Myonecrosis induced in vivo by
cardiotoxin,
melittin, and Asp49 and Lys49
phospholipase A2 (PLA2)
myotoxins involves rapid lysis of the sarcolemma, myofibril clumping, and hypercontraction of sarcomeres. In contrast, skeletal muscle
necrosis induced by
crotamine and
myotoxin a is much slower, consisting of mitochondrial and sarcoplasmic reticulum swelling, myofibril degeneration, and lack of sarcolemma or transverse tubule damage. The mechanisms contributing to the myonecrosis induced by these
peptides were evaluated. Two
cardiotoxins and two Lys49 PLA2
myotoxins lysed primary cultures of human skeletal muscle within 24 hr at a concentration of 0.25 microM, while
melittin,
crotamine, and
myotoxin a, and an Asp49 PLA2
myotoxin were non-cytolytic at concentrations up to 5.0 microM, suggesting that cytolysis is not a good measure of
myotoxicity.
Crotamine and the Lys49 PLA2
myotoxin altered Ca2+ ion flux in human heavy sarcoplasmic reticulum by opening the ryanocine receptor. Whole-cell patch-clamp studies demonstrated that administrating
crotamine intracellularly increased Na+ currents.
Free fatty acids, liberated by activation of tissue
phospholipase C or by the PLA2 activity of the
myotoxins, were monitored for
crotamine,
myotoxin a and a Lys49 PLA2
myotoxin in cell cultures in which the
lipids had been radiolabeled. Only the Lys49
myotoxin produced significant amounts of
fatty acid in cell cultures, supporting a potential role for
fatty acid production only in the mechanism of sarcolemma-destroying
myotoxins. These findings, coupled with those in the literature, support a hypothesis in which the
myotoxins and/or products of
lipase activity (e.g.
fatty acids) are acting at a site existing on both the Na+ channel and a
protein involved in Ca2+ release and probably serving a modulatory function for ion regulation. Based on the similarities in mechanisms between the toxins and
fatty acids, the most likely site would be a
fatty acid binding site on the
protein (either similar to that on
fatty acid binding proteins, or an acylated
cysteine residue) or in the membrane.