Studies of beta-cardiotoxin from king cobra venom on rat cardiac functions: Effect on isolated single cardiomyocyte

Abstract

Beta-cardiotoxin (ß-CTX), a novel protein isolated from the King cobra (Ophiophagus hannah) venom has previously been proposed as a beta-blocker candidate. However, cellular mechanisms of ß-CTX on cardiomyocyte is unknown. This study aimed to evaluate the impact of ß-CTX on isolated rat cardiomyocyte function and the cardiac myofibrillar activity, and to explore involvement of ß-adrenergic receptor (ß-AR) signaling pathway. ß-CTX was isolated and purified using two-step chromatographic method and confirmed by N-terminal sequencer. The function of ß-CTX on cardiomyocyte was compared to propranolol in 3 conditions, basal state, with isoproterenol (ISO), and with forskolin (FSK). ß-CTX exhibited more potency than propranolol to suppress the myocyte contraction without altering calcium transient. However, ß-CTX prolonged the calcium decaying resulted in negative lusitropy. Although cardiac functions were blunted by ß-CTX in the presence of either ISO or FSK, the phosphorylation of the downstream ß-AR signaling sites were not affected. The compound was further tested on the cardiac myofibrillar ATPase activity and the kinetics. ß-CTX suppressed the maximal ATPase activity without changing Ca2+-sensitivity of the myofibril. In conclusion, the mechanism of ß-CTX on cardiomyocyte was not mediated through classical ß-AR pathway but directly suppress the actomyosin ATPase activity, depress the myofilament kinetics, and hence, reduce the cardiomyocyte functions.