Purpose: The purpose of this investigation was to examine the plasma to red blood cell (RBC) lactate concentration ([La]) gradient and RBC:plasma [La] ratio during 30 min of steady-state cycle ergometer exercise at work rates below lactate threshold (<LT = approximately 40% of peak cycle ergometer O2 uptake ¿VO2peak¿) and above LT (approximately 70% of VO2peak).
Methods: Eight subjects (cycling VO2peak = 41.6+/-1.6 mL x kg(-1) x min(-1); LT = 57.9 + 1.2% VO2peak) performed 30 min of cycle exercise at intensities < and > LT. Blood samples were taken from a heated forearm vein, immediately cooled to 4 degrees C in a dry-ice ethanol slurry, and centrifuged at 4 degrees C to separate plasma and RBCs.
Results: During >LT, plasma [La] rose to 8.8+/-1.1 mM after 10 min and remained above 6 mM. RBC [La] (4.9+/-0.7 mM) was significantly lower than plasma [La] at 10 min and remained lower throughout exercise. As a result, there was a sizable [La] gradient (approximately 3.5 mM) from plasma to RBC during most of >LT. In <LT, plasma [La] increased only slightly from rest (1.6+/-0.2 mM) after 6 min (2.4+/-0.3 mM) and then declined to approximately 2 mM for the remainder of the trial. The plasma to RBC [La] gradient averaged approximately 0.8 mM throughout <LT. Despite drastically different plasma to RBC [La] gradients in <LT and >LT, the ratio of RBC [La]:plasma [La] was the same for both (0.58+/-0.02) and not significantly different from rest.
Conclusions: These results refuted our hypothesis that the RBC:plasma [La] ratio would decrease at the onset of >LT exercise because of muscle lactate release exceeding the ability of RBCs to take up the lactate. Instead, there appears to be an equilibrium between plasma [La] and RBC [La] in arterialized venous blood from a resting muscle group as evidenced by the constant RBC [La]:plasma [La] ratio.