Lactic acid (C 3 H 6 O 3 ) is a substance produced by the body during normal body metabolism. This synthesis becomes particularly intense in conditions of oxygen deficiency, that is when the metabolic demand of this gas exceeds the availability; it is a juncture characteristic of strenuous physical exercise, but also of particular pathological states, such as those resulting from an airway obstruction.
Biochemical bases
Let us briefly recall that lactic acid is produced from pyruvate, which is the final product of glycolysis (a cytoplasmic process that produces the degradation of glucose in two molecules of pyruvic acid or pyruvate). In the sixth of the ten stages of glycolysis, the 3-phosphoglyceric aldehyde is oxidized thanks to the oxidized NAD (NAD +) which acts as an H + hydrogenion acceptor. The NAD is then reduced to NADH (H +). At this point, if we want the energy to continue to be generated through glycolysis, we must take care to regenerate the oxidized NAD (NAD +), which would otherwise be rapidly depleted until it is depleted. When the availability of oxygen is sufficient, the reoxidation of reduced NAD is entrusted to the Krebs cycle (mitochondrial oxidative phosphorylation), with oxygen consumption, water formation and ATP synthesis. When the oxygen is scarce, the pyruvate that does not enter the krebs cycle is reduced to lactic acid by the enzyme lactate dehydrogenase. From this reaction (see figure), the NAD + necessary for the further reaction of the 3-phosphoglyceric aldehyde is restored; glycolysis can then proceed.
Once produced, at physiological pH, lactic acid tends to dissociate almost entirely in two ions: the lactate ion and the H + ion (according to the reaction shown in the figure).
Since, as the name suggests, an acid is produced, the excessive production of lactate and H + tends to lower the pH inside the cell, contributing (along with many other factors) to the onset of fatigue.
The first mechanism implemented by the cells to defend themselves from the excessive production of lactic acid consists in its efflux towards the extracellular environment and the blood. Not surprisingly, in normal conditions the blood lactate concentration is equal to 1-2 mmol / L, while it rises to over 20 mmol / L during a particularly intense physical exercise.
Disposal of lactic acid
Although at high concentrations lactic acid is a particularly toxic product, which as such must necessarily be disposed of, it cannot and must not be considered a waste. Indeed, once produced, lactic acid can:
- be picked up and used by some tissues for energy purposes, as happens for example in the heart (which prefers to use lactate rather than glucose), but also at the level of the same muscle cells (the white fibers are better at producing it and the red ones at the disposal of it) ;
- be used for the new synthesis of glucose / glycogen (gluconeogenesis, Cori cycle in the liver).
In both cases, lactate must first be converted back into pyruvate, again by the lactate dehydrogenase enzyme, with a reduction of NAD + to NADH (H +). At this point, pyruvate can be completely oxidized in the Krebs cycle or be used for gluconeogenesis.
We have already seen how an excessive synthesis of lactic acid disrupts the metabolism of the cell, which provides to release it outside through specific membrane transporters (MCT). In addition to various defense mechanisms that we will see shortly, there is a priori further control that prevents the excessive accumulation of lactate in the intracellular environment. The drop in pH (acid environment) - due to the accumulation of H + hydrogenions deriving from the dissociation of lactic acid - in fact inhibits the enzyme phosphofruttokinase, which intervenes in the third stage of glycolysis, determining its speed. Consequently, an excessive drop in pH causes a slowing of glycolysis, reducing the speed of lactic acid synthesis (negative feedback).
The excessive decrease of the intracellular pH is however also fought by the buffer systems, among which the most important is the biarbonate / carbonic acid, enhanced by the respiratory activity with elimination of CO2:
As shown in the figure, the intense respiratory activity that occurs during intense exercise reduces the concentration of CO2 and carbonic acid in the blood, blocking the release of the H + product by dissociation of the lactic acid.
The image above shows the time course of blood lactate (lattatemia) during the recovery phase following an intense lactic acid effort. As clearly shown by the graph, the trained subject is able to dispose of lactic acid in a shorter time than the sedentary one. Another important thing to note is that within an hour, at most, the levels of lattemia return to basal conditions; therefore it is wrong to attribute to the accumulation of lactic acid the muscular soreness that accompanies the days following a particularly intense training.
To facilitate the disposal of lactic acid after a maximum effort, the athlete will take care to have the performance followed by a slow-down regeneration phase lasting 15-20 minutes.
