Abstract
Recently, it has been suggested that epinephrine influences blood lactate and the
lactate threshold during incremental exercise through a ß-adrenergic adenylate cyclase
dependent mechanism. We sought to characterize the relationship between the changes
in the ß-adrenergic adenylate cyclase system and blood lactate during incremental
exercise indirectly through the measurement of plasma cAMP. The relationships of plasma
cAMP to blood lactate levels and the lactate threshold were examined in nine untrained
male subjects. Each subject performed an incremental exercise test to volitional exhaustion
on an electronically-braked cycle ergometer. Although plasma cAMP was rising at the
lactate threshold, it did not demonstrate the classic threshold response usually seen
in blood lactate. Pairwise matched t-tests were used as a post-hoc test to determine
if the successive changes in blood lactate and plasma cAMP between 21.4, 38.6, 58.7,
81.2 and 100% of V̇O2max were significant. Plasma cAMP was rising between 38.6% and 58.7% and between 58.7%
and 81.2% of V̇O2max, but these changes in plasma cAMP did not reach statistical significance (p>0.0125)
with Bonferroni adjustment. The change in plasma cAMP compared to its previous value
as well as the change in plasma cAMP above the resting value was not statistically
significant until 81.2% of V̇O2max. A moderate but significant correlation was observed between blood lactate and
plasma cAMP levels (r=0.612) using blood samples obtained at each workstage in all
subjects. The mean correlation between blood lactate and plasma cAMP was 0.75 (S.E.
= 0.05) and ranged between 0.58 and 0.97 in individual subjects. It is concluded that
the extent to which a ß-adrenergic cAMP-dependent mechanism contributes to the change
in blood lactate during incremental exercise demonstrates large variability between
subjects. Moreover, these data suggest that a ß-adrenergic cAMP-dependent mechanism
is not entirely responsible for the lactate threshold.
Key words
Lactate threshold - cAMP - catecholamines
