Ž . European Journal of Pharmacology 348 1998 167–173 Tension generation and increase in voltage-activated Na q current by crotamine Alessandra C.S. Matavel a , Dalton L. Ferreira-Alves b , Paulo S.L. Beirao a , Jader S. Cruz a, ) ˜ a Departamento de Bioquımica e Imunologia, Instituto de Ciencias Biologicas, UniÕersidade Federal de Minas Gerais, ´ ˆ ´ Caixa Postal 486, 30161-970 Belo Horizonte, MG, Brazil b Departamento de Farmacologia, Instituto de Ciencias Biologicas, UniÕersidade Federal de Minas Gerais, ˆ ´ Caixa Postal 486, 30161-970 Belo Horizonte, MG, Brazil Received 2 October 1997; revised 12 February 1998; accepted 17 February 1998 Abstract We performed the present experiments to study the action of crotamine, a toxin isolated from the venom of the South American rattlesnake, Crotalus durissus terrificus, on macroscopic Na q currents in frog skeletal muscle by using the loose patch clamp technique. q Ž . Crotamine at 50 mM increased the peak Na current by 50% P -0.05 . In addition, the voltage dependence of inactivation was shifted q Ž by q8 mV. Other parameters of Na currents reversal potential, voltage-dependence of activation and time courses of inactivation, of . activation and of removal of inactivation were not significantly affected. We suggest that crotamine inhibits the direct transition of channels from closed to inactivated states, thereby forcing their transition through the open states. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Na q channel; Crotamine; Myotoxin; Crotalus durissus terrificus; Skeletal muscle, frog; Patch clamp, loose; Myonecrosis 1. Introduction Na q channels are the main targets for many toxins of widely diverse structure that exist in the venom of arthro- pods, coelenterates, microorganisms, fish, plants, but gen- Ž erally not in snake venoms reviewed by Adams and . Swanson, 1996 . One reported exception for this rule is crotamine, which has been suggested to increase the Na q permeability of skeletal muscle membranes. Crotamine induces a depolarization that is Na q -dependent and blocked by tetrodotoxin, and potentiates the depolarizing effects of Ž veratrine, batrachotoxin and grayanotoxin I Chang et al., 1983; Cheymol et al., 1964, 1971a,b; Hong and Chang, . 1983, 1989; Vital Brazil and Fontana, 1993 . Crotamine, which is obtained from the venom of the Ž . South American rattlesnake Crotalus durissus terrificus , is a single-chain polypeptide consisting of 42 amino-acid Ž residues but lacks alanine, valine and threonine Laure, ) Corresponding author. Tel.: q55-31-499-2663; fax: q55-31-441- 5963; e-mail: jader@mono.icb.ufmg.br . 1975 . It shares high primary sequence homology with Ž other myonecrosis-causing toxins Smith and Schmidt, . Ž . 1990 such as peptide C Maeda et al., 1978 ; myotoxin a Ž . Ž Fox et al., 1979 and myotoxins I and II Bieber et al., . 1987 . Crotamine and the myotoxins have a well character- Ž ized myonecrotic effect Ownby et al., 1988; Cameron and . Tu, 1978 . Crotamine induces contracture of mammalian Ž skeletal muscle, both in situ and in vitro Cheymol et al., . Ž . 1971b; Chang and Tseng, 1978 . Chang and Tseng 1978 Ž . and Vital Brazil and Fontana 1993 , using rat neuromus- cular preparations, have suggested that the membrane de- polarization induced by crotamine is the cause of the contracture. However, the primary mechanism by which crotamine acts has not yet been fully clarified. The purpose of the present study was to investigate further the effects of crotamine by directly assessing skele- tal muscle Na q channel function under voltage-clamp conditions. An improved loose patch-clamp technique was used as a convenient method to study macroscopic Na q Ž . currents I in intact muscle fibers. Na Some of these data have been presented in abstract form Ž . XIIth International Biophysical Congress, 1996 . 0014-2999r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.