Effects of the Infusion of Insulin-like Growth Factor I in a Child with Growth Hormone Insensitivity Syndrome (Laron Dwarfism)

The patient was the second of dizygotic twin boys born to a Polish couple after an uneventful 37-week pregnancy. The patient's birth weight was 2.75 kg (35th percentile), and apart from cryptorchidism, for which he underwent bilateral orchiopexy at the age of four years, physical examination at birth was normal. The birth weight of the patient's twin was 2.8 kg, and his subsequent growth and development were normal. There were no other siblings. The father's height was 172 cm, and the mother's 159 cm. There was no family history of growth or developmental disorders and no Jewish ancestry or consanguinity.

Growth failure became evident in the patient after the age of six months. His basal serum growth hormone concentrations were high on two occasions (10 and 10.5 μg per liter) and increased to 40 μg per liter after insulin-induced hypoglycemia at the age of 3.5 years and to 67.2 μg per liter in response to the administration of clonidine at the age of 6 years. A six-month trial of growth hormone therapy at that time (0.3 and then 0.5 U per kilogram of body weight per week for two consecutive three-month periods) had no effect on the rate of increase in height (4.4 cm per year before treatment vs. 3.2 cm per year during treatment). Serum IGF-I concentrations before and after four daily subcutaneous injections of growth hormone (0.3 U per kilogram each day) were 6 and 5 μg per liter, respectively (normal range for children 6 to 8.9 years old, 88 to 474).

At 8.9 years, when he was enrolled in this study, the patient's height was 105.8 cm (50th percentile for the age of 4.5 years), and his weight was 18.8 kg (50th percentile for 5.2 years). The ratio of his upper to his lower segment (1.03) was normal for age. He had a cherubic face, prominent forehead, depressed nasal bridge, small mandible, mild truncal obesity, ripply subcutaneous fat, and poor muscle bulk, all features typical of growth hormone insensitivity syndrome. Roentgenograms of his sella turcica were normal, and his bone age was 5.5 years. High-performance liquid chromatography of serum growth hormone—binding activity¹⁵ in the presence and absence of excess unlabeled growth hormone revealed no growth hormone—binding protein in the serum of the patient, whereas it was present in the serum of his twin brother.

Figure 1. Figure 1. Fasting Serum Concentrations and 24-Hour Urinary Excretions of Urea Nitrogen, Calcium, Phosphate, and Sodium; Serum IGF-I Concentrations; and the Dose of IGF-I Infused in a Child with Growth Hormone Insensitivity Syndrome before, during, and after the Administration of Growth Hormone or IGF-I.

The base-line period was days 1 to 6; the period of growth hormone administration (arrows), days 7 to 13; the period of IGF-I infusion (shaded area), days to 15 to 25; and the period after IGF-I infusion, days 26 to 32. Serum values are indicated by connected dots, and urine values by bars. Significant changes were observed during the IGF-I infusion as compared with base line in the serum concentration and urinary excretion of urea nitrogen (P<0.001 and <0.005, respectively) and in urinary calcium (P<0.001), phosphate (P<0.01), and sodium (P<0.05) excretion. After the IGF-I infusion, urinary urea nitrogen excretion remained lower than base-line values (P<0.005), and urinary calcium excretion dropped below base-line values (P<0.001 for the comparison with the infusion period and P<0.005 for the comparison with base line). GH denotes growth hormone.

The experimental protocol was approved by the University of North Carolina Committee for the Protection of the Rights of Human Subjects, and informed consent was obtained from the parents. During five weeks of hospitalization in the General Clinical Research Center the patient was offered a diet of 6275 kj (1500 kcal) per day (335 kj[80 kcal] per kilogram per day) based on his usual diet, and his actual daily intake was calculated according to the weight of the food on the returned meal trays. The patient was observed without treatment for six days, and he then received subcutaneous injections of somatrem (Protropin recombinant growth hormone, Genentech, S. San Francisco) on days 7, 9, and 11 (0.1 mg per kilogram per dose). On days 15 through 25 he received a continuous intravenous infusion of recombinant IGF-I (Genentech) diluted in physiologic saline. The initial dose was 2 μg per kilogram per hour; the infusion rate was gradually increased to a dose of 16 μg per kilogram per hour (achieved by the second day), and the maximal rate of 24 μg per kilogram per hour was reached on the fourth day (Fig. 1). From day 4 on, IGF-I was infused at the rate of 16 μg per kilogram per hour. Observation continued for one week after completion of the infusion.

Approximately every other day, blood samples were collected after an overnight fast for the measurement of serum glucose, sodium, potassium, urea, creatinine, calcium, phosphorus, insulin, C peptide, growth hormone, IGF-I, and insulin-like growth factor II (IGF-II). Serum osteocalcin was measured at the end of the week before treatment and on day 11 of the IGF-I infusion. During the infusion of IGF-I, blood glucose was measured in samples collected by finger stick (Chemstrip BG, Accu-Check II, Boehringer–Mannheim, Indianapolis) before each meal and snack, and urine was tested for ketones and glucose (Ketodiastix, Ames, Miles Laboratories, Elkhart, Ind.). On the last day of the IGF-I infusion (day 11 ) and six days thereafter, blood was collected 30, 60, 120, and 180 minutes after breakfast and lunch for the measurement of serum glucose and insulin concentrations.

Daily 24-hour urine samples were collected for the measurement of urea nitrogen, sodium, potassium, calcium, phosphate, and creatinine throughout the study. Creatinine clearance was calculated from serum and urine values, and renal tubular reabsorption of phosphate (expressed as the ratio of the maximal rate of renal tubular reabsorption of phosphate to the glomerular filtration rate) was derived from a nomogram.¹⁶

Serum glucose, sodium, potassium, urea, creatinine, calcium, and phosphate and urinary sodium, calcium, phosphate, urea, and creatinine excretion were measured by standard automated techniques. Serum concentrations of IGF-I and IGF-II were measured by radioimmunoassay after the binding proteins had been eliminated from the serum by C-18 Sep-pak extraction (Waters Associates, Milford Mass.), as described previously.^{17 , 18} Serum growth hormone, insulin, and C peptide were measured with standard radioimmunoassays, and serum osteocalcin was measured by radioimmunoassay at the Nichols Institute (San Juan Capistrano, Calif.).¹⁹ We calculated the mean (±SD) of each biochemical measurement in each study period using all the results obtained in that study period. The Tukey—Kramer analysis of variance (Systat, Evanston, Ill.) was used to determine the significance of the differences in the serum and urine measurements between study periods.

Table 1. Table 1. Mean (±SD) Serum and Urine Values in a Child with Growth Hormone Insensitivity Syndrome.

The child's actual intake of the constant diet offered decreased throughout the study. Comparing study periods, we found no significant difference in his daily intake of calories, fat, carbohydrate, calcium, and sodium, but his daily intake of protein and phosphate was lower during and after the infusion of IGF-I than during the week of base-line observations (P<0.05). There were, however, no differences in protein or phosphate consumption between the infusion period and either the week growth hormone was administered or the week after IGF-I was infused (protein intake was 47±7 g per day at base line, 37±5 g per day during growth hormone administration, 30±7 g per day during IGF-I infusion, and 29±8 g per day after IGF-I infusion; and phosphate intake was 885±307 mg per day at base line, 657±140 mg per day during growth hormone administration, 538±151 mg per day during IGF-I infusion, and 489±131 mg per day after IGF-I infusion). Weight and vital signs were stable throughout the study. Urinary volume increased by 48 percent during the infusion of IGF-I (P<0.01) (Table 1).

Figure 2. Figure 2. Fasting Serum Concentrations of Growth Hormone, (GH), IGF-II, Glucose, and C Peptide, According to Study Period, in a Child with Growth Hormone Insensitivity Syndrome.

The arrows indicate growth hormone injections, and the shaded area the period of IGF-I infusion. The asterisk indicates a sample that may not have been collected during fasting. Serum growth hormone concentrations decreased during the IGF-I infusion and rebounded afterward. Serum IGF-II concentrations rose during the administration of growth hormone (P<0.02), fell during the infusion of IGF-I (P<0.005 for the comparison with the period of growth hormone administration), and rose again after the IGF-I infusion (P<0.05 for the comparison with the infusion period). Serum glucose levels were lower during the infusion of IGF-I than at base line (mean [±SD], 3.4±0.8 vs. 4.7±0.1 mmol per liter; P<0.05). Serum C-peptide concentrations were lower during the infusion period than during the administration of growth hormone (850±303 vs. 1500±345 pmol per liter; P<0.05).

The administration of growth hormone was not associated with significant biochemical change, apart from a 50 percent increase in the serum level of IGF-II (252±16 μg per liter vs. 168±28 μg per liter at base line; P<0.02) (Fig. 2).

The infusion of IGF-I was well tolerated. The mean serum IGF-I concentration increased from a base-line value of 13±4 μg per liter to 448 μg per liter when 20 μg per kilogram per hour was infused, and to 235±23 μg per liter when 16 μg per kilogram per hour was infused (Fig. 1). The serum IGF-I returned to base-line levels within 24 hours after the infusion was stopped. The secretion of growth hormone was inhibited by the second day of the IGF-I infusion, and it increased promptly when IGF-I was discontinued (Fig. 2). Serum IGF-II decreased to 80 percent of the baseline concentration during the infusion of IGF-I (134± 30 μg per liter; P<0.005 for the comparison with the period of growth hormone administration), and increased again during the week after infusion (218±44 μg per liter; P<0.05 for the comparison with the infusion period) (Fig. 2).

The infusion of IGF-I produced a 56 percent decrease in the serum urea nitrogen concentration within 24 hours (P<0.001), and a 47 percent decrease in urinary urea nitrogen excretion in three days (P<0.005) (Fig. 1 ^, Table 1). After cessation of the infusion, serum urea nitrogen returned to the base-line level within 24 hours, but urinary urea nitrogen excretion remained below the base-line level (P<0.005). Urinary calcium excretion increased 2.5-fold during the infusion of IGF-I (P<0.001), whereas serum calcium remained constant (Fig. 1 ^, Table 1). After the IGF-I infusion was discontinued, urinary calcium excretion decreased to 42 percent of the base-line value (P<0.005) within 24 hours. Urinary phosphate excretion decreased by 30 percent during the infusion (P<0.01 for the comparison with base line), whereas serum phosphate concentrations did not change (Fig. 1 ^, Table 1). Serum sodium did not change, but urinary sodium excretion decreased by 27 percent during the infusion period (P<0.05 for the comparison with values before treatment) (Fig. 1 ^, Table 1). Serum and urine potassium did not change (data not shown). Creatinine clearance increased by 20 percent during the infusion of IGF-I (P<0.05) (Table 1). The 11 percent increase in the ratio of the maximal rate of renal tubular reabsorption of phosphate to the glomerular filtration rate was not significant (Table 1). Serum osteocalcin levels did not change (22.7 μg per liter at base line and 24.9 μg per liter on day 11 of the IGF-I infusion).

Morning fasting serum glucose values were lower during the infusion of IGF-I than during the other periods, and asymptomatic morning fasting hypoglycemia was documented on two occasions (Fig. 2). Seven bedside measurements of blood glucose at 3 a.m. indicated normoglycemia (mean blood glucose level, 5.9±0.6 mmol per liter). Fasting serum C-peptide concentrations decreased during the IGF-I infusion (P<0.05) (Fig. 2). Fasting serum insulin concentrations were at or below the limits of detection of the assay (19.5 pmol per liter) throughout the study (data not shown). Fasting morning ketonuria was detected on five of the eight days during which 16 μg of IGF-I per kilogram per hour was infused.

Figure 3. Figure 3. Serum Glucose (•) and Insulin (○) Concentrations in a Child with Growth Hormone Insensitivity Syndrome before and after Breakfast and Lunch on Day 11 of the IGF-I Infusion (Top Panel) and Six Days after Discontinuation of the Infusion (Bottom Panel).

The same meals were presented on both days, but the carbohydrate consumed during each meal differed; during the IGF-I infusion it was 50.9 g at breakfast and 58.7 g at lunch, and after the infusion period it was 36.5 g at breakfast and 40.2 g at lunch. The limit of detection of the insulin assay was 19.5 pmol per liter.

In contrast with the tendency toward hypoglycemia during fasting, nonfasting bedside measurements before meals and snacks during the infusion of IGF-I revealed frequent episodes of hyperglycemia (mean serum glucose level, 7.8±3.2 mmol per liter; range, 3.8 to 16; n = 25). During the eight days when 16 μg of IGF-I per kilogram per hour was infused, 27 of 32 urine specimens tested positive for ketones and 18 of 32 were positive for glucose one to four hours after the most recent meal. In the week after the infusion, all urine samples, both fasting and nonfasting, were negative for glucose and ketones. On day 11 of the infusion of IGF-I, serum glucose increased from a low fasting level of 2.5 mmol per liter to a peak of 14.8 mmol per liter one hour after breakfast; however, serum insulin concentrations did not increase appropriately and remained low despite prolonged postprandial hyperglycemia (Fig. 3, top panel). When the same meals were served six days after the end of the infusion, glucose and insulin responses were normal (Fig. 3, bottom panel).

We found that IGF-I has anabolic potential in patients with growth hormone insensitivity syndrome. With the exception of the increase in serum IGF-II, for which we have no explanation, the administration of growth hormone had no notable biochemical effects. On the other hand, the infusion of IGF-I produced a variety of metabolic responses. The prompt, marked decrease in serum and urine urea nitrogen, the increase in urinary calcium excretion, and the decrease in urinary phosphate and sodium excretion are effects that are recognized to occur when growth hormone is given to patients with growth hormone deficiency.²⁰ These results therefore suggest that IGF-I may be effective in promoting linear growth in children who have growth hormone insensitivity syndrome, and they provide additional evidence that IGF-I mediates the growth-related effects that occur when growth hormone—sensitive patients are treated with growth hormone.

The decrease in serum and urine urea nitrogen in our patient during the infusion of IGF-I is similar to that found when children with growth hormone deficiency²¹ and normal adults are treated with growth hormone,²² confirming that the well-known nitrogen-sparing effect of growth hormone is mediated by IGF-I. Normal adults infused with IGF-I had a 40 percent decrease in plasma urea nitrogen within two days, but no change in urinary urea nitrogen excretion.¹⁰ There is evidence from studies in rats infused with IGF-I and [l-¹⁴C]leucine that the rapid decrease in serum urea nitrogen may be due to an acute inhibitory effect of IGF-I on proteolysis.²³ The persistent decrease in both serum and urine values in our patient suggests that long-term treatment with IGF-I may promote net accretion of protein.

Treating patients with growth hormone deficiency and normal adults with growth hormone increases urinary calcium and decreases urinary phosphate excretion,^{22 , 24 , 25} effects that occurred in our growth hormone—insensitive patient in response to IGF-I. Two mechanisms for growth hormone—associated hypercalciuria have been proposed: increased bone turnover, suggested by associated increases in serum osteocalcin concentrations and urinary hydroxyproline excretion²⁴; and increased intestinal absorption of calcium mediated by calcitriol, the production of which is thought to be stimulated by a parathyroid hormone—independent effect of growth hormone on renal 1-alpha-hydroxylation of vitamin D.^{26 , 27} There is evidence that IGF-I may mediate both these effects. IGF-I increases osteocalcin synthesis by osteoblasts in fetal rat calvaria in vitro²⁸; and IGF-I infusion increases serum calcitriol concentrations in hypophysectomized rats in vivo.²⁹ The decrease in urinary phosphate excretion during growth hormone treatment in children with growth hormone deficiency is associated with an increase in the renal tubular reabsorption of phosphate.²⁵ Although the ratio of the maximal rate of phosphate reabsorption to the glomerular filtration rate did not increase significantly in our patient, the infusion of IGF-I in hypophysectomized rats does increase it.²⁹ There is evidence from in vitro studies that this increase is due to stimulation by IGF-I of sodium-dependent phosphate transport in the renal brush border.²⁹

The infusion of IGF-I in this growth hormone—insensitive child and in normal adults¹¹ decreased urinary sodium excretion and increased creatinine clearance, as has been described in both adults with growth hormone deficiency and normal adults who have been treated with growth hormone.^{30 31 32} Growth hormone and IGF-I may, however, have different effects on renal water handling. As judged by the stability of body weight and vital signs, the infusion of IGF-I in our patient (and in healthy adults^{10 , 11}) did not produce the increase in the volume of extracellular fluid known to be associated with growth hormone treatment.^{30 , 32} The explanation for this may lie with the increased urinary output we observed during the IGF-I infusion, an effect that does not occur in response to treatment with growth hormone.^{30 , 32} Our patient's glucosuria is unlikely to have contributed substantially to this apparent diuresis, since there were concurrent decreases in urinary sodium and phosphate excretion.

The ability of IGF-I to reduce fasting blood glucose levels is well recognized^{10 , 14 , 33} and is undoubtedly related to its insulin-like effect on target cells. The postprandial hyperglycemia found in our patient, however, was a previously unreported effect of IGF-I and appeared to result primarily from the suppression of insulin secretion. Such an effect on insulin has been found both in isolated rat pancreatic tissue perfused concurrently with IGF-I and glucose³⁴ and in rats in which euglycemia was maintained during IGF-I infusion.^{23 , 35} A secondary reason for the postprandial hyperglycemia is that IGF-I is only 1 to 2 percent as efficient as insulin in disposing of a glucose load, and even less effective in its insulin-like suppression of hepatic glucose production.^{23 , 35} There is evidence in rats³⁵ that the suppression of insulin secretion induced by IGF-I is dose-dependent; the hyperglycemia may therefore be eliminated by a lower IGF-I dose. The fact that our patient had both fasting and nonfasting ketonuria confirms that IGF-I is also a less efficient antilipolytic agent than insulin.^{23 , 35} IGF-I, like growth hormone, thus has the potential to be anabolic without increasing fat stores.

The differential effects of IGF-I on carbohydrate metabolism may be related to the distribution of type I growth factor receptors among tissues. In skeletal muscle, in which type I receptors are relatively abundant,³⁶ IGF-I may stimulate glucose disposal through its own receptor.³⁵ In liver and fat, however, in which type I receptors are few,^{37 38 39} IGF-I's inhibition of glucose production and lipolysis may be weak because it must act through the insulin receptor.^{23 , 35 , 39}

The suppression of growth hormone secretion observed in our patient during the infusion of IGF-I could have been due to a direct suppressive effect of IGF-I on pituitary somatotrophs⁴⁰ or to an indirect effect involving stimulation of the secretion of somatostatin by the hypothalamus.⁴¹ IGF-I—mediated stimulation of somatostatin secretion might also be responsible for the suppression of insulin secretion. Arguing against this, however, is the observation that IGF-I did not attenuate the glucagon response to hypoglycemia,^{33 , 34} which might have been expected had pancreatic concentrations of somatostatin been increased.

Our patient's growth hormone-like anabolic response to the infusion of IGF-I indicates that IGF-I has promise as a means of stimulating growth in children with growth hormone insensitivity syndrome. Further studies are needed to determine whether a lower dose or a different treatment regimen can preserve the growth hormone-like response while limiting the inhibitory effect of IGF-I on insulin secretion.