Mechanisms of heat injury
On the basis of all the above evidence, what is the Mechanism of heat injury? According to Crisan (1973), some 25 hypotheses have been proposed to explain it. In view of the many kinds of heat injury described above, any attempt to identify a single mechanism seems futile. The secondary, heat-induced, water stress which injures via desiccation is discussed in Vol.2, Chapter 5, and will not be considered here. The following are the remaining possibilities.
The five kinds of primary, indirect heat injury, occurring during continued exposure (hours or days) to moderately high temperature are all metabolic in nature and, therefore, may conceivably have a single basic mechanism. The first strain produced by high temperature is kinetic-an increase in reaction rates. As in the case of chilling injury differences in slope of Arrhenius plots for different metabolic reactions may reverse relative reaction rates and lead to an increase in net breakdown and therefore may produce a decrease in concentration or complete absence of an essential metabolite at heat injury temperatures. The resulting injury may be starvation, biochemical lesions, protein hydrolysis, and growth reduction or cessation. Conversely, it may produce an increase in concentration of a toxic substance normally not detectibly present, or in too low a concentration to be injurious. In the case of soluble enzymes, the differences in slope may simply be due to differences in activation energy for different reactions, In the case of membrane enzymes, breaks in the curve may be due to a phase transition or increased mobility of the membrane lipids associated with the enzyme. Membrane proteins are embedded in the membrane lipids, and due to their higher hydrophobicity than that of the soluble proteins, are not so likely to be denatured by high temperatures at which hydrophobic bonds are strengthened. Involvement of nucleic acids has also been suggested, but the evidence to date is negative. The above kinds of metabolic damage, occurring during hours or days at moderately high temperatures are unable to explain the primary, direct injury due to heat shocks for seconds or minutes at somewhat higher temperatures. A different mechanism must, therefore, be involved. The direct observations of heat shock injury, as well as the leakage of ions and amino acids, and the effects of salts, narcotics, etc., all point to membrane damage