The Role Of Lactate In The Exercise-induced Growth Hormone Response

Abstract

PURPOSE During the last thirty years the contention that lactate is the major stimulus for the exercise induced hGH response (EIGR) has undergone a cycle of popularity and rejection. Currently, there is a growing body of evidence of the ubiquity of roles for lactate and in this regard the EIGR is no exception. McArdles disease is a type V inherited glycogen storage disease in which moyphorylase is absent. Accordingly these individuals do not produce lactate during exercise and hence represent an ideal model to explore the role of lactate in the EIGR. The present study examined the acute hGH response to a standardised graded exercise stimulus in McArdles patients. METHODS Eleven patients entered the study. All have McArdles disease as confirmed by both DNA and muscle biopsy analysis. Participants completed an incremental exercise test on a motorised treadmill. This comprised walking, beginning at 1.5 kph and increasing by 0.5 kph with each 3-minute stage. A 60s rest interval was given between successive stages for blood sampling. The test was terminated either after completion of 3 minutes at 6.5 kph or when a CR10 pain scale rating of “4” was achieved. Capillary (earlobe) blood samples for lactate and hGH determination were taken before the test and after each stage. HR was measured continuously throughout exercise. RESULTS Six individuals demonstrated no EIGR whilst five individuals demonstrated a modest rise in growth hormone concentration during exercise with peak values ranging from 0.73 to 8.40 μg.1−1. The two groups which arose from this demonstrated a significantly higher (p<0.05) peak heart rate in those who had an EIGR (169.25 ± 10.34 bpm vs 131.83 ± 24.00, mean ± SD). Although peak heart rate as a percentage of age-predicted maximal heart rate demonstrated a non-significant difference between the two groups, calculation of effect size revealed a meaningful difference (1.36) suggesting that those who produce hGH in response to exercise have a higher relative heart rate than those who do not. CONCLUSION The dichotomous response observed in McArdles patients in the present study suggests that lactate is not the primary stimulus for the EIGR. However, by virtue of a modest response in half the participants, lactate may be a co-stimulus of the EIGR along with other factors that have yet to be fully identified.