Question

Q: Blood Lactate Levels during Vigorous Exercise. The concentrations of lactate in blood plasma before, during, and after a 400 m sprint are shown in the flowing graph.

a. What causes the rapid rise in lactate concentration?

b. What causes the decline in lactate concentration after completion of the sprint? Why does the decline occur more slowly than the increase?

c. Why is the concentration of lactate not zero during the resting state?

Answer 1

Ans. 1. During vigorous exercise, ATP is hydrolyzed at greater speed to compensate the demand of energy for muscle activity. Simultaneously, the muscles also face hypoxic condition because- I. the rate of oxygen supply to skeletal muscles does not increase significantly during strenuous exercise, and, II. Myoglobin in not very much efficient O2 transporter like hemoglobin in RBCs.

So, under low pO₂ (limited O₂ availability) in muscles, myocytes can’t carry out citric acid cycle and electron transport chain in mitochondria because O₂ acts as the terminal electron acceptor. In this case, the only process in action producing ATP is glycolysis with net gain of 2 ATP.

However, glycolysis requires NAD⁺ for the process to continue which is being reduced to NADH during formation of pyruvate. Therefore, to regenerate NAD⁺, the cells convert pyruvate into lactate with regeneration of NAD⁺, a process being catalyzed by lactate dehydrogenase. NAD⁺ regenerated through lactate fermentation, is re-used in glycolysis.

Therefore, the process of lactate fermentation under hypoxic (anaerobic) condition acting to regenerate NAD⁺ for compensating energy demand through glycolysis causes lactate concentration to rise considerably.

Ans. b. After completion of sprint, following events take place-

I. Relatively greater availability of O₂ to myocytes resumes TCA and ETC that produces large amount of ATP from each glucose (30 ATPs, compared to 2 ATP in glycolysis) through oxidative phosphorylation. This process stops lactate fermentation to large extent and inhibits further formation of lactate.

2. Excess lactate produced during various muscle activities is transported into liver. In hepatocytes, lactate dehydrogenase (LDH) catalyzes the reverse reaction (lactate to pyruvate) using Cori cycle. Cori cycle is a multi-step reaction (compared to single step reaction of pyruvate to lactate) requires relatively longer time to convert lactate back into pyruvate. Therefore, due to change in site of pyruvate regeneration from lactate (liver) and relatively longer time consuming Cori cycle, the rate of conversion of lactate into pyruvate is very slow compared to the reverse reaction.

Ans. c. In muscles, myoglobin transports O₂. Myb is less efficient than Hb (has only 1 O₂ binding site compared to 4 in Hb). Moreover, muscles (smooth and skeletal) are always in action (normal day to day movement/ activities) and require relatively higher ATP consumption rate. However, because of relatively limited O₂ in muscles and limited [NAD⁺], muscles almost continuously produce a small amount of lactate, it’s normal. Thus, even at rest, there is little amount of lactate traceable in the circulation.