Plasma Cyclic AMP and Blood Lactate Responses to Incremental Cycling in Untrained Male Subjects

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