NO ROLE FOR ENZYMATIC-ACTIVITY OR DANTROLENE-SENSITIVE CA2(+) STORES IN THE MUSCULAR EFFECTS OF BOTHROPSTOXIN, A LYS49 PHOSPHOLIPASE A(2) MYOTOXIN

The role of low levels of phospholipase A(2) (PLA(2)) activity and int racellular Ca2+ stores in the pharmacological action of bothropstoxin (BthTX), a myotoxic Lys49 PLA(2) homologue isolated from the venom of Bothrops jararacussu, was investigated. We examined the muscular effec ts of BthTX in the mouse diaphragm and its PLA, activity in radiolabel ed human and rat primary cultures of skeletal muscle. Although it is a Lys49 PLA(2) homologue, BthTX had a low, but easily detectable, level of enzymatic activity relative to two Asp49 PLA(2) 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 replaceme nt of 1.8 mM Ca2+ by 4 mM Sr2+ did not alter the BthTX-induced contrac ture and blockade of the muscle twitch tension. In addition, Sr2+ decr eased by 50% the time required to cause 50% paralysis, and evoked appr oximately a four-fold increase in the number of spontaneous spikes. In isolated sarcoplasmic reticulum preparations, BthTX opened the intrac ellular Ca2+ release channel (ryanodine receptor) and lowered the thre shold of Ca2+-induced Ca2+ release by a second, as yet unidentified, m echanism. 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 PL A(2) activity, or dantrolene-sensitive intracellular Ca2+ stores, in t he action of BthTX. The mechanism whereby Sr2+ stimulates the pharmaco logical activity of BthTX remains to be clarified.