[Effects of glucose and arginine on the secretion of insulin in Cushing's syndrome]

Glucose Metabolism Abnormalities in Cushing Syndrome: From Molecular Basis to Clinical Management

An impaired glucose metabolism, which often leads to the onset of diabetes mellitus (DM), is a common complication of chronic exposure to exogenous and endogenous glucocorticoid (GC) excess and plays an important part in contributing to morbidity and mortality in patients with Cushing syndrome (CS). This article reviews the pathogenesis, epidemiology, diagnosis, and management of changes in glucose metabolism associated with hypercortisolism, addressing both the pathophysiological aspects and the clinical and therapeutic implications. Chronic hypercortisolism may have pleiotropic effects on all major peripheral tissues governing glucose homeostasis. Adding further complexity, both genomic and nongenomic mechanisms are directly induced by GCs in a context-specific and cell-/organ-dependent manner. In this paper, the discussion focuses on established and potential pathologic molecular mechanisms that are induced by chronically excessive circulating levels of GCs and affect glucose homeostasis in various tissues. The management of patients with CS and DM includes treating their hyperglycemia and correcting their GC excess. The effects on glycemic control of various medical therapies for CS are reviewed in this paper. The association between DM and subclinical CS and the role of screening for CS in diabetic patients are also discussed.

Study of glucose tolerance in consecutive patients harbouring incidental adrenal tumours. Study Group of Incidental Adrenal Adenoma

OBJECTIVE

Clinically silent cortisol hypersecretion has been frequently observed in recent series of adrenal 'incidentalomas'. A significant body of data indicates that a spectrum of cortisol excess exists. Up to 90% of patients with cortisol hypersecretion are glucose intolerant. The aim of this study was (1) to assess glucose tolerance in consecutive patients with 'nonfunctioning' adrenal adenomas, and (2) to study the influence of the status of the hypothalamo-pituitary-adrenal axis (HPAA) on carbohydrate tolerance.

PATIENTS AND METHODS

Sixty-four consecutive patients with nonfunctioning adrenal adenomas (non-hypersecretory) diagnosed between September 1995 and July 1996 in five hospitals were included in the study. The prevalence of glucose intolerance or diabetes mellitus was determined with an oral glucose tolerance test (oGTT) according to the recommendations of the National Diabetes Data Group. Twenty-three consecutive unselected patients diagnosed with 'nonfunctioning' adrenal adenoma were enrolled in a study to determine the influence of HPAA status on carbohydrate tolerance. In these patients, in addition to basal and post-dexamethasone serum and urinary free cortisol concentrations, assessment of peripheral sensitivity to insulin (estimated during the oGTT according to a previously validated method) and adrenal sensitivity to ACTH (calculated from the CRH test) were performed.

RESULTS

Twenty-five patients were considered to have normal glucose tolerance (95% confidence interval (95% CI) for the mean of 2 h glucose, 5.5-6.5 mmol/l); 17 showed glucose intolerance (95% CI 8.5-9.3 mmol/l); and 22 were classified as having diabetes mellitus (95% CI 12.1-14.6 mmol/l), including six patients with previously known diabetes mellitus. These three groups were comparable in age, sex, body mass index, waist-hip ratio and concomitant diseases. Thus, the prevalence of disturbed glucose tolerance was 39/64 (61%), well above the prevalence of NIDDM described in a population of similar age in our area. Among the 23 patients included in the study of HPAA status, the size of the tumour correlated with serum cortisol after dexamethasone (DXM) (r = 0.52, P < 0.01) and 24-h urinary free cortisol (UFC) after DXM (r = 0.55, P < 0.01), and negatively with DHEAs (r = -0.42, P = 0.04). The area under the curve for ACTH after hCRH (AUCacth) correlated negatively with both UFC (r = -0.40, P = 0.04) and serum cortisol after dexamethasone (r = -0.47, P = 0.02). The degree of peripheral sensitivity to insulin (SI) positively correlated with adrenal sensitivity (r = 0.56, P = 0.005).

CONCLUSIONS

A high prevalence (61%) of disturbed glucose tolerance was found among consecutive patients harbouring incidental 'nonfunctional' adrenal adenomas. Therefore, patients with incidental adrenal tumours should be tested for glucose tolerance. The positive correlation between insulin sensitivity and adrenal sensitivity to ACTH suggests that, in these patients, insulin resistance hampers ACTH action at the level of the adrenal.

Effect of glucocorticoid replacement therapy on glucose tolerance and intermediary metabolites in hypopituitary adults

BACKGROUND AND OBJECTIVES

Excess impaired glucose tolerance and diabetes mellitus have been reported in hypopituitary adults on conventional replacement therapy including glucocorticoids. We investigated the effect of the glucocorticoid component on glucose tolerance and intermediary metabolites in hypopituitary adults.

DESIGN

A 3-hour 75-g oral glucose tolerance test (OGTT) was performed on two study days, at least one week apart. On one study day, the glucocorticoid replacement morning dose was taken 60 minutes before the OGTT, and on the other it was left until after the OGTT. All other pituitary replacement therapies were kept unchanged on the two study days.

PATIENTS

Eight hypopituitary adults (3 males and 5 females; aged 46-76 years) on conventional replacement therapy were studied. Their duration of hypopituitarism was mean (range) 15 (5-31) years. Their mean body mass index (BMI) was 28.4 (24.1-35.1) kg/m2. Their total daily cortisol dose was 26 (15-30) mg.

MEASUREMENTS

Plasma glucose, insulin, non-esterified fatty acids (NEFA), glycerol and 3-hydroxybutyrate were measured at 30-minute intervals and plasma cortisol levels were measured hourly.

RESULTS

Fasting glucose and insulin concentrations were similar on the glucocorticoid day (GD) and the non-glucocorticoid day (NGD) (glucose (mean +/- SD) 4.9 +/- 0.9 vs 4.4 +/- 0.5 mmol/l; insulin (median (range)) 5 (1-17) vs 2 (1-15) mU/l, respectively). Post-glucose glycaemia was higher on the GD than on the NGD with a significantly higher glucose area under the curve (AUC) (45.0 +/- 8.2 vs 38.9 +/- 11.7 mmol/l h, P < 0.05). Post-glucose insulinaemia was also higher on the GD than on the NGD with significantly higher insulin AUC (270 (47-909) vs 207 (46-687) mU/l h, P < 0.02). Impaired glucose tolerance was found in three patients on the GD, one of whom continued to have impaired glucose tolerance on the NGD. The areas under the curves of NEFA, glycerol and 3-hydroxybutyrate were not significantly different on the two days. On the NGD, plasma cortisol levels were undetectable (< 50 nmol/l) in all patients and on the GD the median (range) peak was 500 (330-740) nmol/l dropping to 125 (60-330) nmol/l at 180 minutes. The difference in glucose AUC between the two days correlated with the maximal plasma cortisol levels (Spearman's p = 0.83, P < 0.01).

CONCLUSIONS

Glucocorticoid replacement therapy taken pre-prandially in hypopituitary adults induces mild elevations in circulating glucose and insulin levels even with acceptable plasma cortisol concentrations. Optimal regimens for glucocorticoid replacement require more study.

Diabetes and adrenal disease

Disorders of the adrenal cortex and medulla can result in glucose intolerance or overt diabetes mellitus. Cushing's syndrome, characterized by excessive secretion of glucocorticoids, impairs glucose tolerance primarily by causing insulin resistance at the post-receptor level. On the other hand, phaeochromocytoma and hyperaldosteronism, via the respective actions of catecholamines and hypokalaemia on the pancreatic beta-cell, impair glucose tolerance primarily by inhibiting insulin release. The glucose intolerance associated with these adrenal disorders is usually only mild to moderate in severity. Marked hyperglycaemia, glycosuria, and polyuria are uncommon and ketosis is rare. Moreover, the late complications of diabetes mellitus are distinctly uncommon in patients with these disorders, and the prognosis for morbidity and death is usually that of the underlying disease and not that of diabetes mellitus. The impaired glucose tolerance induced by all three of these adrenal disorders usually returns to normal once the underlying aetiology has been cured. These factors must guide the clinician in treatment of these secondary forms of diabetes, and suggest that tight (near normal) blood glucose control may not be an appropriate goal in patients with these disorders. The relationship between adrenal androgens and glucose tolerance is more uncertain. Several studies in humans have demonstrated an acute decline in serum concentrations of the adrenal steroids DHEA and DHEA-sulfate in response to experimentally-induced hyperinsulinaemia, but the regulatory role of insulin on adrenal androgen metabolism in normal physiology or disease remains speculative. In several animal models DHEA appears to exert potent anti-obesity and anti-diabetogenic actions, but such effects have yet to be demonstrated in humans. Human studies of DHEA are limited, and more research needs to be conducted to determine whether the observations made in animal models will prove applicable to man.

Hyperalimentation as cause of markedly worsened metabolic control in a patient with autoantibodies to the insulin receptor

Patients with type B insulin resistance and acanthosis nigricans have autoantibodies to their insulin receptors and usually have signs and symptoms of other autoimmune diseases. The first case demonstrating that hyperalimentation markedly disturbs blood glucose control in type B insulin-resistant patients is described. Neither prednisone, insulin (up to 240 units per hour), nor tolbutamide appeared to help this patient's metabolic control. After the addition of cyclophosphamide for one week, the anti-insulin receptor autoantibody titer dropped from greater than 1:1,000 to 1:1. Six months later, the patient had a complete remission, which is rare, with only three other reported remissions in these patients with type B insulin resistance.

Hormonal and metabolic rhythms in Cushing's syndrome

Hormone and metabolite profiles were investigated over a 12-hr period in six patients with Cushing's syndrome, ten age- and sex-matched normal controls, and six moderately obese subjects matched for weight with the patient group. Mean diurnal plasma cortisol levels were 563 +/- 74 nmole/liter in the patients, 275 +/- 22 nmole/liter in normal controls and 241 +/- 32 nmole/liter in obese subjects, with total loss of diurnal changes in Cushing's syndrome. Fasting blood glucose concentration was similar in all groups although mild hyperglycemia occurred after meals in the Cushing's patients compared with normal and obese subjects (mean 12-hr blood glucose: Cushing's 6.31 +/- 0.39 mmole/liter; normal controls, 5.32 +/- 0.14 mmole/liter, p < 0.01; obese subjects, 5.41 +/- 0.18 mmole/liter, p < 0.05) despite marked hyperinsulinemia (mean 12-hr serum insulin: Cushing's 57.3 +/- 18.2 mU/liter; normal controls, 19.7 +/- 2.5 mU/liter, p < 0.02; obese subjects, 18.1 +/- 4.0 mU/liter, p < 0.05). Concentrations of the gluconeogenic precursors lactate, pyruvate, and alanine were raised in Cushing's syndrome, particularly postprandially. Plasma nonesterified fatty acids (NEFA), blood glycerol, and blood ketone body concentrations were comparable in all three groups although the normal diurnal variation in circulating NEFA and ketone body levels was lost in Cushing's syndrome. Serum triglyceride (TG) concentrations were grossly elevated in the Cushing's patients (mean 12-hr serum TG: Cushing's 3.51 +/- 1.23 mmole/liter; normal controls 0.89 +/- 0.19 mmole/liter, p < 0.02; obese subjects, 0.93 +/- 0.23 mmole/liter, p < 0.05) and correlated positively with serum insulin levels. Plasma glucagon concentrations were raised in Cushing's syndrome (mean 12-hr plasma glucagon: Cushing's 23.2 +/- 3.7 pmole/liter; normal controls 12.3 +/- 1.5 pmole/liter p < 0.01; obese subjects 12.2 +/0 2.0 pmole/liter, p < 0.02) and correlated positively with the serum cortisol but not with blood alanine, suggesting that some stimulatory factor other than alanine was responsible. The metabolic effects of chronic glucocorticoid excess thus may not be explained on the basis of obesity alone. Compensatory hyperinsulinemia limits the disturbance of carbohydrate and lipid metabolism in Cushing's syndrome but may be important in production of the hypertriglyceridemia observed.