Plasma ghrelin levels during exercise in healthy subjects and in growth hormone-deficient patients

Abstract

Objective: To characterise plasma levels of the recently identified endogenous ligand for the GH secretagogue receptor (ghrelin) during submaximal aerobic exercise in healthy adults and in GH-deficient adults. Design: Eight healthy males (mean^S.E. age, 40:8^2:9 years) and eight hypopituitary males with verified GH deficiency (mean^S.E. age, 40:8^4:7 years) underwent a baseline test of their peak aerobic capacity (VO 2 peak) and lactate threshold (LT) on a cycle ergometer, as well as an evaluation of body composition. The patients were then studied on two occasions in random order when they exercised for 45 min at their LT. On one occasion, GH replacement had been discontinued from the evening before, whereas on the other occasion they received their evening GH in addition to an intravenous infusion of GH (0.4 IU) during exercise the following day. The healthy subjects exercised at their LT on one occasion without GH. Results: The patients were significantly more obese and had lower VO 2 max (corrected for body weight) and LT as compared with the control subjects. Exercise induced a peak in serum GH concentrations after 45 min in the control group ð11:43^3:61 mg=lÞ: Infusion of GH in the patients resulted in a peak level after 45 min, whereas no increase was detected when exercising without GH (9:77^2:40 (GH) vs 0:11^0:07 mg=l (no GH)). Plasma ghrelin levels did not change significantly with time in either study, and no correlations were detected between ghrelin levels and parameters such as GH and IGF-I levels, age or body composition. Plasma ghrelin levels were significantly lower during the study period with GH as compared with the study with no GH. Conclusions: Submaximal aerobic exercise of an intensity sufficient to stimulate GH release was not associated with significant alterations in plasma ghrelin concentrations, which indicated that systemic ghrelin is not involved in the exercise-induced stimulation of GH secretion. The observation that ghrelin levels were lower during GH replacement suggests that GH may feedback-inhibit systemic ghrelin release.

Exercise in rats does not alter hypothalamic AMP-activated protein kinase activity

“…In humans, exercise has been reported not to influence plasma ghrelin levels even at high exercise intensities [28][29][30]. We are unaware of other published data in which the plasma ghrelin response to exercise has been studied in rats.…”

Influence of resistance and aerobic exercise on hunger, circulating levels of acylated ghrelin, and peptide YY in healthy males

“…The majority of studies have focused on aerobic (cardiovascular) exercise (31), with only three studies examining the effects of resistance (muscle strengthening) exercise, and these have reported contradictory effects (21,33,47). Resistance exercise is a key component of exercise recommendations for weight control (2) and public health (23, 42); thus, it is important to clarify the effects of resistance exercise on hunger and gut hormones.The effect of acute exercise bouts on total plasma ghrelin concentrations is controversial, with studies reporting no changes either during or postexercise (10,15,26,27,32,39,43,45,47), as well as increases (14,17,25,44) and decreases (21,33,48,50). Acylation of ghrelin is thought to be essential for appetite regulation because only the acylated form of the hormone can cross the blood-brain barrier (41).…”

The negative association between plasma ghrelin and IGF-I is modified by obesity, insulin resistance and type 2 diabetes

“…Pubertal GH surge is not associated with ghrelin levels [27]. Furthermore, GH pulsatility is not explained by changes in ghrelin concentration [43] and acute GH response in exercise is not associated with the changes in ghrelin levels [25,31]. Interestingly, ghrelin-deficient mice are viable and exhibit a normal growth rate, demonstrating that ghrelin is not critical for normal growth [23].…”

“…Even though IGF-I is not universally regarded as an ideal discriminator of GH status [44], IGF-I measurement is currently the best indirect method to assess GH secretion [45]. The recent studies have reported negative association between ghrelin and IGF-I concentrations in children and adolescents [27][28][29][30] whereas nihilistic results has been reported in adult subjects [31][32][33].…”

“…If ghrelin has a significant role in the long-term regulation of GH concentration, there should be a detectable association between ghrelin and IGF-I, the most important peripheral effector protein of GH ac-tion. However, the results of the recent studies concerning the association between ghrelin and IGF-I are somewhat conflicting showing negative correlation in children and adolescents [27][28][29][30] but no correlation at all in adult subjects [31][32][33]. Sample sizes of these studies have been relatively small.…”