Levels of GH binding activity, IGFBP-1, insulin, blood glucose and cortisol in intensive care patients

The endocrine response to critical illness

The endocrine response to stress is complex. Elevations in the serum concentrations of the "classic" stress hormones, epinephrine and cortisol, occur following many kinds of physiologic challenge and are accompanied by elevations in corticotropin, GH, and glucagon levels. These changes are probably responsible for the hyperglycemia and hypercatabolism common to most critical illness. If volume depletion is present, vasopressin, renin, and aldosterone secretion are also likely to be stimulated. These hormones, if present in excess, may produce fluid retention and hyponatremia. In some critically ill patients, there is a dissociation of renin and aldosterone production called hyperreninemic hypoaldosteronism, but the clinical importance of this syndrome is poorly understood. Thyroid hormone metabolism is commonly affected by critical illness, which results in characteristic abnormalities of thyroid function testing known as the euthyroid sick syndrome. The reproductive axis is exquisitely sensitive to physiologic stress; hypogonadotropic hypogonadism is a common finding in critical illness. The ongoing challenge to the clinician is to determine whether seemingly abnormal hormone measurements in critically ill patients reflect an appropriate homeostatic response to severe illness or, instead, whether they denote an independent metabolic disorder that might actually cause or contribute to the patient's unstable condition. In view of the exceedingly complex (and poorly understood) interactions involved in the human response to a severe illness, a thoughtful approach to the whole patient is essential and far preferable to indiscriminate hormone testing. Such testing, at best, may be uninterpretable in light of the clinical circumstances or, at worst, may lead to therapeutic misadventures.

An audit of the insulin tolerance test in adult subjects in an acute investigation unit over one year

OBJECTIVE

We audited our practice of insulin tolerance testing (ITT) in terms of safety and technical success. We reviewed the results of those tests performed over a 12-month period. By relating peak cortisol response to 0900 h screening cortisol level, we determined whether we could reduce the number of tests performed.

DESIGN

The results of all ITTs performed on our unit between 1 January and 31 December 1991 were reviewed.

PATIENTS AND MEASUREMENTS

Patients were prescreened by measurement of serum cortisol and thyroxine, and recording of an electrocardiogram. A subnormal serum thyroxine (< 58 nmol/l), 0900 h serum cortisol (< 100 nmol/l) or an abnormal ECG were taken as contraindications to the test. Minimum glucose and maximum cortisol levels were recorded, along with peak GH responses when measured. The peak cortisol response was compared to the screening and basal cortisol levels.

RESULTS

A total of 161 tests were performed, 135 of which fulfilled our inclusion criteria. The test was technically successful in all but 5 of these; a significant adverse event occurred in one patient with full recovery after reversal of hypoglycaemia. Thirty-two patients had a suboptimal serum cortisol response to hypoglycaemia: screening cortisol level ranged from 128 to 493 nmol/l and 0900 h serum cortisol measured prior to the ITT from 97 to 431 nmol/l. The lowest level of screening cortisol above which all patients would be expected to achieve the normal peak cortisol of 580 nmol/l or over is therefore 494 nmol/l. If this cut-off level had been adopted, 10 (8%) ITTs need not have been performed if their only purpose had been to assess cortisol reserve. Altering the criterion for the necessary peak cortisol to 500 nmol/l did not affect the number of ITTs required. Our lower limit for testing could not be revised upwards from 100 nmol/l. Adequacy of cortisol reserve did not predict a normal GH response to insulin-induced hypoglycaemia.

CONCLUSIONS

When performed in an experienced endocrine unit with adequate supervision, the insulin tolerance test is a safe procedure. According to the current sample, fewer tests would be performed without detriment to patient care if those with a screening cortisol of greater than 500 nmol/l did not proceed to testing, unless the purpose of the test was also to exclude GH deficiency. A lower limit of 100 nmol/l appears reasonable and need not be revised upwards.

The regulation of insulin-like growth factor binding protein (IGFBP)-1 during prolonged fasting

OBJECTIVE

Insulin-like growth factor binding protein (IGFBP)-1 levels increase overnight, being inversely related to changes in insulin. With prolonged fasting IGFBP-1 levels increase further. In animal studies high IGFBP-1 levels increase plasma glucose levels possibly by regulating the insulin-like actions of 'bio-available' plasma IGF. Following prolonged fasting, there is an increase in insulin requirement. A proportion of this reversible insulin resistance may be due to inhibitory effects of high IGFBP-1 levels on IGF action. This study examined the regulation of IGFBP-1 in the presence of reversible insulin resistance.

SUBJECTS

Nine normal adult volunteers, seven female and two male (mean age 27.6 +/- SD 2.6 years, range 21.7-46.0 years) of normal body mass index were studied.

METHODS

Subjects fasted from 2200 h day 0 to 0900 h day 3 (59 hours), the fast being completed with a 75-g glucose meal. At least one week later, an 11-hour overnight fast was performed, followed by a repeat glucose meal. Blood samples were taken at regular intervals from 0900 h day 1 and for 5 hours during both glucose meal studies via an indwelling cannula.

MEASUREMENTS

Serum levels of IGFBP-1, insulin, GH, glucose, IGF-I and cortisol were measured at varying intervals during the fast and both glucose meal studies.

RESULTS

Following the initial 11-hour overnight fast IGFBP-1 levels rose from (mean +/- SEM) 32 +/- 5 micrograms/l to reach a maximum of 144 +/- 24 micrograms/l after 32 hours of fasting. IGFBP-1 levels then fluctuated, falling in the morning (93 +/- 8 micrograms/l) and then rising overnight (126 +/- 9 micrograms/l), but not regaining the initial peak levels. The increase of IGFBP-1 from overnight fasting levels was associated with a fall in plasma insulin from 5.7 +/- 0.7 to 2.2 +/- 0.2 mU/l. In comparison, 30 minutes after termination of the fast with the glucose meal, IGFBP-1 levels fell from 120 +/- 11 to 24 +/- 2 micrograms/l within 4 hours. After an overnight fast IGFBP-1 levels fell from 35 +/- 5 to 13 +/- 2 micrograms/l within 3 hours. There was glucose intolerance and increased insulin levels following the glucose meal preceded by the 59-hour fast when compared with the overnight fast. The fall of IGFBP-1 levels after the glucose meal was best expressed, taking into account subject variation, by the following regression equations: Glucose meal preceded by 11-hour fast: log [IGFBP-1] = 1.64-0.255 log [1 h previous insulin] (R2 0.51); Glucose meal preceded by 59-hour fast: log [IGFBP-1] = 1.41-0.265 log [1 h previous insulin] + 0.557 log [current glucose] (R2 0.82).

CONCLUSION

In man, insulin appears to regulate circulating IGFBP-1 levels in all circumstances, this regulation being unaffected by the resistance to insulin action induced by prolonged fasting. The high IGFBP-1 levels were statistically related to the higher glucose levels and may have directly contributed to the increased insulin requirement observed after prolonged fasting.

Administration of human recombinant insulin-like growth factor-I to patients following major gastrointestinal surgery

OBJECTIVE

The aim was to study the pharmacokinetic parameters and biological activity of a single dose of human recombinant IGF-I (rhIGF-I) administered to patients following major gastrointestinal surgery.

DESIGN

A double blind placebo controlled externally randomized study of 30 patients; the study commencing 24 hours after major colonic or gastric surgery.

MEASUREMENTS

After a baseline blood sampling day, IGF-I (40 micrograms/kg by single subcutaneous dose, n = 20) or placebo (n = 10) was administered and serum and urine samples collected over the ensuing 72 hours. Serum IGF-I, IGF-II, IGF binding proteins (IGFBP-1, IGFBP-3), GH and insulin were measured by radioimmunoassay. Serum IGF bioactivity was assessed using a validated porcine cartilage bioassay. Serum and urinary electrolytes were measured by standard methodology.

RESULTS

Serum immunoreactive IGF-I levels peaked at 4 hours following injection of IGF-I (1.09 +/- 0.12 U/ml mean +/- SEM), remained elevated for 15 hours and returned to basal levels by 24 hours after injection. IGF bioactivity was increased by 57% 6 hours after IGF-I injection. Mean levels of IGFBP-1 and IGFBP-3, IGF-II and GH were unaffected by IGF-I administration. Insulin levels were suppressed at 30 minutes following injection of IGF-I compared with the placebo group (16.9 +/- 3.0 mU/I vs 32.3 +/- 7.1, P = 0.02); thereafter, there were no differences in insulin levels. The mean change in serum creatinine following IGF-I (-6.3 +/- 3.0 mmol/l) was significantly different from that in the control group (+7.2 +/- 6.2, P = 0.03). Creatinine clearance rose from a mean of 71.6 +/- 7.5 ml/min to 83.2 +/- 7.6 ml/min after IGF-I treatment (P = 0.02). In the IGF treated patients, cholesterol levels consistently fell (-0.20 +/- 0.05 mmol/l); this was not observed in the placebo group (+0.20 +/- 0.14, P = 0.006). Basal serum potassium levels in the IGF treatment group (4.1 +/- 0.1 mmol/l) fell to 3.8 +/- 0.1 at 4 hours (P = 0.002) and 3.6 +/- 0.1 at 10 hours (P = 0.001) returning to a level of 4.0 +/- 0.1 (P = 0.293) at 24 hours after injection. There were no other observed differences in serum or urinary electrolytes or serum free fatty acids and triglycerides. Pharmacokinetic parameters derived from baseline adjusted IGF-I measurements revealed a slow absorption of the administered dose with a Tmax of 5.0 +/- 0.43 hours and an elimination half-life of 10.8 +/- 1.2 hours. The computed volume of distribution was 0.33 +/- 0.05 I/kg and the clearance on average 25 ml/min.

CONCLUSION

A single subcutaneous dose of IGF-I normalized circulating IGF-I levels in post-operative patients, was well tolerated and without side-effects. IGF bioactivity was increased and associated with a fall in serum cholesterol, potassium and creatinine levels and a rise in creatinine clearance. Further long-term studies are now required to assess the anabolic effects of rhIGF-I in this type of patient group.