Acute regulation of plasma leptin by isoprenaline in lean and obese fasted subjects

AIM

In human obesity, there is some evidence for impaired adrenergic sensitivity with respect to catecholamine-induced lipolysis. The beta-adrenoceptor agonist isoprenaline has been shown to suppress plasma leptin levels in lean humans in vivo. We hypothesized that a reduced adrenergic sensitivity in obese humans would result in impaired suppression of leptin secretion.

METHODS

Eight obese [Ob, body mass index (BMI) = 33.3 kg/m2] and seven lean (Ln, BMI = 21.8 kg/m2) men were studied after an overnight fast. Intravenous isoprenaline infusion was initiated at a rate of 8 ng/kg/min, titrated up to 24 ng/kg/min over 30 min and continued at this rate for a further 120 min with continuous electrocardiogram monitoring.

RESULTS

Baseline fasting plasma leptin was higher in obese compared with lean subjects (Ob 12.2 +/- 1.8, Ln 2.6 +/- 0.6 ng/ml, p < 0.05 unpaired t-test). Baseline fasting glycerol as a measure of lipolysis was similar in both groups (Ob 62.9 +/- 7.6, Ln 42.4 +/- 8.9 micromol/l) and increased from baseline to 150 min by equivalent amounts (Ob +66.9%, Ln +81.2%, p = NS). Plasma leptin decreased from baseline to 150 min with similar relative changes in both groups (Ob -29.2%, Ln -27.8%).

CONCLUSIONS

Obese subjects show a similar lipolytic and leptin response to acute isoprenaline infusion compared with lean subjects. Impaired beta-adrenergic-induced inhibition of leptin secretion does not appear to contribute to hyperleptinaemia in obese human subjects.

Gender differences in growth hormone response to exercise before and after rhGH administration and the effect of rhGH on the hormone profile of fit normal adults

OBJECTIVES

Exercise is a potent physiological stimulus of GH secretion. We hypothesized that exogenous recombinant human growth hormone (rhGH) administration through an increase in GH and IGF-I levels would blunt the GH response to exercise. The aim of the study was to examine and compare the impact of rhGH on the exercise-induced GH response in healthy normal men and women.

DESIGN AND MEASUREMENTS

Sixty-nine subjects (36 men, 33 women) were randomized to receive low-dose rhGH (0.1 U/kg/day), high dose rhGH (0.2 U/kg/day), or placebo. Subjects were matched for age (24 +/- 3.1), and body mass index (BMI). rhGH was given as a single subcutaneous (s.c.) injection for the first 28 days. All subjects exercised to exhaustion (maximal oxygen consumption--VO2max) before rhGH treatment (Test 1), and on day 28 (Test 2). GH was measured before exercise (time 0), immediately after exercise (time 0') and at 15, 30, 60, 90 and 120 min postexercise. Baseline IGF-I levels were measured before exercise on days 0 and 28.

RESULTS

Baseline IGF-I levels showed no gender differences (42.3 women vs. 38.8 nmol/l men) but basal GH values were higher in women (9.9 vs. 1.8 mU/l, P < 0.001). The areas under the GH response curve, for Test 1 were similar in men and women. Peak GH values were higher in women than men (37.9 vs. 23.5 mU/l, but this did not quite reach statistical significance (P = 0.055). In men, administration of rhGH resulted in a significant increase in IGF-I levels over the basal state in both the LD and HD groups (P < 0.0001). In women, the increase in lGF-I levels reached significance only in the HD group (P < 0.0001). On day 28, GH secretion in response to exercise was calculated from the areas under the GH response curve correcting for an exogenous rhGH component (delta AUC). In men, the delta AUC, for Test 2 were similar in all three groups. In women, the delta AUC was higher in the placebo group, than in the HD group (P < 0.02). Free T4 levels decreased significantly in men, and free T3 increased in both men and women, in HD group after the rhGH administration. TSH levels were suppressed only in women. No changes in sex hormones were found in men or women in any of the treatment groups. Conclusions In terms of IGF-I, men are more responsive to rhGH treatment than women. In addition, as men, but not women, were able to overcome the negative feedback control of the elevated IGF-I levels, it seems that exercise may be a more robust stimulus to GH release in men compared to women.