Effects of a cardiotoxin from Naja naja kaouthia venom on skeletal muscle: Involvement of calcium-induced calcium release, sodium ion currents and phospholipases A2 and C

Snake venom cardiotoxin (CTX) fractions induce contractures of skeletal muscle and hemolysis of red blood cells. The fractions also contain trace amounts of venom-derived phospholipase A₂ (PLA₂) contamination and activate tissue phospholipase C (PLC) activity. The present study examines the mechanisms of action of a CTX fraction from Naja naja kaouthia venom in skeletal muscle. Sphingosine competitively antagonized CTX-induced red blood cell hemolysis, but not skeletal muscle contractures. CTX rapidly lowered the threshold for Ca²⁺-induced Ca²⁺ release in heavy sarcoplasmic reticulum fractions, as monitored with arsenazo III. There was also a slower time-dependent reduction of Na⁺ currents, as assessed by whole cell patch-clamp techniques. The CTX fractions elevated levels of free fatty acids and diacylglycerol for 2 hr in primary cultures of human skeletal muscle by a combined action of venom-derived PLA₂ contamination in the fraction and activation of endogenous PLC activity. The activation of tissue PLC activity could be readily distinguished from the contribution of the venom PLA₂ by p-bromophenacyl bromide treatment of CTX fractions. The mechanism of action involved in contractures of skeletal muscle appears to be related to the immediate and specific effect of CTX (Ca²⁺ release by the sarcoplasmic reticulum), while the mechanisms involved in hemolysis of red blood cells and decreased Na⁺ currents in skeletal muscle most likely relate to long-term effects on lipid metabolism.