Basal insulin-level oscillations in normotensive individuals with genetic predisposition to essential hypertension exhibit an irregular pattern

Islet Hypersensitivity to Glucose Is Associated With Disrupted Oscillations and Increased Impact of Proinflammatory Cytokines in Islets From Diabetes-Prone Male Mice

Pulsatile insulin release is the primary means of blood glucose regulation. The loss of pulsatility is thought to be an early marker and possible factor in developing type 2 diabetes. Another early adaptation in islet function to compensate for obesity is increased glucose sensitivity (left shift) associated with increased basal insulin release. We provide evidence that oscillatory disruptions may be linked with overcompensation (glucose hypersensitivity) in islets from diabetes-prone mice. We isolated islets from male 4- to 5-week-old (prediabetic) and 10- to 12-week-old (diabetic) leptin-receptor-deficient (db/db) mice and age-matched heterozygous controls. After an overnight incubation in media with 11 mM glucose, we measured islet intracellular calcium in 5, 8, 11, or 15 mM glucose. Islets from heterozygous 10- to 12-week-old mice were quiescent in 5 mM glucose and displayed oscillations with increasing amplitude and/or duration in 8, 11, and 15 mM glucose, respectively. Islets from diabetic 10- to 12-week-old mice, in contrast, showed robust oscillations in 5 mM glucose that declined with increasing glucose. Similar trends were observed at 4-5-weeks of age. A progressive left shift in maximal insulin release was also observed in islets as db/db mice aged. Reducing glucokinase activity with 1 mM D-mannoheptulose restored oscillations in 11 mM glucose. Finally, overnight low-dose cytokine exposure negatively impacted oscillations preferentially in high glucose in diabetic islets compared with heterozygous controls. Our findings suggest the following: 1) islets from frankly diabetic mice can produce oscillations, 2) elevated sensitivity to glucose prevents diabetic mouse islets from producing oscillations in normal postprandial (11-15 mM glucose) conditions, and 3) hypersensitivity to glucose may magnify stress effects from inflammation or other sources.

Episodic hormone secretion: a comparison of the basis of pulsatile secretion of insulin and GnRH

Rhythms govern many endocrine functions. Examples of such rhythmic systems include the insulin-secreting pancreatic beta-cell, which regulates blood glucose, and the gonadotropin-releasing hormone (GnRH) neuron, which governs reproductive function. Although serving very different functions within the body, these cell types share many important features. Both GnRH neurons and beta-cells, for instance, are hypothesized to generate at least two rhythms endogenously: (1) a burst firing electrical rhythm and (2) a slower rhythm involving metabolic or other intracellular processes. This review discusses the importance of hormone rhythms to both physiology and disease and compares and contrasts the rhythms generated by each system.

Evidence of diminished glucose stimulation and endoplasmic reticulum function in nonoscillatory pancreatic islets

Pulsatility is a fundamental feature of pancreatic islets and a hallmark of hormone secretion. Isolated pancreatic islets endogenously generate rhythms in secretion, metabolic activity, and intracellular calcium ([Ca(2+)](i)) that are important to normal physiological function. Few studies have directly compared oscillatory and nonoscillatory islets to identify possible differences in function. We investigated the hypothesis that the loss of these oscillations is a leading indicator of islet dysfunction by comparing oscillatory and nonoscillatory mouse islets for multiple parameters of function. Nonoscillatory islets displayed elevated basal [Ca(2+)](i) and diminished [Ca(2+)](i) response and insulin secretory response to 3-28 mm glucose stimulation compared with oscillatory islets, suggesting diminished glucose sensitivity. We investigated several possible mechanisms to explain these differences. No differences were observed in mitochondrial membrane potential, estimated ATP-sensitive potassium channel and L-type calcium channel activity, or cell death rates. Nonoscillatory islets, however, showed a reduced response to the sarco(endo)plasmic reticulum calcium ATPase inhibitor thapsigargin, suggesting a disruption in calcium homeostasis in the endoplasmic reticulum (ER) compared with oscillatory islets. The diminished ER calcium homeostasis among nonoscillatory islets was also consistent with the higher cytosolic calcium levels observed in 3 mm glucose. Inducing mild damage with low-dose proinflammatory cytokines reduced islet oscillatory capacity and produced similar effects on glucose-stimulated [Ca(2+)](i), basal [Ca(2+)](i), and thapsigargin response observed among untreated nonoscillatory islets. Our data suggest the loss of oscillatory capacity may be an early indicator of diminished islet glucose sensitivity and ER dysfunction, suggesting targets to improve islet assessment.

Insulin resistance: a proinflammatory state mediated by lipid-induced signaling dysfunction and involved in atherosclerotic plaque instability

The dysregulation of the insulin-glucose axis represents the crucial event in insulin resistance syndrome. Insulin resistance increases atherogenesis and atherosclerotic plaque instability by inducing proinflammatory activities on vascular and immune cells. This condition characterizes several diseases, such as type 2 diabetes, impaired glucose tolerance (IGT), impaired fasting glucose (IFG), obesity, hypertension, dyslipidemia, and other endocrinopathies, but also cancer. Recent studies suggest that the pathophysiology of insulin resistance is closely related to interferences with insulin-mediated intracellular signaling on skeletal muscle cells, hepatocytes, and adipocytes. Strong evidence supports the role of free fatty acids (FFAs) in promoting insulin resistance. The FFA-induced activation of protein kinase C (PKC) delta, inhibitor kappaB kinase (IKK), or c-Jun N-terminal kinase (JNK) modulates insulin-triggered intracellular pathway (classically known as PI3-K-dependent). Therefore, reduction of FFA levels represents a selective target for modulating insulin resistance.

The effects of exercise on haemodynamic function in relation to the familial hypertension risk model

Offspring hypertensives are characterized by a hyperactive sympathetic nervous system and other early cardiovascular abnormalities that increase the risk of developing hypertension. A physically active lifestyle is associated with a lower risk of hypertension, although the mechanisms are incompletely understood and likely to be multifactorial. One aspect that has received little attention is the interaction of exercise with familial hypertension risk. The present review examines the effects of exercise on haemodynamic function in relation to the familial hypertension risk model. Paradoxically, exercise may be viewed as potent stressor to the cardiovascular system, although recent studies are beginning to show that cardiovascular adaptations, primarily mediated by changes in sympatho-vagal balance, following both acute and chronic exercise may be particularly important for individuals with familial risk of hypertension. Future studies that focus on inflammatory, metabolic, and genetic pathways may uncover further beneficial effects of exercise in relation to familial risk.

Mitochondrial metabolism reveals a functional architecture in intact islets of Langerhans from normal and diabetic Psammomys obesus

The cells within the intact islet of Langerhans function as a metabolic syncytium, secreting insulin in a coordinated and oscillatory manner in response to external fuel. With increased glucose, the oscillatory amplitude is enhanced, leading to the hypothesis that cells within the islet are secreting with greater synchronization. Consequently, non-insulin-dependent diabetes mellitus (NIDDM; type 2 diabetes)-induced irregularities in insulin secretion oscillations may be attributed to decreased intercellular coordination. The purpose of the present study was to determine whether the degree of metabolic coordination within the intact islet was enhanced by increased glucose and compromised by NIDDM. Experiments were performed with isolated islets from normal and diabetic Psammomys obesus. Using confocal microscopy and the mitochondrial potentiometric dye rhodamine 123, we measured mitochondrial membrane potential oscillations in individual cells within intact islets. When mitochondrial membrane potential was averaged from all the cells in a single islet, the resultant waveform demonstrated clear sinusoidal oscillations. Cells within islets were heterogeneous in terms of cellular synchronicity (similarity in phase and period), sinusoidal regularity, and frequency of oscillation. Cells within normal islets oscillated with greater synchronicity compared with cells within diabetic islets. The range of oscillatory frequencies was unchanged by glucose or diabetes. Cells within diabetic (but not normal) islets increased oscillatory regularity in response to glucose. These data support the hypothesis that glucose enhances metabolic coupling in normal islets and that the dampening of oscillatory insulin secretion in NIDDM may result from disrupted metabolic coupling.

Rarefaction of skin capillaries in normotensive offspring of individuals with essential hypertension

BACKGROUND

Rarefaction of skin capillaries in people with intermittent borderline essential hypertension suggests a primary or an early abnormality that may antedate the onset of sustained hypertension.

OBJECTIVE

To compare skin capillary density in subjects with and without a family history of essential hypertension.

SUBJECTS

21 normotensive individuals, one or both of whose parents had essential hypertension (mean age 39.3 years; blood pressure 124/79 mm Hg); 21 normotensive controls with no family history of hypertension (age 46.3 years; blood pressure 124/78 mm Hg).

METHODS

The skin of the dorsum of the fingers was examined by intravital capillary microscopy before and after venous congestion at 60 mm Hg for two minutes.

RESULTS

By analysis of variance, both baseline and maximum skin capillary density were lower in subjects with a family history of essential hypertension than in those with no family history (baseline: 67 v 79 capillaries per field, p = 0.008; maximum: 74 v 93 capillaries per field, p < 0.0005).

CONCLUSIONS

Capillary rarefaction in essential hypertension may occur before the increase in blood pressure and could, at least in part, reflect a primary rather than a secondary abnormality.

Hemostasis and fibrinolysis factors in first-degree relatives of patients with Type 2 diabetes without hypertension

First-degree relatives of type 2 diabetic patients with or without a family history of hypertension are at increased risk for cardiovascular diseases. The aim of this study was to verify some possible hemostatic alterations in first-degree relatives of type 2 diabetic, normotensive and hypertensive patients. In 78 non-diabetic, normotensive first-degree relatives of type 2 diabetic patients (47 without a family history of hypertension and 31 with a family history of hypertension) and in 36 normoglycemic, normotensive subjects with no family history of hypertension, we evaluated plasma levels of fasting glucose and insulin, tissue-type plasminogen activator (t-PA), plasminogen activator-inhibitor (PAI-1), D-dimer (DD) and prothrombin fragment 1 + 2 (F1+2). Insulin resistance, calculated by the HOMA model, and plasma levels of t-PA and PAI-1 were significantly higher in relatives of diabetics compared to controls. As far as the thrombin activation indexes are concerned, we detected a significant increase in DD and F1+2 in relatives of diabetics with hypertension compared to other study subjects. In conclusion, our data indicate that familial predisposition may influence the hemostatic system in first-degree relatives of diabetic and/or hypertensive patients.

Insulin resistance and insulin pulsatility in essential hypertension

OBJECTIVE

Studies in normal humans and in patients with type 2 diabetes mellitus have demonstrated a close inverse relationship between peripheral insulin sensitivity and the frequency of short-term insulin secretory pulses in the systemic circulation. Our objective was to study this relationship in essential hypertension.

DESIGN

Study of insulin sensitivity and insulin pulse characteristics in hypertensive subjects and normotensive controls using well-established techniques.

METHODS

Twelve subjects with essential hypertension and 12 age- and sex-matched normotensive controls were recruited. Insulin action was measured using the glucose clamp technique combined with isotope dilution methodology. Insulin pulsatility in the peripheral circulation was assessed by sampling every 2 min for 90 min after an overnight fast Pulses were identified using the computer program Pulsar.

RESULTS

Insulin sensitivity index (glucose infusion rate/ serum insulin) was lower in the hypertensive patients (P= 0.01) and fasting insulin was increased (P= 0.008) compared to controls. The frequency and amplitude of insulin pulses were similar in the two groups. Insulin pulse frequency and insulin sensitivity were inversely related in the normotensive group (r= -0.68, P= 0.015), but not in the hypertensive group (r= -0.23, P= 0.48). Insulin clearance was reduced in the hypertensive group (P= 0.03), and was inversely related to insulin pulse frequency in the two groups combined (r = -0.51, P= 0.01).

CONCLUSIONS

Insulin action was not related to insulin pulse frequency in essential hypertension, in contrast to the situation in normal man.

Pathophysiology of impaired pulsatile insulin release

Plasma insulin displays 5-10 min oscillations. In Type 2 diabetes the regularity of the oscillations disappears, which may lead to insulin receptor down-regulation and glucose intolerance and explain why pulsatile delivery of the hormone has a greater hypoglycemic effect than continuous delivery. The rhythm is intrinsic to the islet. Variations in metabolism, cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), other hormones, neuronal signaling and possibly beta-cell insulin receptor expression have been implicated in the regulation of plasma insulin oscillations. Most of these factors are important for amplitude-regulation of the insulin pulses. Although evidence exists supporting a role of both metabolism and [Ca(2+)](i) as pacemakers of the pulses, metabolic oscillations probably have a primary role and [Ca(2+)](i) oscillations a permissive role. Results from islets from animal models of diabetes suggest that altered plasma insulin pattern could be due to lowering of pulse amplitude of insulin oscillations rather than alterations in their frequency. Supporting a role of metabolism, altered plasma insulin oscillations were found in MODY2, MIDD and glycogenosis Type VII, which are linked to alterations in glucokinase, mitochondrial tRNALeu(UUR) and phosphofructokinase. Plasma insulin oscillations require coordination of islet secretory activities in the pancreas. The intrapancreatic ganglia have been suggested as coordinators. The diabetes-associated neuropathy may contribute to the deranged pattern as indicated by glucose intolerance in chagasic patients. Continued investigation of the role and regulation of pulsatile insulin release will lead to better understanding of the pathophysiology of impaired pulsatile insulin release, which could lead to new approaches to restore normal plasma insulin oscillations in diabetes and related diseases.

Insulin resistance, exercise capacity and body composition in subjects with two hypertensive parents

OBJECTIVE

To study insulin resistance in subjects with strong genetic predisposition to essential hypertension, compared with non-disposed subjects.

SUBJECTS

Thirty normotensive subjects aged 18-35 years whose parents both had essential hypertension, and 30 age- and sex matched subjects whose parents were both normotensive, were studied. Subjects or parents with diabetes and morbid obesity were excluded.

METHODS

The study comprised (1) a frequent sampling oral glucose tolerance test; (2) an isoglycemic hyperinsulinemic clamp study; (3) an analysis of body composition by dual-energy X-ray absorptiometry; (4) an exercise test with gas exchange analysis; and (5) investigation of composition of usual diet by diet registration for 5 days.

RESULTS

The 24-h diastolic blood pressure was higher in subjects predisposed to hypertension compared with the controls: 78.1 versus 74.0 mmHg (confidence interval for the difference between the means; -0.5; -7.9), but the insulin sensitivity index was similar: 312 versus 362 I(2) min(-1) pmol(-1) kg(-1) (28; -129). The two groups were similar in terms of body composition, exercise capacity and composition of usual diet. Resting and 24-h diastolic blood pressures were correlated to abdominal fat mass but not to insulin sensitivity.

CONCLUSION

Subjects with a strong genetic predisposition to essential hypertension had increased diastolic blood pressure compared with subjects with normotensive parents, but they were not insulin resistant. This may be due to the subjects being highly selected as to confounding factors. The increased blood pressure in the hypertension prone subjects could not be attributed to differences in body composition, exercise capacity or dietary habits.