LACTATE DISTRIBUTION IN THE BLOOD DURING STEADY-STATE EXERCISE

Purpose: The purpose of this investigation was to examine the plasma t o red blood cell (RBC) lactate concentration ([La]) gradient and RBC:p lasma [La] ratio during 30 min of steady-state cycle ergometer exercis e at work rates below lactate threshold (<LT = approximate to 40% of p eak cycle ergometer O-2 uptake {(V) over dot O-2peak}) and above LT (a pproximate to 70% of (V) over dot O-2peak). Methods: Eight subjects (c ycling (V) over dot O-2peak = 41.6 +/- 1.6 mL.kg(-1).min(-1); LT = 57. 9 +/- 1.2% (V) over dot O-2peak) performed 30 min of cycle exercise at intensities < and > LT. Blood samples were taken from a heated forear m 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 Fl 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 pl asma [La] at 10 min and remained lower throughout exercise. As a resul t, there Nas a sizable [La] gradient (approximate to 3.5 mM) from plas ma to RBC during most of >LT. In <LT, plasma [La] increased only sligh tly from rest (1.6 +/- 0.2 mM) after 6 min (2.4 +/- 0.3 mM) and then d eclined to approximate to 2 mM for the remainder of the trial. The pla sma to RBC [La] gradient averaged approximate to 0.8 mM throughout <LT . Despite drastically different plasma to RBC [La] gradients in <LT an d >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 d ecrease 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 arter ialized venous blood from a resting muscle group as evidenced by the c onstant RBC [La]:plasma [La] ratio.