type 2 diabetic subjects are not similarly entrained. We conclude that loss of entrainment of spontaneous high-frequency insulin oscillations in type 2 diabetes is a highly sensitive manifestation of beta-cell secretory dysfunction.

Counterregulatory hormones oscillate during steady-state hypoglycemia

During hypoglycemia, the magnitude of the counterregulatory response depends on the extent of plasma glucose reduction. However, our clinical observations during steady-state hypoglycemia indicate that symptom severity can change independently of plasma glucose concentrations, i.e., symptoms appeared to fluctuate despite stable glucose levels. This study was therefore designed to test the hypothesis that hormonal and symptomatic responses to hypoglycemia are pulsatile. Seven healthy subjects had serial blood sampling at 3-min intervals during 90 min of insulin-induced hypoglycemia. Mean +/- SE plasma glucose levels plateaued at 62 +/- 3 mg/dl. Counterregulatory hormones were significantly elevated (P < 0.05-0. 01, except norepinephrine) and strikingly pulsatile. Cluster analysis revealed pulses of large magnitude in plasma glucagon, epinephrine, and norepinephrine concentrations. Amplitudes were, respectively, 72 +/- 4, 64 +/- 8, and 48 +/- 3% of the mean. Interpeak intervals were 27 +/- 7, 19 +/- 4, and 25 +/- 5 min, respectively. Symptom score and cardiovascular responses were also pulsatile; their peaks were found to coincide with epinephrine peaks. We conclude that hormonal and symptomatic counterregulation in hypoglycemia, while critically driven by plasma glucose levels, is also influenced by an endogenous pulsatility that exists despite steady-state glucose concentrations.

Erratic oscillatory characteristics of plasma insulin concentrations in patients with insulinoma: mechanism for unpredictable hypoglycemia

Patients with insulin-producing tumors may have hypoglycemic symptoms at unpredictable times. This study evaluated whether plasma insulin oscillations, known to occur in normal individuals but not explored in patients with insulinomas, could be an underlying mechanism for such events. Nine normal subjects and five patients with proven insulinomas were studied in the fasting state. Serial sampling of arterialized blood over 80-100 min, at 2- or 3-min intervals was performed. In normal subjects, mean plasma glucose and insulin concentrations were 5.3 +/- 0.1 mmol/L and 58 +/- 9 pmol/L, respectively. Regular, low-amplitude plasma insulin oscillations were observed, with a period of 10-17 min. The subjects with insulinomas had lower mean plasma glucose and higher insulin concentrations than controls, 3.6 +/- 0.3 mmol/L (P = 0.01) and 150 +/- 42 pmol/L (P = 0.01), respectively. They also had insulin oscillations that appeared unstable as a result of variability in duration and amplitude compared with controls. The insulin pulses were irregular, and interpeak intervals varied between 4-54 min in different subjects; in some subjects, the amplitude was also variable, with sudden spontaneous pulses as high as 565 pmol/L, with an associated glucose decrement. We conclude that large spontaneous bursts of insulin secretion occur in patients with insulinomas as part of an erratic pattern of oscillatory insulin secretion, and these can account for unpredictable occurrences of hypoglycemia.

An overnight insulin infusion algorithm provides morning normoglycemia and can be used to predict insulin requirements in noninsulin-dependent diabetes mellitus

Initial insulin requirements in noninsulin-dependent diabetes mellitus (NIDDM) are difficult to estimate because of individual variability in insulin sensitivity and secretion. We evaluated a simple, nurse-managed algorithm for overnight delivery of insulin, for its ability to provide morning near-normoglycemia and as a means to predict initial insulin requirements in NIDDM. Twenty-seven patients with poorly controlled NIDDM were studied on 30 occasions. A 12-h iv insulin infusion was begun at 2000 h, and bedside blood glucose concentrations were measured at hourly intervals. The rate of insulin infusion was adjusted according to blood glucose levels. We estimated the preprandial insulin dose requirement for the following day in 16 patients based on overnight insulin requirements to maintain normoglycemia. Preprandial insulin doses were adjusted for prevailing blood glucose concentrations. At 2000 h, the mean (+/-SEM) blood glucose concentration was 265.7 +/- 10.8; at 0300 h, it was 122.8 +/- 3.4; and at 0700 h, it was 123.8 +/- 5.1 mg/dL. On the next day, mean blood glucose levels (before and 2 h after a meal) were: breakfast, 102.5 +/- 5.9 and 177.3 +/- 19.2; lunch, 138.9 +/- 15.5 and 136.3 +/- 11.4; dinner, 105.7 +/- 7.2 and 178.1 +/- 15.7 mg/dL. There was no significant difference between mean calculated and administered total insulin dosage the next day (84.2 +/- 7.0 vs. 78.2 +/- 8.2 U). Thus, a weight-based algorithm for iv insulin infusion induced near-normoglycemia in NIDDM and successfully predicted the insulin dose requirement. We conclude that initiating insulin therapy in NIDDM patients can be achieved rapidly and efficiently based on a nurse-managed overnight insulin infusion.

Pseudohypohyperparathyroidism: A Case Study of Differential Resistance to Parathyroid Hormone

OBJECTIVE

To present a rare case of a combination of hyperparathyroidism and hypoparathyroidism in a 30-year-old woman.

METHODS

We review the laboratory, radiographic, and pathologic findings in a healthy-appearing woman who sustained a patellar fracture after a simple fall at home.

RESULTS

Our patient had features of hypoparathyroidism--that is, tetanic crises and hypocalcemia--and also hyperparathyroidism--fracture of the patella, multiple bone cysts, and confirmed osteitis fibrosa cystica on bone biopsy specimens. These features were associated with a high serum level of intact parathyroid hormone (PTH) and parathyroid hyperplasia. A lack of response of nephrogenic cyclic adenosine monophosphate (cAMP) to PTH stimulation was observed along with subnormal serum cAMP responses. In contrast, urine phosphate excretion increased after administration of PTH.

CONCLUSION

These results demonstrate a state of renal PTH resistance in a patient with osteitis fibrosa cystica.

Applications of segmented regression models for biomedical studies

In many biological models, a relationship between variables may be modeled as a linear or polynomial function that changes abruptly when an independent variable obtains a threshold level. Usually, the transition point is unknown, and a major objective of the analysis is its estimation. This type of model is known as a segmented regression model. We present two methods, Gallant and Fuller's (J Am. Stat. Assoc. 68: 144-147, 1973) method and Tishler and Zang's (J. Am. Stat. Assoc. 76: 980-987, 1981) method, using nonlinear least-squares techniques for estimating the transition point. We give the following three examples: a hypoglycemia study, a testosterone study, and an estimate of age-cortisol relationship. Simulation techniques are used to compare the two methods. We conclude that these models provide useful information and that the two methods studied produce essentially equivalent results. We recommend that both methods be used to analyze a data set if possible to avoid problems due to local minima and that if the results do not agree, then evaluation of the likelihood function in the range of the estimates be used to determine the best estimate.

Very-low-energy diets alter the counterregulatory response to falling plasma glucose concentrations

A consequence of short-term very-low-energy diets (VLEDs) in lean subjects is reactive hypoglycemia. We therefore tested the responses of overweight women on prolonged (14 d) VLEDs. Subjects lost 4.8 +/- 0.2 kg (mean +/- SEM, n = 13, P < 0.001). Group A (n = 6) was challenged with an oral-glucose-tolerance test (OGTT) and group B (n = 7) with an oral-sucrose-tolerance test (OSTT) on days 1 and 14. In group A, mean nadir plasma glucose after the OGTT was lower on day 14, 3.75 +/- 0.16 vs 4.7 +/- 0.19 mmol/L (P < 0.01), because of an accelerated rate of glucose decline (RGD, 26.7 +/- 3.3 vs 17.2 +/- 3.9 mumol.l-1.min-1, P < 0.05) late in the OGTT. Plasma insulin was also lower (P < 0.03) and the VLED suppressed two growth hormone (GH) peaks on day 14 (P < 0.05 for each). In group B on day 14, a greater RGD was also observed late in the OSTT, 16.9 +/- 4.1 vs 6.5 +/- 2.0 mumol.L.min-1 (P < 0.03). GH peaks were also significantly suppressed. We conclude that a VLED results in altered glucose regulation late after carbohydrate loading, characterized by an accelerated decline in plasma glucose and GH suppression. Patients on a VLED may be at risk for abnormally low plasma glucose concentrations when ingesting high carbohydrate loads.

Accuracy of plasma glucose measurements in the hypoglycemic range

OBJECTIVE

This study was designed to evaluate three different enzymatic methods for glucose measurement in plasma samples with special emphasis on glucose concentrations in the hypoglycemic range.

RESEARCH DESIGN AND METHODS

Glucose dehydrogenase (Hemo-Cue analyzer), glucose oxidase (YSI analyzer), and hexokinase (Abbott analyzer) methods were used to measure plasma samples that were obtained during research studies.

RESULTS

Mean glucose concentrations (n = 240) were 5.3 +/- 0.2, 5.4 +/- 0.2, and 5.6 +/- 0.2 mM (95.6 +/- 3.9, 96.7 +/- 3.9, and 101.6 +/- 4.0 mg/dl) using glucose dehydrogenase, glucose oxidase, and hexokinase, respectively (NS). In the hypoglycemic range, mean glucose concentrations with each method retained the same hierarchy of measurements: 2.7 +/- 0.05, 2.8 +/- 0.04, and 2.9 +/- 0.03 mM (48.4 +/- 0.9, 50.6 +/- 0.8, and 52.3 +/- 0.6 mg/dl) by glucose dehydrogenase, glucose oxidase, and hexokinase, respectively (P < 0.005). Individual glucose dehydrogenase measurements (n = 240) correlated well with glucose oxidase and hexokinase, r = 0.99, and were considerably easier to perform at the bedside. The differences between the glucose measurement methods were consistent and similar in low, normal, and high concentration ranges.

CONCLUSIONS

We conclude that any interpretation or comparison of critical clinical and research measurements of glucose in different settings take into account methodological differences, particularly in the hypoglycemic range.

Evidence for pancreatic pacemaker for insulin oscillations in low-frequency range

Circulating insulin concentrations oscillate in regular fashion, with periods that fall into a high-frequency (period of 5-17 min) or low-frequency (period of 50-150 min) range. Only the high-frequency oscillations have so far been reported in vitro, suggesting that these derive from a primary pancreatic source. This study tested whether the low-frequency insulin oscillations could also be identified in vitro. Rat islets of Langerhans were perifused for 20 h using RPMI medium with 5.5 mM glucose. Perifusate fractions were collected at 9.9-min intervals. Mean insulin concentrations at the outset were 21.4 +/- 2.9 microU/ml, increased to 32.5 +/- 4.6 (P < 0.05) between 13 and 17 h after the start of perifusion, and then either leveled off or decreased to baseline. Superimposed on this general trend, we found sustained insulin oscillations with a period of 50-100 min. The mean amplitude was 14.2 +/- 4.2 microU/ml, and the amplitude/mean ratio was 64.6 +/- 12%. Spectral analysis revealed significant peaks at periods that were close to either 50 or 100 min and a smaller peak at 24-37 min. These data, using in vitro methodology and constant glucose concentrations, indicate the presence of sustained, spontaneous, low-frequency, ultradian insulin oscillations in the pancreatic islets. This provides evidence for a pancreatic component that may participate in the previously described in vivo ultradian insulin oscillations. This finding may also provide a mechanism for the apparent escape from glucose entrainment of serum insulin oscillations in non-insulin-dependent diabetes mellitus.

Plasma glucose thresholds for counterregulation after an oral glucose load

Glucose counterregulation (GCR) plays an important role in the transition between exogenous and endogenous glucose delivery after an oral glucose load. This response is initiated when plasma glucose concentrations are decreased below threshold levels, previously defined in studies of insulin-induced hypoglycemia. In this study, we tested the plasma glucose thresholds for activation of the GCR response under more physiologic circumstances, ie, after glucose ingestion. We studied 20 normal subjects for 300 minutes after 75 g of oral glucose. Between 150 and 300 minutes, blood samples and symptom scores were obtained at 10-minute intervals. After oral glucose, individual glucose nadirs were observed over a wide time range (160 to 290 minutes). Mean glucose concentrations decreased from 5.3 +/- 0.2 mmol/L at 30 minutes before the nadir (-30 minutes) to 3.8 +/- 0.2 mmol/L at the nadir (0 minutes). Mean plasma epinephrine concentrations increased from 210 +/- 35 pmol/L, were significantly elevated at -10 minutes (P < .05), and peaked at +20 minutes (1,008 +/- 184 pmol/L, P < .001). Mean plasma glucagon concentrations were significantly increased over baseline (100%) at +10 minutes (P < .001) and peaked at +30 minutes (122% +/- 7%, P < .001). Seven subjects (out of 15 tested) developed symptoms. Quantitative evaluation revealed a peak in the mean symptom score at +20 minutes, an increase from 0.4 +/- 0.3 to 2.6 +/- 0.1 arbitrary units (P < .06).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of high fat and high glucose meals on the growth hormone response to exercise

The health promoting, anabolic effects of physical activity may be mediated, in part, by an exercise-associated increase in GH. However, little is known about the acute effects of diet on exercise-induced GH release. We hypothesized that a single meal could attenuate the GH response to exercise by modulating substances like somatostatin, insulin, or glucose. Eleven healthy young adults performed 10 min of high intensity, standardized cycle ergometry in the morning following an overnight fast. On separate days they ingested a noncaloric placebo liquid meal or an isovolemic, isocaloric liquid meal high in either fat or glucose. Venous blood samples were obtained before and for 90 min after exercise began, whereas gas exchange data were measured breath by breath. Peak mean oxygen consumption (VO2) was, on average, 9-fold greater than preexercise levels in all groups. Although there was no difference in preexercise GH levels, mean peak, postexercise GH was 54% lower after the high-fat meal compared with placebo (P < 0.01). Modest decreases in GH response to exercise after the high-glucose meal were not statistically significant. Mean serum somatostatin was significantly higher after the high-fat meal compared with both high glucose and placebo meals. This study demonstrates that exercise-induced GH release can be significantly attenuated by the contents of a single preexercise meal. The high fat meal increased circulating somatostatin and was associated with an inhibition of the GH secretion. The data provide a possible specific mechanism to explain how diet can acutely modulate the anabolic effects of exercise.

A mathematical model of oscillatory insulin secretion

Insulin is secreted in sustained oscillatory fashion from isolated islets of Langerhans. This finding has led to the assumption of an underlying synchronizing process that coordinates insulin oscillations. This assumption was tested by developing a mathematical model of oscillatory insulin secretion in which we included degree of synchrony as a parameter. We first evaluated insulin oscillations in perifused isolated rat islets, using spectral analysis to determine their regularity and frequency. A parsimonious mathematical model was developed to account for these characteristics. The model postulates a group of secretory units discharging at discrete intervals with the same underlying period. Variation from two sources, phase differences between units (synchrony) and regularity within units, is introduced by adding two normally distributed random variables with standard deviations (Sg and Si, respectively) to the secretory period. Sets of 100 simulations for different values of Sg and Si were run. Results of the simulations suggest that the system tolerates a relatively large degree of asynchrony yet still demonstrates regularity of oscillations on spectral analysis. Comparison with perifusion data suggests that a moderate degree of asynchrony between islets can best account for the pattern of insulin oscillations observed. This model provides a theoretical basis for the study of mechanisms for insulin oscillations.

Oscillations of lactate released from islets of Langerhans: evidence for oscillatory glycolysis in beta-cells

Oscillations in the glycolytic process have been demonstrated in a number of different biological systems. However, their presence has never been demonstrated in insulin-secreting beta-cells. We used lactate as a marker for glycolysis and measured lactate and insulin concentrations in the effluent of isolated perifused rat islets of Langerhans. Sustained regular oscillations in lactate concentrations with an average period of 16-20 min were observed in islets that were perifused with medium containing 5.5 or 16.7 mM glucose. Sustained oscillations of insulin concentrations secreted by the islets were also observed in these experiments, and the average period of oscillation was 14.6 +/- 2.3 min at 16.7 mM glucose. Mean insulin concentrations at 5.5 mM glucose were too low to permit analysis of oscillations. Spectral analysis confirmed the regularity of the lactate and insulin oscillations and showed peaks that were consistent with the average periods obtained using the Clifton program. Moreover, spectral analysis demonstrated marked similarity between the patterns of lactate and insulin oscillation during perifusion with 16.7 mM glucose. Cross-correlation analysis found these oscillations not to be consistently in phase. In conclusion, sustained oscillations in lactate released from islets of Langerhans suggest that the glycolytic process in beta-cells also oscillates. The similarity of the periods of lactate and insulin raises the possibility that oscillations in glycolysis may provide a mechanism for pulsatile insulin secretion.

Sustained pulsatile insulin secretion from adenomatous human beta-cells. Synchronous cycling of insulin, C-peptide, and proinsulin

The endocrine pancreas secretes insulin in a pulsatile fashion. This rhythm is generated at a site within the pancreas, although its precise location has not been determined. With an in vitro system, we tested the possibility that beta-cells might generate spontaneous pulsatile insulin secretion in the absence of any external influence. Human insulinoma tissue from five patients was perifused for 7-10 h with RPMI-1640 medium and constant concentrations of glucose (5.5 mM). Insulin, C-peptide, and proinsulin were measured in the effluent collected at 3.3-min intervals. All three peptides demonstrated pulsatility of secretion in a similar, synchronous fashion that was sustained throughout each study. The Clifton cycle detection program demonstrated cycling in all five tumors, with an average period for all tumors of 28, 29, and 26 min for insulin, C-peptide, and proinsulin, respectively. Spectral analysis confirmed the regularity and consistency of the hormonal secretory patterns. Mean hormone concentrations secreted by different tumors varied, but insulin and C-peptide were secreted in a nearly 1:1 ratio. This study demonstrates 1) that beta-cells are able to generate spontaneous pulsatile insulin secretory activity, which is independent of innervation or the presence of other islet cells, and 2) proinsulin secretion from the beta-cell also has an inherent pulsatility. The synchrony observed in the cycles of proinsulin and its peptide products confirms their common secretory pathway in the beta-cell. We conclude that the beta-cell may be the originator of insulin cycling.

Catecholamine concentrations in the hepatic portal system: effect of surgical stress upon portal levels

The biological effects of a hormone are dependent on the concentration delivered to the target tissue. This is generally best reflected in the arterial concentration. However, the liver is unique in that it receives substantial additional blood flow from the portal venous system. This may be important in the case of the catecholamines, where extraction or spillover from the splanchnic circulation may occur. In this study we examined portal venous catecholamine concentrations in anesthetized, laparotomized rabbits. We compared the values with simultaneously sampled arterial levels to evaluate the effects of the splanchnic tissues upon a wide range of catecholamine concentrations delivered to the liver during a state of stress. Spillover of norepinephrine and extraction of epinephrine were observed in all rabbits. Mean (+/- SEM) arterial norepinephrine concentrations were elevated, 716 +/- 167 pg/ml (n = 11); mean portal concentrations were 178 +/- 37% higher (p less than 0.01), at 1,425 +/- 301 pg/ml, representing net spillover from splanchnic tissues. In addition, significant extraction of epinephrine was observed; arterial concentrations were 2,144 +/- 580 pg/ml (n = 11). Portal levels were 1,205 +/- 382 pg/ml, 38 +/- 7.45% lower than corresponding arterial concentrations (p less than 0.02). Furthermore, there was a concentration-dependent effect upon norepinephrine spillover; the highest arterial norepinephrine concentrations were associated with the lowest splanchnic spillover. This resulted in a negative correlation between the arterial norepinephrine levels and the percent increase from spillover into the portal vein (r = -0.81, p less than 0.003). We conclude that portal venous catecholamine concentrations are significantly different from arterial levels in anesthetized, laparotomized rabbits over a wide range of concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Oscillations of circulating plasma insulin concentrations in the rat

In previous studies, we found that insulin is secreted in a pulsatile fashion in vitro in isolated rat pancreatic islets. This study evaluated whether similar plasma insulin fluctuations occur in the rat in vivo. Freely moving rats were implanted with a chronic jugular catheter and serial blood samples were obtained 48-72 hrs post surgery. Blood was sampled at 3 min intervals for 60 mins with volume replacement using a red cell preparation. Plasma insulin concentrations were observed to fluctuate around a mean of 10.6 +/- 1.1 uU/ml, with an amplitude of 4.7 +/- 0.5 uU/ml and a period of 13.3 +/- 1 mins (n = 6). This was similar to the cycling observed in isolated islets at similar glucose concentrations. Sampling during the dark phase of the light-dark cycle in the rat was associated with an increase in the mean plasma level, amplitude and period of insulin oscillations compared with values obtained during the light phase (n = 3). These data are the first in vivo demonstration of oscillatory circulating insulin concentrations in the rat and show that the pulsatility in this species is similar to that observed in other mammals including man. We conclude that the chronically catheterised rat is a useful model for the evaluation of oscillating insulin concentrations in vivo, and may provide interesting insights by comparison with in vitro data in the same species.

Fetal catecholamine responses to maternal hypoglycemia

The present studies were designed to determine the fetal catecholamine and metabolic responses to insulin-induced maternal hypoglycemia. Maternal hypoglycemia was induced by a primed constant infusion of insulin and glucose administered to pregnant ewes to maintain maternal glucose at 20-25 mg/dl. Maternal and fetal samples for measurement of catecholamine, glucose, and free fatty acid levels and arterial blood gas analysis were collected before insulin infusion and at intervals thereafter for 6 h. Maternal and fetal plasma catecholamine levels increased significantly in response to hypoglycemia (analysis of variance, P less than 0.01). Fetal insulin and glucagon levels did not change despite a 50% reduction in fetal whole blood glucose concentration. Fetal free fatty acid levels increased significantly during hypoglycemia (P less than 0.05). There were no significant changes in maternal or fetal heart rate, blood pressure, or arterial blood gases during hypoglycemia. These results suggest that the fetus is capable of responding to hypoglycemia with an increase in catecholamine and free fatty acid levels. These results are consistent with the interpretation that the fetus is able to mobilize alternative energy substrates in response to maternal insulin-induced hypoglycemia.

Sulphonylurea effects on insulin secretion in islets desensitized to glucose

Prolonged, continuous exposure of the islets of Langerhans to high glucose concentrations results in desensitization of the beta cell to glucose stimulation. This study tested the ability of a sulphonylurea to stimulate insulin secretion in this setting. Normal isolated rat islets were cultured for 18-20 h in RPMI-1640 with 300 mg/dl glucose to induce desensitization or with 100 mg/dl as a control. Islets were then placed into a perifusion system and perifused with 60 mg/dl glucose followed by a stimulus. After preincubation at 300 mg/dl, a significant 50% suppression of glucose-induced insulin secretion compared with secretion in the control group preincubated at 100 mg/dl glucose was observed (p less than 0.025-0.001). This confirmed the occurrence of desensitization to glucose in this in vitro model. In contrast, stimulation of insulin secretion by glyburide (500 ng/ml) was unaffected compared with control. We also tested whether glyburide corrects the defective response to glucose stimulation in glucose-desensitized islets. Control islets (preincubated at 100 mg/dl) were stimulated with 300 mg/dl glucose or with this glucose concentration plus glyburide. Peak incremental insulin responses were similar (0.81 +/- 0.07 and 0.77 +/- 0.12 microU/ml.islet). After preincubation at 300 mg/dl, perifusion with 300 mg/dl glucose alone or with glyburide was associated with smaller, but similar, peak insulin responses (0.53 +/- 0.13 and 0.62 +/- 0.06 microU/ml.islet). In conclusion, islets in which the insulin-secretory response is compromised by desensitization to glucose are nevertheless completely responsive to the direct stimulatory effects of a sulphonylurea. However, the sulphonylurea does not correct the defect in glucose-induced insulin secretion.

A glucose reduction challenge in the differential diagnosis of fasting hypoglycemia: a two-center study

Investigation of patients with suspected or proven hypoglycemia is often a time-consuming and expensive process. We describe a glucose reduction challenge test which may be useful as an out-patient screening procedure. Insulin is infused for 3 h at 40 mU/kg.h. Plasma glucose was monitored at the bedside during the test, and blood samples were collected for measurement of C-peptide. Responses were examined in 17 normal controls, and 6 patients with insulinomas. In normal subjects, mean plasma glucose fell to a plateau value of 3.2 +/- 0.2 mmol/L (57 +/- 2.6 mg/dL) and remained at that level with few symptoms. In contrast, five of six patients with insulinomas developed severe hypoglycemia, with plasma glucose levels between 1.9 (34 mg/dL) and 2.2 mmol/L (39 mg/dL). Plasma C-peptide concentrations were suppressed to 0.08 pmol/mL or less in normal subjects, but in insulinoma patients remained at 0.32-1.6 pmol/mL i.e. outside the normal range, and diagnostic of nonsuppressible insulin secretion. These data demonstrate that moderate reduction of serum glucose maintained for a prolonged period results in marked suppression of plasma C-peptide, permitting improved discrimination between normal subjects and patients with insulinomas. This glucose reduction challenge can, therefore, be used as a test of glucose-regulating ability, where failure (hypoglycemia) per se represents a measurable abnormality. C-Peptide measurements will determine whether the cause of hypoglycemia is due to hyperinsulinemia.

Pulsatile insulin secretion in isolated rat islets

The pancreas secretes insulin in an oscillatory fashion, but the precise site of the pacemaker for pulsatile insulin secretion has not been identified. These studies were designed to determine whether islets also secrete insulin in a pulsatile fashion if they are isolated from their pancreatic milieu. Isolated rat islets (80-100) were perifused 8 h in culture medium after overnight incubation, and samples were collected at 3.3-min intervals. Insulin secretion was evaluated for pulsatility with the Clifton Cycle Detection Program. Perifusion of islets was associated with a spontaneous, persistent, and regular pulsatility of insulin secretion, which was observed in all conditions tested. Perifusion with medium containing 5.5 mM glucose (n = 11) demonstrated oscillations with a mean periodicity of 17.6 +/- 1.1 min and a mean amplitude of 4.8 +/- 0.4 microU/ml when overall mean insulin concentration was 16.7 +/- 2.4 microU/ml. When the glucose concentration was 16.7 mM (n = 9), overall mean insulin concentration was 54.4 +/- 2.6 microU/ml, with increases in periodicity (22.0 +/- 1.3 min) and amplitude (10.7 +/- 0.5 microU/ml). All measurements were significantly different from those observed during perifusion with 5.5 mM glucose (P less than 0.02-0.001). Theophylline (1 mM) also enhanced the overall mean insulin concentration and amplitude (69.4 +/- 10.4 and 14.2 +/- 1.2 microU/ml, respectively) compared with control studies without theophylline (16.7 +/- 5.3 and 4.3 +/- 0.5 microU/ml) (P less than 0.01). The period of the cycle was also increased from 17.5 +/- 1.1 to 26.4 +/- 6.3 min, but this was not significantly different from the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentamidine-induced beta cell toxicity is not preventable by high glucose

The incidence of beta cell damage attributable to pentamidine treatment of pneumocystis pneumonia is increasing in frequency because of the AIDS epidemic. We carried out in vitro studies in perfused rat islets using insulin secretion as an index of beta cell damage to study the effects of pentamidine and to test whether glucose can prevent toxicity in this physiologic model. Isolated islets were cultured for 16-18 hours of static incubation, in a culture medium containing 100 mg/dl glucose, with or without pentamidine (10(-6) M, a therapeutic concentration). Islets were then perfused with media containing 60 mg/dl followed by 300 mg/dl glucose concentrations to study the insulin secretory response. Incubation of islets with pentamidine was associated with subsequent basal hypersecretion of insulin (0.40 +/- 0.05 microU/islet .5 minute vs. 0.18 +/- 0.04 microU/islet .5 minute, p less than .005), and an insulin secretory response to glucose which was completely abolished (0.05 +/- 0.04 microU/islet .5 minute versus 1.12 +/- 0.02 microU/islet .5 minute, p less than .005). To determine whether glucose may protect against the effects of pentamidine, islets were then exposed to high glucose concentrations during simultaneous incubation with pentamidine. Coincubation with high glucose did not prevent these insulin secretory defects. A more extended culture of pentamidine-treated islets in the absence of pentamidine and at a glucose concentration of 100 mg/dl did not result in any recovery of insulin secretion. We conclude that pentamidine-induced beta cell damage is irreversible, not preventable by incubation with high glucose concentrations, and may therefore result from a mechanism different to that of alloxan.

Sparing of cognitive function in mild hypoglycemia: dissociation from the neuroendocrine response

Central nervous system function during insulin-induced reductions in plasma glucose was studied by measuring plasma epinephrine concentrations and testing cognitive function. Mild glucose reduction [mean plasma glucose, 62 +/- 3 (+/- SEM) mg/dL (3.4 +/- 0.2 mmol/L)] was induced with an iv insulin infusion at the rate of 40 mU/kg.h for 180 min in 7 normal subjects. Despite a marked increase in mean plasma epinephrine concentrations, which peaked at 426 +/- 68 pm/mL (2325 +/- 371 pmol/mL; P less than 0.001), no significant differences in cognitive function occurred as determined by a series of trail-making tests compared with the results of serial tests in a group of 17 control subjects. In contrast, when hypoglycemia was induced (plasma glucose, less than 42 mg/dL; 2.3 mmol/L) by bolus injection of insulin in 4 normal subjects, cognitive function was impaired in every subject, as demonstrated by a delay in completion of the trail-making test. The mean completion time was prolonged to 107 +/- 16% of the baseline at the time of hypoglycemia vs. 74 +/- 4% in control subjects (P less than 0.01). These findings suggest that cognitive function may be spared during mild plasma glucose reductions and is dissociated from the neuroendocrine adrenergic response that is activated under these conditions. This dissociation may be part of a homeostatic process in which overall brain function is maintained during glucoprivation, although counterregulation has already been triggered to prevent a further decrease in plasma glucose.

Somatostatin impairs clearance of exogenous insulin in humans

Somatostatin has been widely used to suppress endogenous pancreatic hormone secretion in research studies. Many of these studies required the simultaneous infusion of a hormone together with somatostatin. A critical assumption for its use in metabolic investigation is that somatostatin has no effect on the action or clearance of a concomitantly infused hormone. To test whether clearance of an exogenously infused hormone is affected, we infused insulin with or without somatostatin in two sets of studies. Insulin (40 mU X kg-1 X h-1) was infused for 100 min (n = 6). Plasma glucose levels fell to 55 +/- 4.1 mg/dl with insulin alone and significantly lower, to 44 +/- 1.9 mg/dl, when somatostatin (250 micrograms/h) was also infused (P less than .01). Plasma immunoreactive insulin (IRI) rose to 57 +/- 12.5 microU/ml with insulin alone, which was significantly different from 88 +/- 15 microU/ml when insulin was infused together with somatostatin (P less than .01). When a smaller dose of insulin (30 mU X kg-1 X h-1) was infused for 100 min (n = 4), similar results were observed. When somatostatin was infused together with insulin, plasma glucose fell to lower levels (41 +/- 4.2 vs. 62 +/- 9.5 mg/dl; P less than .01) and plasma IRI rose higher (39 +/- 8.5 vs. 27 +/- 5.9 microU/ml; P less than .01) than when insulin was infused alone. C-peptide was equally suppressed by hypoglycemia regardless of whether somatostatin was administered, indicating suppression of endogenous insulin during these studies. We conclude that somatostatin infusion impairs the clearance of exogenous insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone decreases centrally induced hyperglycemia in dogs. Evidence for an opioid role in glucose homeostasis

Intracerebroventricular (ICV) instillation of morphine and beta-endorphin causes centrally induced hyperglycemia. Locally active, endogenous opioids in the central nervous system may, therefore, also be involved in the elevation of blood sugar. This possibility was tested by examining the glucoregulatory response to central glucoprivation induced by ICV administration of 2-deoxy-D-glucose (2DG) in dogs. Administration of 2DG resulted in a rise in plasma glucose and immunoreactive glucagon (IRG) of 108 +/- 19 mg/dl and 70 +/- 20 pg/ml, respectively. These changes were attenuated by the simultaneous central infusion of the opiate antagonist naloxone: plasma glucose levels increased by 77 +/- 14 mg/dl and IRG by 43 +/- 3 pg/ml, both significantly different from the effect of 2DG alone (P less than 0.05-0.01). These findings suggest that opiate receptors participate in the counterregulatory response to central glucoprivation. They also provide a mechanism by which endogenous opioid peptides may play a role in the central regulation of glucose homeostasis.

Circulating somatostatin acts on the islets of Langerhans by way of a somatostatin-poor compartment

Somatostatin perfused in canine pancreases at 10 to 20 picograms per milliliter or 10 to 20 percent of the pancreatic vein somatostatin concentration inhibited insulin and glucagon secretion. This suggests that the high local concentration of endogenous somatostatin is not in contact with somatostatin receptors of the islets. The integrity of this separation may determine the sensitivity of islet cells to circulating somatostatin.

Central control of peripheral circulating somatostatin in dogs: effect of 2-deoxyglucose

Circulating plasma somatostatin concentrations are known to fluctuate in response to nutrients and hormones. However, little is known about neural or central nervous system (CNS) control of somatostatin secretion. To test whether peripheral circulating somatostatin is influenced by a central stimulus, 2-deoxyglucose (37.5 mg/kg) was infused into a lateral cerebral ventricle of six conscious dogs over a period of 15 min. Plasma somatostatin levels rose from a base line of 105 +/- 6 pg/ml (mean +/- SE) to a peak of 154 +/- 10 pg/ml (P less than 0.005) at 30 min after the onset of the infusion. Somatostatin levels were still significantly elevated (P less than 0.025) at 60 min (119 +/- 6 pg/ml) and thereafter gradually returned toward base line. Plasma glucose and glucagon levels increased in response to intraventricular 2-deoxyglucose. Glucose concentrations rose from 105 +/- 5 mg/dl to peak at 203 +/- 16 mg/dl (P less than 0.005) at 80 min and remained elevated to 120 min. The concentration of plasma glucagon increased from 41 +/- 6 to 92 +/- 18 pg/ml at 60 min (P less than 0.05) and then declined. In marked contrast to intraventricular 2-deoxyglucose, similar concentrations of 2-deoxyglucose administered intravenously (n = 4) resulted in a slight fall in plasma somatostatin. Intraventricular saline did not result in a change in plasma somatostatin. It is concluded that peripheral circulating somatostatin may be susceptible to central nervous system control.

Effect of somatostatin on determinants of bile flow in unanesthetized dogs

Seven dogs each underwent cholecystectomy, ligation of the accessory pancreatic duct, and insertion of a Thomas duodenal cannula opposite the ampulla of Vater. After full recovery, bile secretions were studied in the unanesthetized dogs by opening the cannula and placing a ureteric catheter through the papilla into the common bile duct. All animals received, throughout study, constant infusions of taurocholic acid to replace losses caused by interruption of the enterohepatic circulation and 14 C-erythritol for measurement of erythritol clearance. After bile flow stabilized somatostatin 800 ng/kg/minute was infused for 100 minutes and bile flow declined from 3.0 +/- 0.7 ml/10 minutes (SD) to 1.19 +/- 0.47 ml/10 minutes (p less than 0.001) and 14C-erythritol clearance fell from 3.6 +/- 1.14 to 1.77 +/- 0.43 ml/10 minutes (p less than 0.001). Bile salt output was unchanged, indicating that somatostatin inhibited bile salt-independent canalicular flow (BSICF). In other experiments animals underwent intraduodenal acidification which resulted in a marked increase in bile flow. Somatostatin infusion again causes a sharp fall in bile flow (p less than 0.05) suggesting that somatostatin also inhibited ductular flow. Infusion of somatostatin did not inhibit choleresis produced by exogenous secretin administration. Thus, somatostatin inhibits 1) ductular flow by inhibiting secretin release and 2) BSICF by a direct effect or by decreasing the release of hormones which induce canalicular flow.

Morphine-induced hyperglycemia: role of insulin and glucagon

An iv bolus injection of 0.5 mg/kg morphine, about twice the therapeutic dose, caused plasma glucose to rise more than 120 mg/dl in alloxan-diabetic conscious dogs but had little effect on conscious normal dogs. Plasma glucagon rose in the diabetic and nondiabetic groups by 30 +/- 10 and 100 +/- 29 pg/ml, respectively, but insulin levels increased significantly only in the nondiabetics. The hyperglycemic action on morphine may, at least in part, be the result of an increase in glucagon secretion without a sufficient accompanying release of insulin.

Somatostatin analogs inhibit somatostatin release

To determine if, like insulin, somatostatin inhibits its own secretion from the pancreas, nonimmunoreactive analogs of somatostatin were perfused in an isolated dog pancreaticoduodenal preparation using a nonrecirculating system. [D-Trp8-D-Cys14]somatostatin, at a concentration of 200 ng/ml, blocked the response of somatostatin-like immunoreactivity (SLI) to cholecystokinin and arginine. When perfusion of the analog was discontinued, SLI release increased. At a concentration of 0.1 ng/ml, des Asn5-[D-Trp8]somatostatin lowered SLI levels significantly without significantly reducing glucagon levels. At a concentration of 1 ng/ml, des Asn5-[D-Trp8]somatostatin significantly inhibited SLI as well as insulin and glucagon release. Perfusion of glucagon at a concentration of 10 ng/ml failed to overcome the blockade of SLI and insulin release caused by 50 ng/ml des Asn5-[D-Trp8]somatostatin. The results are compatible with a direct inhibitory effect of somatostatin analogs upon SLI release and raise the possibility of a self-inhibiting action of the native hormone.

Bombesin stimulates the release of insulin and glucagon, but not pancreatic somatostatin, from the isolated perfused dog pancreas

Synthetic bombesin, perfused in the isolated canine pancreas at a rate of 340-380 ng/min for 10 min, elicited a 4-fold rise in insulin to a peak at 2 min; a rapid decline followed discontinuation of bombesin. Glucagon rose by 50% to a peak at 6 min, but remained elevated after discontinuation of the bombesin. Somatostatin-like immunoreactivity was not significantly affected by perfusion with bombesin.

Properties of endogenous somatostatin-like immunoreactivity and synthetic somatostatin in dog plasma

Somatostatin-like immunoreactivity (SLI) in the peripheral venous plasma of dogs and in their pancreatic and gastric venous effluents was characterized and compared with synthetic somatostatin. Both endogenuous plasma SLI and somatostatin added to plasma were eluted from Sephadex gels at pH 8.8 in the 150,000--200,000-mol wt region but at pH 2.5 both appeared in the 1,500--2,000-mol wt region. The SLI released from the isolated dog pancreas perfused with plasma-free buffer was eluted entirely as a 1,600-dalton polypeptide, but when the pancreas was perfused with plasma, SLI was eluted in the 150,000--200,000-mol wt zone. Affinity chromatography of plasma samples on immobilized antibodies directed against the central portion of the somatostatin molecule (residues 5--9 and 11) removed approximately equal to 90% of both endogenous SLI and somatostatin added to plasma, but neither was removed by affinity chromatography on antibodies directed against the NH2-terminal region of somatostatin (residues 1--4). The SLI from plasma and from pancreas perfusate isolated by affinity chromatography was identical in molecular size, charge, and immunometric properties to synthetic somatostatin. It is concluded that endogenous SLI is secreted by the pancreas and stomach in a form not distinguishable from synthetic somatostatin, but circulates in plasma bound to large molecular weight components; the NH2-terminal residues of somatostatin appear to be important in this binding.