Acute effects of insulin on cardiovascular function and noradrenaline uptake and release

Anorexia nervosa versus hyperinsulinism: therapeutic effects of neuropharmacological manipulation

Background:

We have demonstrated that anorexia nervosa is underpinned by overwhelming adrenal sympathetic activity which abolishes the neural sympathetic branch of the peripheral autonomic nervous system. This physiological disorder is responsible for gastrointestinal hypomotility, hyperglycemia, raised systolic blood pressure, raised heart rate, and other neuroendocrine disorders. Therefore, we prescribed neuropharmacological therapy to reverse this central and autonomic nervous system disorder, in order to normalize the clinical and neuroendocrine profile.

Methods:

The study included 22 female patients with anorexia nervosa (10 restricted type, 12 binge-eating type) who received three months of treatment with amantadine 100 mg/day. We measured blood pressure, heart rate, and circulating neurotransmitters, (noradrenaline, adrenaline, dopamine, platelet serotonin, free plasma serotonin) during supine resting, one minute of orthostasis, and a five-minute exercise test before and after one, two, and three months of treatment with amantadine, a drug which abrogates adrenal sympathetic activity by acting at the C1(Ad) medullary nuclei responsible for this branch of the peripheral sympathetic activity.

Results:

We found the amantadine abolished symptoms of anorexia nervosa from the first oral dose onwards. Normalization of autonomic and cardiovascular parameters was demonstrated within the early days of therapy. Abrupt and sustained increases in the plasma noradrenaline:adrenaline ratio and disappearance of abnormal plasma glucose elevation were registered throughout the three-month duration of the trial. Significant and sustained increases in body weight were documented in all cases. No relapses were observed.

Conclusion:

We have confirmed our previously published findings showing that the anorexia nervosa syndrome depends on the hypomotility of the gastrointestinal tract plus hyperglycemia, both of which are triggered by adrenal sympathetic hyperactivity. The above neuroendocrine plus neuroautonomic and clinical disorders which underpinned anorexia nervosa were abruptly suppressed since the first oral dose of amantadine, a drug able to revert the C1(Ad) over A5(NA) pontomedullary predominance responsible for adrenal and neural sympathetic activity, respectively.

Anorexia nervosa depends on adrenal sympathetic hyperactivity: opposite neuroautonomic profile of hyperinsulinism syndrome

OBJECTIVE

The aim of our study was to determine the central and peripheral autonomic nervous system profiles underlying anorexia nervosa (AN) syndrome, given that affected patients present with the opposite clinical profile to that seen in the hyperinsulinism syndrome.

DESIGN

We measured blood pressure and heart rate, as well as circulating neurotransmitters (noradrenaline, adrenaline, dopamine, plasma serotonin, and platelet serotonin), using high-performance liquid chromatography with electrochemical detection, during supine resting, one minute of orthostasis, and after five minutes of exercise. In total, 22 AN patients (12 binge-eating/purging type and 10 restricting type) and age-, gender-, and race-matched controls (70 ± 10.1% versus 98 ± 3.0% of ideal body weight) were recruited.

RESULTS

We found that patients with AN had adrenal sympathetic overactivity and neural sympathetic underactivity, demonstrated by a predominance of circulating adrenaline over noradrenaline levels, not only during the supine resting state (52 ± 2 versus 29 ± 1 pg/mL) but also during orthostasis (67 ± 3 versus 32 ± 2 pg/mL, P < 0.05) and after exercise challenge (84 ± 4 versus 30 ± 3 pg/mL, P < 0.01).

CONCLUSION

Considering that this peripheral autonomic nervous system disorder depends on the absolute predominance of adrenomedullary C1 adrenergic nuclei over A5 noradrenergic pontine nucleus, let us ratify the abovementioned findings. The AN syndrome depends on the predominance of overwhelming adrenal sympathetic activity over neural sympathetic activity. This combined central and autonomic nervous system profile contrasts with that registered in patients affected by hyperinsulinism, hypoglycemia, and bulimia syndrome which depends on the absolute predominance of neural sympathetic activity.

Insulin infusion normalizes cardiovascular responses and plasma noradrenaline after oral glucose in type 1 (insulin-dependent) diabetes

We examined whether the abnormal regulation of the cardiovascular system and plasma noradrenaline observed after oral glucose in insulin-dependent diabetic patients could be normalized by intravenous infusion of insulin. Eight patients with type 1 (insulin-dependent) diabetes were examined after an oral glucose load with and without simultaneous infusion of insulin. Insulin infusion increased plasma insulin from 0.07 to 0.31 nmol/l. In the control experiment (glucose only), mean heart rate and mean arterial systolic blood pressure remained unchanged and plasma noradrenaline (NA) decreased (p less than 0.05). After oral glucose plus intravenous insulin, mean heart rate increased by 11% and mean systolic blood pressure by 5% (p less than 0.05, p less than 0.01), whereas plasma NA did not change significantly. The present study indicates that physiologic increments in plasma insulin concentration are of importance in the regulation of the cardiovascular system and plasma NA following an oral glucose load.

Hyperinsulinemia and insulin resistance are independently associated with plasma lipids, uric acid and blood pressure in non-diabetic subjects. The GISIR database

BACKGROUND AND AIMS

We evaluated whether hyperinsulinemia and/or insulin resistance are independently associated with plasma lipids, uric acid and blood pressure in non-diabetic subjects.

METHODS AND RESULTS

A database of non-diabetic Italian subjects has recently been set up using data from hyperinsulinemic euglycemic clamp studies carried out using the standard technique (40 mU per min per square meter of body surface area). In this database we evaluated the relationships between fasting plasma insulin (FPI), glucose metabolized during clamp (M) and plasma levels of triglycerides (TG), high density lipoprotein cholesterol (HDL-C), uric acid (UA) as well as blood pressure (BP) in non-diabetic subjects with fasting plasma glucose <6.1 mmol/l. Parallel analyses were conducted in all subjects in the database (n=1093) and in those with all variables available (n=309). In the univariate analysis both FPI and M were significantly correlated with TG, HDL-C, UA and BP (systolic, diastolic and mean). Multivariate regression analyses including center, sex, age, body mass index (BMI), FPI and M as independent variables showed that: (1) TG and UA were positively correlated with FPI and negatively correlated with M; (2) HDL-C was negatively correlated with FPI and positively correlated with M; and (3) BP was negatively correlated with both FPI and M. Analyses of covariance showed that, after adjusting for center, sex, age and BMI, subjects with isolated hyperinsulinemia or isolated insulin resistance had higher TG and UA and lower HDL-C. Subjects with isolated insulin resistance had also higher BP whereas subjects with isolated hyperinsulinemia had lower BP. Subjects with both defects had a worse profile.

CONCLUSIONS

Hyperinsulinemia and insulin resistance might contribute with distinct and independent mechanisms to the development of several metabolic and hemodynamic disorders often clustering in the same individual. In particular, hypertriglyceridemia, low HDL-cholesterol and hyperuricemia seem to be related to both hyperinsulinemia and insulin resistance, whereas hypertension seems to be related only to insulin resistance.

The metabolic and cardiovascular effects of intravenous infusion of glucose or intralipid in normal humans

This study was performed to determine whether the administration of intravenous fluids, isosmolar with plasma, activated the sympathetic nervous system, thereby causing changes in cardiovascular variables. On four separate occasions, six young, healthy men were studied for 30 min before and, for 40 min after a 60 min period of either (a) intravenous infusion of 500 ml of 5% dextrose, (b) intravenous infusion of 500 ml of a mixture of 10% Intralipid and saline (154 mmol NaCl 1(-1)), (c) intravenous infusion of 500 ml of saline (154 mmol NaCl 1(-1)) or (d) no intravenous infusion. Venous plasma noradrenaline and adrenaline concentrations did not change significantly during any of these interventions. The venous plasma insulin level rose during infusion of 5% dextrose (p<0.001). The respiratory exchange ratio rose during the dextrose infusion and fell during the infusion of the Intralipid and saline mixture (p<0.01). Hand and calf blood flows and vascular resistances were not significantly affected by any procedure. Similar, small, changes in heart rate, arterial blood pressure, metabolic rate, core temperature and mean skin temperature were observed during the four protocols. The provision of small amounts of metabolic substrate, as either glucose or fat, led to rapid changes in fuel utilisation. However, under the conditions of the present experiments, there was no evidence of activation of the sympathetic nervous system.

Central nervous system circuitry involved in the hyperinsulinism syndrome

Raised plasma levels of insulin, glucose and glucagon are found in patients affected by 'hyperinsulinism'. Obesity, hypertension, mammary plus ovary cysts and rheumatic symptoms are frequently observed in these patients. Sleep disorders and depression are also present in most subjects affected by this polysymptomatic disorder. The simultaneous increases of glucose, insulin and glucagon plasma levels seen in these patients indicate that the normal crosstalk between A cells, B cells and D cells is disrupted. With respect to this, it is well known that glucose excites B cells (which secrete insulin) and inhibits A cells (which secrete glucagon), which in turn excites D cells (which secrete somatostatin). Gastrointestinal hormones (incretins) modulate this crosstalk both directly and indirectly throughout pancreatic and hepatobiliary mechanisms. The above factors depend on autonomic nervous system mediation. For instance, acetylcholine released from parasympathetic nerves excites both B and A cells. Noradrenaline released from sympathetic nerves and adrenaline secreted from the adrenal glands inhibit B cells and excite A cells, which are crowded with beta(2)- and alpha(2)-receptors, respectively. Noradrenaline released from sympathetic nerves also excites A cells by acting at alpha(1)-receptors located at this level. According to this, the excessive release of noradrenaline from these nerves should provoke an enhancement of glucagon secretion which will result in overexcitation of insulin secretion from B cells. That is the disorder seen in the so-called 'hyperinsulinism', in which raised plasma levels of glucose, insulin and glucagon coexist. Taking into account that neural sympathetic activity is positively correlated to the A5 noradrenergic nucleus and median raphe serotonergic neurons, and negatively correlated to the A6 noradrenergic, the dorsal raphe serotonergic and the C1 adrenergic neurons, we postulate that this unbalanced central nervous system circuitry is responsible for the hyperinsulinism syndrome.

Insulin resistance in hypertensives: effect of salt sensitivity, renin status and sodium intake

OBJECTIVE

Homeostasis Model Assessment (HOMA index) is predictive of insulin sensitivity in normal and diabetic patients. This study was designed to see if insulin resistance in hypertensives, measured using the HOMA index, differs, based on salt sensitivity, renin status and sodium intake.

METHODS

Fasting insulin and glucose were determined in subsets of 426 essential hypertensives, and normotensives. HOMA was calculated as fasting glucose (mmol) x fasting insulin (muU/ml)/22.5.

RESULTS

Four hundred and twenty-six essential hypertensives and normotensives from four HERMES centers form the basis of this report. There was no difference in the HOMA index between hypertensives and normotensives (P= 0.291) or between hypertensives grouped according to blood pressure salt sensitivity (P = 0.153). However, when essential hypertensives were subgrouped by renin status, the low-renin group had significantly lower (P< 0.01) HOMA index than the normal/high-renin group. When normal/high-renin group was divided into modulators and non-modulators, the nonmodulators had significantly higher HOMA index (P< 0.001) than other hypertensive subsets. The effect of sodium intake on the HOMA index was significant only for non-modulators (P< 0.002), with salt restriction increasing insulin resistance.

CONCLUSION

Insulin sensitivity differs among subsets of essential hypertension, non-modulators being most insulin resistant and the low-renin subset insulin sensitive. Salt restriction might have an adverse effect on insulin sensitivity in non-modulators. The reduction in cardiovascular risk seen in low-renin hypertensives may be related to their increased insulin sensitivity; in contrast, the clustering of cardiovascular risk factors seen in nonmodulators may be due to increased insulin resistance.

Effects of chronic insulin treatment on blood pressure in rats

There is an evident epidemiological association between plasma insulin levels and blood pressure. The mechanism that relates insulin to blood pressure and the role of insulin in the pathogenesis of arterial hypertension have not been clearly defined. The present study was designed to examine the effects of chronic hyperinsulinism on blood pressure and to determine different related morphological variables. WistarKyoto rats were subcutaneously injected with insulin (25 UI/Kg of weight) daily during the eight weeks of the experiment. Data were collected on systolic and diastolic arterial pressures and heart rate by plethysmography and direct recording (in the last week), and on morphological variables. A statistically significant elevation of systolic arterial pressure was produced after the sixth week of hyperinsulinaemia. At the end of the treatment, the systolic arterial pressure was 173.7 +/- 26.1 in the hyperinsulinaemic rats versus 153.09 +/- 21.7 in the control group. The values obtained by direct recording and by plethysmography did not differ. These results indicate that chronic hyperinsulinism produces a significant elevation in systolic blood pressure levels in the rats studied.

Endocrine and lipid effects of oral L-arginine treatment in healthy postmenopausal women

As a substrate for nitric oxide synthesis, L-arginine may give the same protection as estrogen, but its other biologic effects may adversely affect atherogenesis. Therefore, possible endocrine and lipid effects of L-arginine were investigated in a double-blind, placebo-controlled, single crossover study. After randomization, oral L-arginine (9 g) or placebo was given daily for 1 month, with crossover to the alternate therapy after a 1-month washout period, to 10 postmenopausal women receiving no estrogen. Compared with placebo, L-arginine increased growth hormone (1.5+/-1.8 mg/L vs. 0.6+/-0.6 mg/L, P = .04) but had no effect on insulin and catecholamines. Total cholesterol, triglyceride, apolipoprotein E, and low-, very-low-, and high-density lipoprotein cholesterol levels were also unaffected. Lipoprotein(a) measured by an immunoturbidimetric method was increased by L-arginine in 9 of 10 women relative to placebo (0.46+/-0.35 g/L vs. 0.38+/-0.30 g/L, P = .053), and the changes in lipoprotein(a) levels significantly correlated with the relative increase in growth hormone (r = 0.85, P = .03). However, lipoprotein(a) measured by an enzyme-linked immunosorbent assay failed to demonstrate significant changes. Lack of an increase by L-arginine in lipoprotein(a) with a verifiable apolipoprotein(a) isoform-independent method, despite an increase in growth hormone, questions the validity of previous observations for growth hormone-induced increases in lipoprotein(a). The observed lack of effect on major endocrine hormones and lipid profile support the safety of oral L-arginine administration.

Postprandial blood pressure changes in the elderly

Postprandial blood pressure reductions have been observed in frail elderly individuals. This study evaluated blood pressure and heart rate changes in 10 healthy elderly subjects at intervals before and after a morning meal. Subjects did not exhibit significant reductions in systolic blood pressure or increases in heart rate over time, rather, such changes were due to variation within subjects. They did not exhibit significant changes in diastolic blood pressure. Age, health, and meal choice may have influenced results. Further research will determine which groups of elderly are most at risk for postprandial blood pressure reductions and consequently at risk for injury from falls due to dizziness and syncope.

Disparate effects of antidiabetic drugs on arterial contraction

Type II diabetic patients and others with insulin resistance are at risk for development of hypertension characterized by elevated peripheral vascular resistance and loss of insulin's normal vasodilating activity. Oral antidiabetic drugs have recently been recognized to have disparate effects on arterial pressure in such patients and in related rodent models. Sulfonylureas (e.g., glyburide), which stimulate insulin secretion, have been reported either to increase or not to affect arterial pressure, whereas nonsulfonylurea agents with insulin-sensitizing properties, the biguanide metformin and various thiazolidinediones (eg, pioglitazone), have been reported to decrease arterial pressure in humans and rodents. To help elucidate these disparate effects, we investigated these agents for direct actions on arterial vascular contractility and its sensitivity to insulin. Preincubation of intact rat tail arterial tissue rings for 2 hours with known therapeutically effective antidiabetic concentrations of metformin and pioglitazone significantly attenuated the force of contractions produced by either potassium (membrane depolarization) or norepinephrine ([NE] adrenergic receptor activation). Glyburide did not influence these contractions. Preincubation with metformin also induced an attenuating (vasodilating-like) action of insulin on arterial tissue rings contracted by potassium. Conversely, glyburide induced an accentuating action of insulin on potassium-mediated contractions. These results are consistent with measures of vascular function obtained in the past after oral administration of the drugs, which suggested but did not prove that they may exert direct effects on arterial vascular contractility. Thus, metformin and thiazolidinediones may decrease arterial pressure partly by direct vasorelaxant mechanisms, with metformin having an additional effect of inducing vasorelaxation by insulin. In contrast, sulfonylureas may directly induce a paradoxical vasoconstrictor response to insulin.

The splanchnic circulation and postural hypotension in diabetic autonomic neuropathy

Postural hypotension results from sympathetic failure to cause superior peripheral vasoconstriction. The importance of the splanchnic circulation was studied by measuring mesenteric artery blood flow with duplex Doppler scanning. Nine normal and 9 Type 1 diabetic controls were compared to 8 Type 1 patients with autonomic neuropathy whose pressure fell 40-113 mmHg (range) on tilting. Measurements were made supine and after vertical tilt, fasting without insulin and after a 550 kcal meal. Superior mesenteric artery diameter decreased on tilting in normal controls but not in diabetic control or neuropathy groups (supine vs tilted: controls. 6.3 +/- 0.9 to 5 +/- 0.9 mm, p = 0.004, diabetic controls: 6.0 +/- 0.6 to 6.0 +/- 1.0 mm, and neuropathy group: 6.4 +/- 0.9 to 5.6 +/- 0.9 mm), but proportional blood flow changes were similar in all subjects (controls: 407 +/- 154 to 255 +/- 67 ml min-1 (-31%, p = 0.03), diabetic controls: 379 +/- 140 to 306 +/- 149 ml min-1 (-8%, p = 0.28), neuropathy group: 639 +/- 371 to 435 +/- 142 ml min-1 (-23%, p = 0.10). Postprandially supine superior mesenteric artery flow increased in all subjects but this did not affect the degree of systolic blood pressure drop on tilting (fasting vs postprandial blood flow: controls: 407 +/- 154 to 775 +/- 400 ml min-1 (p = 0.04), diabetic controls: 379 +/- 140 to 691 +/- 262 ml min-1 (p = 0.01), neuropathy group: 639 +/- 371 to 943 +/- 468 ml min-1 (p < 0.06)). The similarity of superior mesenteric artery responses to tilting in the three groups, and the lack of exacerbation of postural hypotension in the presence of postprandial hyperaemia indicates that control of splanchnic blood flow is less important in the aetiology of diabetic autonomic postural hypotension than previously thought.

Postural hypotension in diabetic autonomic neuropathy: a review

Postural hypotension is uncommon in diabetes but can occur secondary to autonomic neuropathy. Symptoms are rare and include dizziness, weakness, blurred vision, tiredness, and loss of consciousness. The pathophysiology of postural hypotension is not clear, but changes in intravascular volume, heart rate, cardiac output, and splanchnic vascular resistance are similar in patients and controls. The main factors producing hypotension are a blunted catecholamine response to standing, and failure of lower limb vascular resistance to increase adequately. Treatment for symptomatic postural hypotension includes avoidance of dehydration, adequate salt intake, and fludrocortisone. Other treatments are reviewed but are less helpful. Patients with postural hypotension have intermittent symptoms over the years but rarely become severely disabled. They have a poorer prognosis than patients with symptomatic autonomic neuropathy without postural hypotension.

The effect of metformin and insulin on sympathetic nerve activity, norepinephrine spillover and blood pressure in obese, insulin resistant, normoglycemic, hypertensive men

To evaluate the effect of metformin on insulin sensitivity and to further examine the relationship between insulin resistance, sympathetic nerve activity and blood pressure, 6 obese insulin resistant, normoglycemic hypertensive men were investigated (age 49 +/- 2 years, BMI 27.6 +/- 1.2, mean +/- SEM). The study had a placebo controlled, double blind, cross over design with 6 weeks' metformin treatment (850 mg b.i.d) vs placebo. Blood pressure was measured weekly. At the end of each treatment period, glucose infusion rate (GIR), muscle sympathetic nerve activity (MSA) and renal and total body norepinephrine (NE) kinetics (radioisotope dilution) were examined during euglycemic hyperinsulinemic clamp. Fasting insulin was 13 +/- 3 and 10 +/- 2 mU/l and fasting glucose 5.3 +/- 0.2 and 5.1 +/- 0.1 mmol/l after placebo and metformin treatment, respectively (ns). GIR during the last hour of the insulin clamp was 3.7 +/- 0.6 vs 3.6 +/- 0.6 mg/kg x min (ns). Resting MSA, total body and right renal NE spillover did not differ significantly after placebo and metformin treatment. Systolic and diastolic blood pressures were 151 +/- 10/95 +/- 5 mmHg after placebo and 146 +/- 5/94 +/- 5 mmHg after metformin treatment (ns). Thus metformin treatment did not have any significant effect on insulin sensitivity, blood pressure or sympathetic activity in this small group of patients. Renal plasma flow and MSA increased significantly during the insulin clamp, whereas renal NE and total body NE spillover remained unchanged, suggesting nonuniform regional sympathetic nerve responses to acute hyperinsulinemia.

Insulin treatment improves microalbuminuria and other cardiovascular risk factors in patients with type 2 diabetes mellitus

OBJECTIVES

Insulin treatment of patients with type 2 diabetes causes hyperinsulinaemia and improves glycaemic control. We have studied how this affects risk factors for cardiovascular disease.

DESIGN

Patients with secondary failure to oral hypoglycaemic agents were studied whilst still taking oral agents and after insulin treatment for 8 weeks in an open study.

SETTING

Department of Internal Medicine, University Hospital, Linköping.

SUBJECTS

Ten consecutive patients with type 2 diabetes and secondary failure to oral hypoglycaemic agents.

INTERVENTIONS

Switching oral treatment to insulin treatment.

MAIN OUTCOME MEASURES

Effect on several cardiovascular risk factors.

RESULTS

Fasting and postprandial plasma insulin concentrations were increased by insulin treatment whereas C-peptide concentrations were lowered. HbA1c was reduced from 8.9 +/- 0.3% (mean +/- SEM) to 6.3 +/- 0.2% after 8 weeks. There was a weight gain of 2.8 +/- 0.7 kg. Plasma concentrations of total- and very-low-density-lipoprotein (VLDL) cholesterol, VLDL-, low density lipoprotein and high-density-lipoprotein triglycerides were all reduced. The plasma concentration of apolipoprotein B was also lowered. Tissue plasminogen activator antigen measured after venous occlusion showed a significant reduction whilst plasminogen activator inhibitor 1 activity was 26.0 +/- 9.8 IU ml-1 on oral treatment and 18.2 +/- 4.7 IU ml-1 on insulin treatment (NS). Albumin excretion in the urine was reduced and the percentage reduction correlated with the percentage lowering of the tissue plasminogen activator antigen concentration after venous occlusion but not with the percentage change of basal tissue plasminogen activator antigen concentration.

CONCLUSIONS

Insulin treatment of patients with type 2 diabetes and secondary failure to oral hypoglycaemic agents causes hyperinsulinaemia and improves or has no unfavourable effect on several cardiovascular risk factors.

Coexistence of increased levels of adiposity, insulin, and blood pressure in a young adult cohort with elevated very-low-density lipoprotein cholesterol: the Bogalusa Heart Study

Clustering and interrelationships of elevated levels (> 75th percentile) of adiposity, insulin, blood pressure, and very-low-density lipoprotein cholesterol (VLDL-C) were examined in a selected subset (n = 89) of a young adult cohort aged 18 to 26 years, whose childhood (ages 10 to 18 years) VLDL-C and/or low-density lipoprotein cholesterol (LDL-C) levels were in the upper or lower percentiles of the distribution. Among the young adults with elevated VLDL-C and LDL-C levels, 23% had increased adiposity and systolic blood pressure, 17% had increased insulin levels and systolic blood pressure, 17% had increased insulin levels and adiposity, and 13% had increased insulin levels, adiposity, and systolic blood pressure; corresponding values among those with elevated VLDL-C and normal (< 75th percentile) LDL-C levels were 29%, 18%, 24%, and 18%. As adults, the group with elevated VLDL-C and LDL-C levels had increased adiposity (P = .02) and systolic blood pressure (P = .002) in comparison to those with normal VLDL-C and LDL-C levels, whereas the group with elevated VLDL-C and normal LDL-C levels had increased fasting plasma insulin levels (P = .05) and systolic blood pressure (P = .007). These 89 individuals had undergone a glucose tolerance test during childhood.(ABSTRACT TRUNCATED AT 250 WORDS)

Fasting insulin, calcium metabolism and the electrocardiogram in hypertensive subjects

Changes in both calcium and insulin metabolism have been described in essential hypertension. Low levels of plasma ionized calcium (Ca2+) and high levels of insulin have previously been associated with vascular complications and coronary heart disease. In the present study, indices of calcium metabolism and fasting serum insulin were related to electrocardiographic (ECG) variables in 58 patients with untreated hypertension. Fasting insulin was found to be related to heart rate (r = 0.47, P < 0.001), diastolic interval (r = -0.39, P < 0.004) and electrical axis (r = -0.29, P < 0.03) while Ca2+ was found to be correlated with the QRS amplitude (r = -0.32, P < 0.03) and diastolic interval (r = 0.37, P < 0.02). Furthermore, non-ionized serum calcium was correlated with the QRS duration (r = 0.36, P < 0.02), ST-segment interval (r = -0.49, P < 0.002) and QT interval (QoT, r = -0.42, P < 0.008). These correlations were still significant when the influences of age, sex, obesity, blood pressure and heart rate were taken into account in the multiple regression analysis. In conclusion, the present study demonstrates that calcium and insulin metabolism are related to several basic characteristic functions of the heart, such as the systolic and diastolic function, as well as to signs of left ventricular hypertrophy.

Autonomic responses to carbohydrate ingestion and head-up tilt in elderly subjects with orthostatic hypotension

To investigate the role of glucose and insulin in the development of hypotension following glucose ingestion in elderly subjects with orthostatic hypotension, the autonomic responses to glucose and xylose ingestion were studied in five elderly subjects with age related orthostatic hypotension (without autonomic failure), five elderly control subjects and three elderly subjects with evidence of autonomic failure. Heart rate, blood pressure, plasma noradrenaline and plasma arginine vasopressin responses to glucose ingestion and to xylose ingestion were investigated. All subjects were supine for 90 min following ingestion of each carbohydrate and were then tilted 45 degrees head-up for 10 min. Blood pressure was maintained in elderly control subjects following ingestion of both carbohydrates and during tilting. The elderly group with orthostatic hypotension, while supine had a fall in systolic blood pressure 60-90 min following both glucose and xylose ingestion. Diastolic blood pressure was lowered 60-90 min after glucose but not xylose. During tilting, blood pressure fell by similar levels following both carbohydrate ingestions; plasma noradrenaline levels after 2 min and plasma arginine vasopressin levels after 10 min tilting were significantly less following glucose ingestion compared to xylose ingestion. The autonomic failure group while supine had a fall in systolic blood pressure 60-90 min following both glucose and xylose ingestion. Diastolic blood pressure was lowered 60-90 min after glucose but not xylose. During tilting, blood pressure fell by similar levels following both carbohydrate ingestion. Plasma noradrenaline and arginine vasopressin levels were unchanged after ingestion of either carbohydrate, and during tilting.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effect of insulin on plasma volume and transcapillary albumin transport

During the past decade it has been demonstrated that insulin, apart from its effects on metabolism and ion fluxes, has acute effects on the cardiovascular system and capillary permeability. Intravenous infusion of insulin in doses which increase plasma insulin to physiological levels, induced vascular dilatation and increased muscle sympathetic nerve activity during a euglycaemic glucose clamp. During similar conditions insulin increased the transcapillary escape rate of albumin and reduced plasma volume. Insulin has also an indirect effect on vascular permeability during hypoglycaemia, which is mediated by the increase in plasma adrenaline. Adrenaline infusion increased haematocrit and decreased plasma volume and intravascular albumin mass. In contrast to insulin adrenaline did not increase the transcapillary escape rate of albumin. Total autonomic blockade during insulin-induced hypoglycaemia abolished the increase in haematocrit, but did not influence the decrease in plasma volume and the increase in the transcapillary escape rate of albumin. Insulin administration may also increase urinary albumin excretion, and this effect was observed during a euglycaemic clamp. The mechanism of the increase in capillary permeability after insulin has not been elucidated. A number of morphological studies indicate that insulin may have effects on endothelial cell morphology and paraendothelial cell permeability. These results indicate that insulin, apart from its effect on peripheral blood flow, may play a role in a normal transfer of macromolecules from the blood to the extracellular space after food intake. This process may be greatly disturbed in insulin-dependent diabetic patients.

Responses of serum insulin and blood pressure to cold and handgrip in obese patients

A close correlation between body weight and blood pressure has been frequently observed in both clinical and epidemiological studies. The aim of this clinical trial was to evaluate whether, in obese patients, there is any relationship between blood pressure, at rest or during sympathetic stimulation, and blood glucose and serum insulin, both while fasting and during an oral glucose challenge. Twenty obese patients (age 26-65 years, body weight 97 +/- 16 kg, 11 normotensive and 9 hypertensive) entered the study. After a 4-week run-in period on an isocaloric diet with normal intake of sodium, blood pressure and heart rate were measured at rest and during sympathetic stimulation induced by cold and isometric testing. Responses of glucose and insulin to a standardized 75 g oral glucose tolerance test were also evaluated. The responses of glucose and insulin to glucose challenge were not statistically different in normotensive and hypertensive obese patients. Levels of insulin in the serum in the serum in the fasting state and during glucose load were significantly correlated with the response of blood pressure to cold and isometric exercise, but not to blood pressure at rest. The response of heart rate to cold was closely related to insulin only in the subgroup of normotensives. The present findings support the hypothesis that the sympathetic nervous system, which influences secretion of insulin and regulation of blood pressure, is involved in the pathophysiology of the association of obesity and hypertension.

Glucose tolerance and insulin action in rats with renovascular hypertension

To test whether hypertension can cause hyperinsulinemia or insulin resistance, we performed intravenous glucose tolerance tests at 1 month and euglycemic clamps at 3 months after induction of two-kidney, one clip renovascular hypertension in rats. At 1 month, systolic pressure was higher in 21 clipped than in 12 control animals (161 +/- 5 mm Hg, range 134-187 mm Hg versus 119 +/- 3 mm Hg, range 108-146 mm Hg; p less than 0.001). Glucose tolerance, assessed as the glucose fractional disappearance rate between 3 and 11 minutes after the glucose injection, was similar in the clipped and sham groups (0.059 +/- 0.002 versus 0.056 +/- 0.002 min-1, respectively; p greater than 0.4). The total area under the insulin curve during glucose tolerance tests was also similar in the clipped and sham groups (926 +/- 95 versus 869 +/- 126 microunits/ml x min; p greater than 0.4). There was no significant relation between systolic blood pressure and insulin area during glucose tolerance tests in the clipped group, but there was a positive rectilinear relation in the control group (r = 0.66; p = 0.01). Fourteen animals had euglycemic clamps 2 months after glucose tolerance tests. At that time, systolic pressure (direct femoral measurement) was higher in the seven clipped animals (189 +/- 13 mm Hg versus 122 +/- 5 mm Hg in controls; p less than 0.001). Insulin infusions of 1 and 4 milliunits/min/kg body wt effected similar plasma insulin levels in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of sympathetic augmentation in response to intravenous load of glucose in rabbits

1. We investigated a link between sympathetic nervous function and carbohydrate metabolism by measuring renal sympathetic nerve activity in response to intravenous load of glucose in alpha-chloralose-urethane anaesthetized rabbits. 2. Intravenous infusion of a 25% glucose solution (0.5 g/kg) over 3 min caused a transient increase in arterial blood pressure and a decrease in renal sympathetic nerve activity. Thereafter, these parameters were restored and remained around preload levels while plasma glucose and insulin concentrations were still elevated. 3. Equimolar mannitol solution produced similar patterns of change in blood pressure and nerve activity without an elevation of plasma glucose and insulin levels. 4. The transient changes in blood pressure and renal nerve activity could be attributed to acute hypervolaemia indicated by similar changes in plasma osmolality and haematocrits in the two groups of treatment. 5. The present study did not support a close relationship between carbohydrate metabolism and the sympathetic nervous system regulating cardiovascular function.

Sympathoadrenal responses during hypoglycemia, hyperinsulinemia, and hypoxemia in the ovine fetus

Interrelations of sympathoadrenal function and changes in glucose and insulin homeostasis were studied in chronically cannulated late gestation fetal sheep. Catecholamine secretory rates (based on direct adrenal sampling) and plasma concentrations were determined in the fetus during 2 h of insulin-induced hypoglycemia, during a period of hypoxemia, and during hyperinsulinemia per se (i.e., without hypoglycemia). Fetal insulin infusion (5-10 mU.kg-1.min-1) resulted in hypoglycemia and a significant rise in secretion of epinephrine but not of norepinephrine. By contrast, fetal hypoxemia caused a prompt and significant increase in adrenal secretion of both norepinephrine and epinephrine. Changes in peripheral plasma catecholamine levels were usually, but not always, qualitatively similar to those in adrenal secretion; the latter was a far more sensitive indicator of adrenal function. Hyperinsulinemia per se caused no change in adrenal secretory rates or plasma concentrations of catecholamines. Nevertheless, insulin infusion caused a fetal tachycardia even in the absence of hypoglycemia and hypoxemia, suggesting either a direct effect on the heart or stimulation of sympathetic nerves.

Hyperinsulinemia produces both sympathetic neural activation and vasodilation in normal humans

Hyperinsulinemia may contribute to hypertension by increasing sympathetic activity and vascular resistance. We sought to determine if insulin increases central sympathetic neural outflow and vascular resistance in humans. We recorded muscle sympathetic nerve activity (MSNA; microneurography, peroneal nerve), forearm blood flow (plethysmography), heart rate, and blood pressure in 14 normotensive males during 1-h infusions of low (38 mU/m2/min) and high (76 mU/m2/min) doses of insulin while holding blood glucose constant. Plasma insulin rose from 8 +/- 1 microU/ml during control, to 72 +/- 8 and 144 +/- 13 microU/ml during the low and high insulin doses, respectively, and fell to 15 +/- 6 microU/ml 1 h after insulin infusion was stopped. MSNA, which averaged 21.5 +/- 1.5 bursts/min in control, increased significantly (P less than 0.001) during both the low and high doses of insulin (+/- 5.4 and +/- 9.3 bursts/min, respectively) and further increased during 1-h recovery (+15.2 bursts/min). Plasma norepinephrine levels (119 +/- 19 pg/ml during control) rose during both low (258 +/- 25; P less than 0.02) and high (285 +/- 95; P less than 0.01) doses of insulin and recovery (316 +/- 23; P less than 0.01). Plasma epinephrine levels did not change during insulin infusion. Despite the increased MSNA and plasma norepinephrine, there were significant (P less than 0.001) increases in forearm blood flow and decreases in forearm vascular resistance during both doses of insulin. Systolic pressure did not change significantly during infusion of insulin and diastolic pressure fell approximately 4-5 mmHg (P less than 0.01). This study suggests that acute increases in plasma insulin within the physiological range elevate sympathetic neural outflow but produce forearm vasodilation and do not elevate arterial pressure in normal humans.

The effects of underfeeding for 7 d on the thermogenic and physiological response to glucose and insulin infusion (hyperinsulinaemic euglycaemic clamp)

The effect of underfeeding for 7 d (at 60 kJ/kg ideal body-weight) on the thermic and physiological responses to glucose and insulin infusions (hyperinsulinaemic euglycaemic clamp) was studied in six healthy women. Underfeeding had no significant effect on baseline metabolic rate, heart rate, forearm blood flow, diastolic blood pressure, blood intermediary metabolites, plasma insulin or catecholamines, but reduced both respiratory exchange ratio (RER; control (C) 0.86 (SE 0.02), underfed (U) 0.75 (SE 0.01); P less than 0.01) and systolic blood pressure (by approximately 10 mmHg, P less than 0.01). Baseline forearm glucose uptake and oxygen consumption were similar in both states. During the final 30 min of the glucose and insulin infusion, metabolic rate rose by 0.43 (SE 0.05) kJ/min in the C state, but no rise was seen in the U state (P less than 0.01). Glucose disposal rate (C 47.9 (SE 1.8), U 47.3 (SE 4.1) mumol/kg per min) and storage rate (C 27.5 (SE 2.4), U 31.6 (SE 3.6) mumol/kg per min) were similar in both states, but glucose oxidation rate was reduced in the U state (C 20.5 (SE 1.7), U 15.4 (SE 0.7) mumol/kg per min; P less than 0.05). RER rose to a higher value in the C state than in the U state (C 0.97 (SE 0.2), U 0.80 (SE 0.01); P less than 0.01). During hyperinsulinaemia, the forearm glucose uptake and O2 consumption rose in both states. No significant differences were seen in the cardiovascular responses to hyperinsulinaemia in either state. Thus underfeeding abolishes the rise in thermogenesis and reduces glucose oxidation during glucose and insulin infusions in healthy women, but does not affect the glucose disposal or storage rates or the other measured responses.

Insulin-induced increase in heart rate and its prevention by propranolol

Acute hyperinsulinaemia in the absence of changes in blood glucose increases heart rate in man. Animal studies have suggested that beta-adrenergic blockade does not prevent the insulin-induced increase in heart rate. The aim of the present study was to investigate the acute effect of insulin on heart rate and blood pressure in non diabetic subjects and, in particular, to determine whether beta-adrenergic receptor blockade would significantly influence the effect. On separate days 9 healthy young volunteers were pretreated with either 80 mg propranolol or placebo p.o. After a 60-90 min period of heart rate and blood pressure stabilization, a placebo injection was given intravenously and heart rate and blood pressure were then monitored every 5 min. After 30 min insulin Actrapid MC 0.2 IU/kg body weight was given i.v. A 20% glucose infusion was given to maintain blood glucose at its fasting level. After insulin administration, a rapid and statistically significant increase in heart rate was observed when the patients were pretreated with placebo; pretreatment with propranolol completely prevented this effect. Serum insulin levels were significantly higher than baseline at all times and there was no significant change in blood glucose. The results are consistent with the hypothesis that the insulin-induced increase in heart rate in man may result from stimulation of cardiac sympathetic activity.

Mental arithmetic stress as a test for evaluation of diabetic sympathetic autonomic neuropathy

The effects of a 1-min mental arithmetic stress test on heart rate change were studied in 72 Type 1 diabetic patients, 36 without and 36 with diabetic autonomic neuropathy (mean age 33 and 44 yr, respectively), and in 80 matched normal subjects. Variation in hand skin temperature was also recorded in 25 normal subjects and 30 diabetic patients without and 32 with autonomic neuropathy. While mental arithmetic rapidly reduced skin temperature of normal volunteers and of patients without autonomic neuropathy, no effect was found in autonomic neuropath (a drop of 0.63 +/- 0.05 (+/- SE), 0.52 +/- 0.04 and 0.16 +/- 0.02 degrees C (p less than 0.001), respectively). In control subjects and in diabetic patients without and with autonomic neuropathy the heart rate increase was 22.9 +/- 6.8 (+/- SD), 21.4 +/- 8.4 and 7.0 +/- 3.7 beats min-1, respectively (p less than 0.001). The ratio between maximum mental arithmetic-induced heart rate and basal heart rate was 1.29 +/- 0.10, 1.24 +/- 0.10 and 1.07 +/- 0.05 (p less than 0.001) for healthy subjects, non-neuropathic patients, and neuropathic patients. Cut-off values (the low normal limit for these variables) are proposed: skin temperature 0.23 degrees C, heart rate increase 11.6 beats min-1 and heart rate ratio 1.12. Anxiety state, blood glucose concentration (excluding hypoglycaemia), body position, basal heart rate, and age did not interfere with responses to mental arithmetic stress.

Apolipoprotein, low density lipoprotein subfraction, and insulin associations with familial combined hyperlipidemia. Study of Utah patients with familial dyslipidemic hypertension

Familial dyslipidemic hypertension (FDH) is a syndrome recently described from sibships selected for early familial hypertension and found to have one or more of three fasting lipid abnormalities [high triglycerides, low high density lipoprotein (HDL) cholesterol, high low density lipoprotein (LDL) cholesterol]. In further analyses of these same 131 hypertensive subjects, apolipoprotein A-I and B, fasting plasma insulin (adjusted for body mass index), and detailed anthropometrics were different in two subgroups of FDH. Of 63 FDH patients, 19 met the criteria for familial combined hyperlipidemia (FCHL); 44 did not, but still had high triglyceride and/or low HDL cholesterol levels. When compared to 20 normolipidemic hypertensive patients, the 19 hypertensive patients with FCHL had 196% higher very low density lipoprotein cholesterol (p = 0.0001), 33% higher apolipoprotein B (p = 0.0002), smaller LDL particles (p = 0.007), and 73% higher fasting insulin (p = 0.003), but no significant differences in body mass index or skinfold thicknesses. The other 44 FDH patients without FCHL had 33% lower HDL (p = 0.0001), with only 8% lower apolipoprotein A-I levels (p = 0.20); significantly higher subscapular skinfolds (p = 0.02), weights (p = 0.002), body mass index (p = 0.006), knee widths (p = 0.0007), and wrist circumferences (p = 0.0009); smaller, denser LDL subfractions (p = 0.001); and increased apolipoprotein B levels (p = 0.01) compared to the normolipidemic hypertensive group. Increased fasting insulin levels were similar to the normolipidemic group and significantly lower than the FCHL group after adjustment for body mass index, suggesting a relationship between obesity and fasting insulin levels only in the non-FCHL group. We conclude that FDH consists of at least two subgroups: 1) FCHL with high apolipoprotein B, small LDL particles, and increased fasting plasma insulin levels, and 2) a less well-defined residual having upper central obesity with low HDL cholesterol and high triglyceride levels. Elevated insulin levels found in both groups, but possibly originating through different physiological mechanisms, may provide the pathophysiological connections between dyslipidemia, obesity, and hypertension.

High blood pressure and hyperinsulinaemia in acromegaly and in obesity

As previously shown, in essential hypertension postprandial plasma insulin concentrations are elevated. In order to determine a relationship of high blood pressure and plasma insulin levels in acromegaly and in obesity 59 subjects with normal glucose tolerance were studied. They were divided into three groups: (I) patients with acromegaly: 7 normotensives and 8 hypertensives, (II) 12 obese normotensives and 12 obese hypertensives and (III) 10 non-obese hypertensives, and 10 healthy subjects. Blood glucose and plasma insulin concentrations were measured in a fasting state and after an oral glucose load of 75 g. The fasting insulin concentrations in all the acromegalics and in all the obese patients were higher than those in healthy subjects. The insulin response to the glucose load was significantly enhanced in all the three groups of hypertensive patients compared with those of matched normotensive controls. The results indicate that insulin may play a role in the regulation of blood pressure in essential hypertension, and in such hyperinsulinaemic disorders as acromegaly and obesity.

Hyperinsulinemia

Patients with mild or early non-insulin-dependent diabetes mellitus often display a delay in insulin response followed by late hyperinsulinemia during oral glucose tolerance testing. Those patients with long-standing disease or elevations of fasting plasma glucose in excess of 140 mg/dl are generally hypoinsulinemic in response to an oral glucose tolerance test. Diabetic patients who do not have an acute response to intravenous glucose may have normal responses to intravenous tolbutamide or intravenous arginine, suggesting that delayed responsiveness to glucose is not due to decreased pancreatic insulin content. An association between hyperinsulinemia and hypertension has been suggested by recent studies from several laboratories. In a homogeneous population of men who suffered traumatic bilateral above-the-knee amputation in the Vietnam War with subsequent development of obesity, it was shown that there was strong correlation between hypertension and hyperinsulinemia during oral glucose tolerance testing despite only mild glucose intolerance. In addition, a subset of hypertensive women who were in their third trimester of pregnancy were markedly hyperinsulinemic during oral glucose tolerance testing in the absence of any abnormalities of glucose tolerance. Thus, the relationship between hyperinsulinemia and hypertension, and the possible reasons for this relationship, are fields of active investigation at present.

Relation of serum calcium concentration to metabolic risk factors for cardiovascular disease

Data from a health screening survey with over 18,000 adult participants were used to determine the relations between serum calcium concentration and the cardiovascular risk factors hypertension, hyperglycaemia, and hyperlipidaemia. Blood pressure and serum glucose and cholesterol concentrations were all positively related to each other independent of age, sex, kidney function, and obesity. Similar relations between the risk factors were found in subjects with hypertension or hyperglycaemia independent of the degree of overweight. These results suggested that there might be a metabolic syndrome of cardiovascular risk factors. Serum calcium concentration was positively related to systolic and diastolic blood pressures and serum glucose and cholesterol concentrations. Thus a common feature in the syndrome is an increased serum calcium concentration. The relations between serum calcium concentrations and the cardiovascular risk factors were not limited to the upper parts of the distribution, being seen over a wide range. Changes in calcium metabolism seem to be related to a metabolic syndrome of hypertension, impaired glucose tolerance, and hyperlipidaemia.

An association between hyperinsulinemia and hypertension during the third trimester of pregnancy

We studied 43 women in their third trimester of pregnancy whose fetuses were at significant risk of intrauterine growth retardation. To define metabolic subtypes for intrauterine growth retardation, a 100 gm glucose load was administered after an overnight fast. Twenty-seven women were normotensive and 16 had hypertension. The glucose tolerance of the hypertensive group was essentially the same as that of the normotensive group. However, 8 of the 16 women with hypertension had marked hyperinsulinemia in response to an oral glucose load. Of the five women with hypertension who gave birth to offspring of low birth weight, three had hyperinsulinemia.

Relationship between blood pressure and plasma insulin in non-obese and obese non-diabetic subjects

In this study, we have measured plasma insulin at fasting and following an oral glucose load and blood pressure after glucose load in 367 (247 non-obese, 120 obese) normotensive and untreated mildly hypertensive subjects. Overall, there was no independent association between fasting plasma insulin levels and blood pressure values. After controlling for age and body weight, a significant relationship between postglucose plasma insulin levels and diastolic blood pressure was found. When non-obese and obese subjects were examined separately, significant relationships were identified between postglucose plasma insulin levels and both systolic and diastolic blood pressure values in the former but not in the latter. A comparison of sex-, age-, and weight-matched hyperinsulinaemic vs normoinsulinaemic subjects showed that the former had significantly higher values of blood pressure only if not obese. These results demonstrate that the plasma insulin response to glucose is independently correlated with blood pressure.

Reduced intraindividual variability of repeated cardiovascular reflex tests: an additional marker of autonomic neuropathy in insulin-dependent (type I) diabetes mellitus?

The particular questions asked in our study were: 1. does the individual reproducibility of the cardiovascular reflex tests differ between healthy controls and patients suffering from type I diabetes mellitus and 2. if there is a difference, do the different cardiovascular reflexes vary in this regard? Nine healthy controls (4 women, 5 men, age 31 +/- 2.1 years) and 11 type I diabetics (4 women, 7 men, age 30.9 +/- 5.6 years, duration of diabetes 3.23 years) underwent the following tests 6 times in a 12-h period (07:00 to 19:00): variation of heart rate during deep breathing (E/I ratio), variation of heart rate during lying and standing (tachycardia/bradycardia or 30/15 ratio), Valsalva maneuver (Valsalva ratio), response of diastolic blood pressure to sustained hand grip, and response of systolic blood pressure to posture. The test results did not indicate a diurnal fluctuation nor were they systematically influenced by antecedent insulin injections or meals, either in diabetic patients or in healthy controls. The 11 diabetics had significantly lower intraindividual variations of E/I and Valsalva ratios than the controls (p less than 0.05, p less than 0.001, respectively). In the diabetics with parasympathetic failure the intraindividual variabilities of all cardiovascular reflex responses were lower than those of the patients with an intact autonomic nervous system as well as those of the control subjects. On the contrary, in the diabetic patients without autonomic neuropathy, only the intraindividual variability of the Valsalva maneuver was significantly attenuated (p less than 0.025), compared with the healthy volunteers. To conclude, the more pathological the single test result, the greater is its reproducibility and its clinical significance.

Reproducibility of cardiovascular autonomic tests in diabetics with and without autonomic dysfunction and in normal controls

The present study evaluates the reproducibility of five cardiovascular reflex tests, deep breathing (DB), Valsalva maneuver (VM), sustained hand-grip (SHG), postural hypotension (PH) and lying to standing (LS) in normal subjects and in insulin dependent (type I) diabetic patients. The study was carried out in 10 normal subjects, in 10 diabetics with autonomic neuropathy and in 10 diabetics without autonomic neuropathy. The five cardiovascular reflex tests were performed five times on five consecutive days by the same investigator and in identical basal conditions. The intraindividual variability of DB, LS and VM was significantly reduced in diabetics with autonomic neuropathy compared with normal controls, but there was no difference between diabetics without neuropathy and normal controls. The intraindividual variability of PH was significantly increased in diabetics with autonomic neuropathy compared with diabetics without autonomic neuropathy and with normal controls. There was no difference among the three groups in the reproducibility of SHG. In normal subjects the intraindividual variability only exceptionally produced a shift from normal to abnormal values or vice versa; in diabetics with autonomic neuropathy this shift was more frequent.

The alpha cell response to glucose change during perfusion of anti-insulin serum in pancreas isolated from normal rats

To determine the effect of neutralization of endogenous insulin upon the glucagon response to a rise and fall of glucose concentration, pancreata isolated from normal rats were perfused with either a potent anti-pork insulin guinea pig serum or a nonimmune guinea pig serum for 30 min. During this period glucose concentration was changed from 100 mg/dl to either 130, 180 or 80 mg/dl for 10 min. Antiserum perfusion at 100 mg/dl caused an approximately two-fold increase in glucagon which was not suppressed by an increase in glucose concentration to either 130 or 180 mg/dl, although glucagon secretion was significantly suppressed in the control experiments in which nonimmune serum was perfused. However, the 0.38 +/- 0.21 ng/min rise in glucagon secretion in response to a reduction in glucose concentration to 80 mg/dl in the control experiments was not abolished by antiserum perfusion but, instead, was enhanced (2.66 +/- 0.60 ng/min). These findings suggest that insulin may be required for glucose-mediated suppression of glucagon in the isolated pancreas of normal rats but not for stimulation of glucagon secretion by mild glucopenia. Alternatively, neutralization of insulin-mediated release-inhibition of glucagon secretion may simply have altered alpha cell responsiveness in a direction that desensitized it nonspecifically to suppression and sensitized it to stimulation.

Reduced noradrenaline turnover in streptozotocin-induced diabetic rats

To clarify whether activity of the sympathetic nervous system is decreased in streptozotocin-induced diabetic rats, noradrenaline turnover, which is a reliable indicator of sympathetic nervous system activity, was measured in the interscapular brown adipose tissue, heart and pancreas of streptozotocin diabetic rats. Results from studies using inhibition of noradrenaline biosynthesis with alpha-methyl-p-tyrosine demonstrated significant reductions (p less than 0.05-0.001) in sympathetic nervous system activity in the interscapular brown adipose tissue, heart and pancreas of streptozotocin (65 mg/kg) diabetic rats, compared with measurements in streptozotocin (35 mg/kg) diabetic and saline-control rats. The daily injections of neutral protamine Hagedorn insulin to streptozotocin (65 mg/kg) diabetic rats prevented the decrease of noradrenaline turnover in the interscapular brown adipose tissue and heart significantly (p less than 0.02), but this was less marked in pancreas, compared with non-treated streptozotocin (65 mg/kg) diabetic rats. Furthermore reduced noradrenaline turnover was also observed in the control rats which showed comparable changes in body weight to the rats injected with streptozotocin (65 mg/kg). These results suggest that poorly controlled streptozotocin diabetic rats may have reduced sympathetic nervous function, and that insulin therapy might prevent this.

Hemodynamic and humoral effects of caffeine in autonomic failure. Therapeutic implications for postprandial hypotension

We examined the effects of caffeine and meals on blood pressure and heart rate in 12 patients with autonomic failure. The influence of caffeine on plasma norepinephrine, epinephrine, and renin activity was also studied. Caffeine 250 mg, raised blood pressure by 12/6 mm Hg, from 129 +/- 25/78 +/- 12 (mean +/- S.D.) to a maximum of 141 +/- 30/84 +/- 16 mm Hg at 45 minutes (P less than 0.01), but did not change heart rate, levels of norepinephrine, or epinephrine, or plasma renin activity. Blood pressure fell by 28/18 mm Hg after a standardized meal, from 133 +/- 32/80 +/- 15 to a minimum of 105 +/- 21/62 +/- 12 mm Hg at 60 minutes (P less than 0.01). After pretreatment with 250 mg of caffeine, the standardized meal induced a fall of only 11/10 mm Hg, from 140 +/- 33/79 +/- 7 to 129 +/- 31/69 +/- 13 mm Hg at 60 minutes (P less than 0.05 vs. values after the control per day for seven days) in five patients, postprandial blood pressures remained higher after caffeine than after placebo (P less than 0.05). We conclude that caffeine is a pressor agent and attenuates postprandial hypotension in autonomic failure, and that this effect is not primarily due to elevations in sympathoadrenal activity or activation of the renin-angiotensin system. Caffeine may be useful in the treatment of orthostatic hypotension due to autonomic failure, especially in the postprandial state.

Pathophysiological interrelations of obesity, impaired glucose tolerance, and arterial hypertension

There is a large amount of epidemiological and clinical evidence for associations among obesity, impaired glucose tolerance, and arterial hypertension; nevertheless, the pathophysiological mechanisms underlying these associations have not yet been elucidated. In this article, some working hypotheses are discussed, and original data are presented from two studies focusing on these pathophysiological interrelations. A case-control study of obese normotensive and hypertensive patients, matched for sex, age, and degree of overweight, has shown that obese patients with associated arterial hypertension have higher fasting serum insulin levels and reduced glucose tolerance compared with their normotensive peers. A second study compared subjects with impaired glucose tolerance with a control group of clinically healthy individuals of comparable sex, age, and body mass index, and it revealed that impaired glucose tolerance is associated with significantly higher blood pressure levels, independent of body weight. The results of the two studies together suggest that the association between hypertension and impaired glucose tolerance is independent of overweight; they also give some support to the hypothesis that hyperinsulinemia may contribute to the development of high blood pressure in obese patients.

Postprandial hyperinsulinemia in patients with mild essential hypertension

Glucose tolerance tests and diurnal profiles of glucose, insulin, free fatty acids, serum triglycerides, total and high-density lipoprotein cholesterol levels were performed in 8 male patients with mild essential hypertension as well as in 20 normotensive subjects. Although glucose tolerance and postprandial glucose levels appeared equal in both groups, the insulin response after a glucose load and after each meal was significantly increased in hypertensive subjects as compared with the controls (p less than 0.01). The levels of free fatty acids were higher in the postabsorptive phase of patients with hypertension in comparison to normotensive subjects, but decreased markedly when plasma insulin levels rose after meals. In both subject groups serum triglyceride levels showed the typical postprandial increase. Total and high-density lipoprotein cholesterol levels showed neither diurnal variations nor differences between hypertensive subjects and normotensive controls. Postprandial hyperinsulinemia in patients with mild essential hypertension possibly may provoke lipid accumulation in the arterial wall and therefore may be a relevant risk factor for atherosclerosis in these subjects.

Hyperinsulinemia. A link between hypertension obesity and glucose intolerance

Hypertension and glucose intolerance, determined in a random population sample (n = 2,475), showed a highly significant (P less than 0.001) association from the mildest levels of both conditions, independent of the confounding effects of age, sex, obesity, and antihypertensive medications. Summary rate ratios for hypertension were 1.48 (1.18-1.87) in abnormal tolerance and 2.26 (1.69-2.84) in diabetes compared with normal tolerance. Altogether, 83.4% of the hypertensives were either glucose-intolerant or obese--both established insulin-resistant conditions. Fasting and post-load insulin levels in a representative subgroup (n = 1,241) were significantly elevated in hypertension independent of obesity, glucose intolerance, age, and antihypertensive medications. The mean increment in summed 1- and 2-h insulin levels (milliunits per liter) compared with nonobese normotensives with normal tolerance was 12 for hypertension alone, 47 for obesity alone, 52 for abnormal tolerance alone, and 124 when all three conditions were present. The prevalence of concentrations (milliequivalents per liter) of erythrocyte Na+ greater than or equal to 7.0, K+ less than 92.5, and plasma K+ greater than or equal to 4.5 in a subsample of 59 individuals with all combinations of abnormal tolerance obesity and hypertension was compared with those in 30 individuals free of these conditions. Altogether, 88.1% of the former vs. 40.0% of the latter group presented at least one of these three markers of internal cation imbalance (P less than 0.001). We conclude that insulin resistance and/or hyperinsulinemia (a) are present in the majority of hypertensives, (b) constitute a common pathophysiologic feature of obesity, glucose intolerance, and hypertension, possibly explaining their ubiquitous association, and (c) may be linked to the increased peripheral vascular resistance of hypertension, which is putatively related to elevated intracellular sodium concentration.