SULFHYDRYL-REAGENTS TRIGGER CA2+ RELEASE FROM THE SARCOPLASMIC-RETICULUM OF SKINNED RABBIT PSOAS FIBERS

1. By analogy with studies on sarcoplasmic reticulum (SR) vesicles, Ca2+ release induced by heavy metals and mercaptans (e.g. cysteine) was investigated in rabbit skinned psoas fibres through measurements of isometric tension. 2. Heavy metals (at 2-5-mu-M) elicited phasic contractions by triggering Ca2+ release from the SR and had the following order of potency: Hg2+ > Cu2+ > Cd2+ > Ag+ > Ni2+. Higher concentrations produced tonic contractions due to maintained high Ca2+ permeability of SR membranes. 3. Contractions induced by heavy metals required a functional and Ca2+-loaded SR, were dependent on [Ca2+]free, blocked by Ruthenium Red (RR), inhibited by free Mg2+ and reduced glutathione (GSH) but not by oxidized glutathione (GSSG). Such contractions were not elicited through direct interaction(s) of heavy metals with the myofilaments. 4. In the presence of catalytic concentrations of Hg2+ or Cu2+ (2-5-mu-M), additions of cysteine (25-100-mu-M) elicited rapid twitches, producing 70% of maximal force with a time to half-peak of 2 s. Contractions induced by cysteine plus a catalyst required a functional SR network, were dependent on free [Mg2+] and were blocked by RR or GSH but not by GSSG. 5. In the presence of Hg2+ (2-5-mu-M), low concentrations of cysteine (10-mu-M) elicited tonic contractures, but subsequent or higher additions of cysteine (50-100-mu-M) caused further SR Ca2+ release and tension, followed by rapid and full relaxation. 6. High cysteine (200-250-mu-M, without Cu2+ or Hg2+) blocked contractions elicited by Cl- induced depolarization of sealed T-tubules. High cysteine probably acted as a sulphydryl reducing agent which promoted rapid relaxation of the fibres through the closure of Ca2+-release channels and ATP-dependent re-uptake of Ca2+ by the SR. 7. In some batches of skinned fibres (approximately 10%), cysteine (5-50-mu-M) alone (without Hg2+ or Cu2+ catalyst) produced rapid twitches. This implied that the catalyst(s) necessary to promote the sulphydryl oxidation reaction with exogenously added cysteine may be present in intact fibres but is usually lost by the skinning procedure. 8. The data demonstrate that skeletal fibres contain a highly reactive and accessible sulphydryl site on an SR protein which can be reversibly oxidized and reduced to respectively, open and close SR Ca2+-release channels. A model of sulphydryl-gated excitation-contraction coupling is proposed where the voltage sensor on the T-tubule membrane directly oxidizes sulphydryl sites on SR Ca2+-release channels.