Discordance between growth hormone (GH) responses after GH-releasing hormone and insulin hypoglycemia in myotonic dystrophy

A cross-sectional study for glucose intolerance of myotonic dystrophy

We made a cross-sectional study to analyze glucose intolerance of myotonic dystrophy type 1 (DM1) with several examination including oral glucose tolerance test (OGTT), insulin tolerance test (ITT) and adiponectin. Ninety-five DM1 patients participated in this study. Health examination data from general people were used as controls. In DM1, homeostasis model assessment-insulin resistance (HOMA-IR) was higher than control even in the lowest fasting blood sugar (FBS) stage (<80 mg/dl) and insulin sensitivity assessed by ITT was low regardless of their FBS. Insulinogenic index of DM1 was positively correlated to HOMA-IR. Insulinogenic index and sum of IRI in OGTT were markedly elevated in the lowest FBS stage and declined along with elevation of FBS. Consequently, as many as 13.3% of DM1 patients with 90-110 mg/dl of FBS exhibited DM pattern, while only 1.9% in control. Adiponectin was higher in DM1 than control. Although age correlated with adiponectin in both control and DM1, its impact was stronger in DM1. DM1 predisposes insulin resistance and compensatory hyperinsulinemia exist even in patients with low FBS. We should pay attention to glucose intolerance of DM1 patients earlier than that of the general population. It seemed that 90 mg/dl of FBS is an important index as an indication of careful managements.

Effect of fasting and growth hormone (GH) administration on GH receptor (GHR) messenger ribonucleic acid (mRNA) and GH-binding protein (GHBP) mRNA levels in male rats

To elucidate whether GHR and GHBP are coordinately regulated or not, we studied the effect of fasting with or without GH administration on the GHR and GHBP mRNAs in the liver as well as in extrahepatic tissues in rats. Tissues were collected from 7-week-old male rats by decapitation 1,3, and 7 days after the start of fasting. Liver GHR mRNA levels were not affected 1 day after the start of fasting but progressively decreased for the subsequent 3 and 7 days of fasting as compared with those in control rats fed ad libitum. In contrast, liver GHBP mRNA levels significantly rose after 1 day fasting, returned to the control level after 3 days and further reduced after 7 days of fasting. Changes in GHBP mRNA level after fasting were different among the tissues. A transient increase in GHBP mRNA levels was observed in muscle and heart as well as liver, while the GHBP mRNA levels in fat tissues did not change throughout 7 days of fasting. Next, bovine GH(bGH) was administered ip to the fasted rats and control fed rats for either 1 day(100 micrograms [corrected], tid) or 5 days(150 micrograms [corrected], daily). In fed rats, liver GHR mRNA level was significantly increased by 1 day bGH treatment, but after 5 days treatment with bGH it was not different from the level in saline-injected control. Accordingly, net increment of plasma IGF-I was 296.0 ng/ml with 1 day bGH treatment and 234.2 ng/ml with bGH administration for 5 days. In fasted rats, liver GHR mRNA level did not changed after 1 day treatment with bGH, but markedly decreased 5 days after bGH administration. Net increment of plasma IGF-I was slightly reduced to 284 ng/ml with 1 day treatment with bGH, and markedly decreased to 37.0 with bGH administration for 5 days. The effect of GH administration on liver GHBP mRNA level was virtually absent in either fasting or fed state. These findings suggest that GHR and GHBP mRNAs in the liver are expressed in different ways and that expression of GHBP mRNA is differently regulated among tissues.

Study on growth hormone and insulin secretion in myotonic dystrophy

Growth hormone (GH) levels were measured in 12 patients with myotonic dystrophy (MD; 7 men and 5 women, aged 21-49 years) and 14 volunteers after administration of 100 micrograms GH-releasing hormone (GHRH; 1-29). A 75-g oral glucose tolerance test was carried out to determine glucose, insulin, plasma C-peptide, and urinary C-peptide. The GH level in six MD patients responded normally to GHRH (group I), with a peak of 17.1 +/- 1.46 micrograms/l, compared with controls (27.8 +/- 19.6 micrograms/l, NS), and that in the other six patients responded subnormally, with a peak of 3.15 +/- 1.46 micrograms/l, lower than in controls and in group I patients (P < 0.001). In group I the insulin response to the glucose tolerance test showed hyperinsulinism and was lower than that in group II patients; stimulated C-peptide was also higher in group II than in group I and in controls; urinary C-peptide levels were parallel to those in previous data. In all MD patients there were a negative correlation between absolute values of GH response to GHRH and insulin response to glucose tolerance test (r = -0.79, P < 0.001). Our data suggest that the failure in GH release and peripheral insulin action is due to a generalized defect in cellular membrane function in MD patients.

GH secretion status in myotonic dystrophy

Frequent endocrine alterations and abnormal growth hormone (GH) secretion have been reported in myotonic dystrophy (MD). To evaluate GH secretion status in MD, GH response to 100 micrograms of growth hormone releasing hormone (GHRH) with or without pyridostigmine pretreatment and its relation with insulin-induced hypoglycemia was investigated in MD patients and compared with normal controls. The mean peak plasma GH response to GHRH was 27.8 +/- 19.2 micrograms/l normal subjects and 11.4 +/- 8.7 micrograms/l in MD patients. In five of seven patients GH reached a mean peak of 12.6 +/- 4.2 micrograms/l after insulin-induced hypoglycemia, compared with 5 +/- 2.8 micrograms/l after GHRH. Conversely, in two patients GH reached a peak of 16.1 and 32 micrograms/l after GHRH, and only 2.5 and 5.3, respectively, after hypoglycemia. Pretreatment with pyridostigmine in nine patients tested potentiated GHRH-induced GH release with a peak of 17.6 +/- 12.5 micrograms/l, compared with 10.05 +/- 6.7 micrograms/l after GHRH alone; IGF-I levels were normal in all patients.

Adrenocorticotropin hyperresponsiveness in myotonic dystrophy following oral fenfluramine administration

Abstract The plasma immunoreactive adrenocorticotropin and cortisol responses to oral fenfluramine hydrochloride (1.5mg/kg body wt) or placebo were examined in 11 patients with myotonic dystrophy, 4 controls with facioscapulohumeral dystrophy, a similarly debilitating muscle wasting disease, and 14 normal controls in single-blind studies performed in mid-afternoon. Mean areas under the adrenocorticotropin response versus time curve were significantly greater in myotonics (2573 + 429 pmol.min/L) than in facioscapulohumeral dystrophy controls (696 + 279 pmol.min/L, P<0.02) and normals (560+/-61 pmol.min/L, P<0.0001). Corresponding cortisol responses were significantly greater in myotonics (35757 + 3949 nmol.min/L) than in normals (21828+/-1669 nmol.min/L, P < 0.001), but not significantly greater than those in facioscapulohumeral dystrophy controls (22830 + 6140 nmol.min/L, P = 0.055). No stressful side-effects which could affect hormone responses, and no significant changes in blood pressure or heart rate were noted. Fenfluramine activates central serotonergic and/or noradrenergic pathways initiating secretion of corticotropin-releasing hormone and possibly arginine vasopressin. We postulate that these fenfluramine-activated pathways are hyperstimulated in myotonics, leading to adrenocorticotropin and cortisol hypersecretion. This may be a manifestation of a general cell membrane defect in myotonic dystrophy. We found a lack of correlation of age (and severity of disease) with adrenocorticotropin response in myotonics, and therefore, the hyperresponse may serve as a useful marker for the disease before development of other overt signs.

Increased plasma membrane fluidity and decreased receptor availability of nonmuscle cells in myotonic dystrophy

Plasma membrane fluidity of intact nonmuscle cells from patients with myotonic dystrophy (MyD) was determined by fluorescence anisotropy measurements. Anisotropy values of the probe diphenylhexatriene were decreased in patient mononuclear cells (0.163 +/- 0.017, n = 13) versus controls (0.181 +/- 0.013, n = 13, P less than 0.01) and in patient platelets (0.087 +/- 0.017, n = 9) versus controls (0.137 +/- 0.015, n = 9, P less than 0.001) indicating increased plasma membrane fluidity in patient nonmuscle cells. Vasopressin plasma concentrations were increased in patients (7.4 +/- 2.1 pg/ml, n = 12) versus controls (4.5 +/- 1.4 pg/ml, n = 22, P less than 0.0005), whereas serum osmolality was normal. These data are compatible with a decreased vasopressin sensitivity in MyD patients. Specific binding of 125I-labelled vasoactive intestinal peptide (VIP) was decreased in patient mononuclear cells (2.9 +/- 0.9%/10(6) cells, n = 8) versus controls (5.2 +/- 1.6%/10(6) cells, n = 9, P less than 0.005) and receptor affinity for VIP was decreased in patient mononuclear cells (Kd = 0.26 +/- 0.05 nM, n = 8) versus controls (Kd = 0.19 +/- 0.02 nM, n = 9, P less than 0.005). In nonmuscle cells of MyD patients, increased membrane fluidity correlated with decreased receptor availability. This might explain the various endocrine defects described in MyD patients.