No role for enzymatic activity or dantrolene-sensitive Ca2+ stores in the muscular effects of bothropstoxin, a Lys49 phospholipase A2 myotoxin.

The role of low levels of phospholipase A2 (PLA2) activity and intracellular Ca2+ stores in the pharmacological action of bothropstoxin (BthTX), a myotoxic Lys49 PLA2 homologue isolated from the venom of Bothrops jararacussu, was investigated. We examined the muscular effects of BthTX in the mouse diaphragm and its PLA2 activity in radiolabeled human and rat primary cultures of skeletal muscle. Although it is a Lys49 PLA2 homologue, BthTX had a low, but easily detectable, level of enzymatic activity relative to two Asp49 PLA2 enzymes from Naja naja kaouthia and Naja naja atra venoms, and this activity was reduced by about 85% in the presence of Sr2+ (4.0 mM). However, the replacement of 1.8 mM Ca2+ by 4 mM Sr2+ did not alter the BthTX-induced contracture and blockade of the muscle twitch tension. In addition, Sr2+ decreased by 50% the time required to cause 50% paralysis, and evoked approximately a four-fold increase in the number of spontaneous spikes. In isolated sarcoplasmic reticulum preparations, BthTX opened the intracellular Ca2+ release channel (ryanodine receptor) and lowered the threshold of Ca(2+)-induced Ca2+ release by a second, as yet unidentified, mechanism. However, in intact muscle, dantrolene, an antagonist of some forms of intracellular Ca2+ release, had no effect on the actions of BthTX. These findings do not support any role for the low levels of PLA2 activity, or dantrolene-sensitive intracellular Ca2+ stores, in the action of BthTX. The mechanism whereby Sr2+ stimulates the pharmacological activity of BthTX remains to be clarified.