Effects of beta-escin and saponin on the transverse-tubular system and sarcoplasmic reticulum membranes of rat and toad skeletal muscle.
Mechanically skinned skeletal muscle fibres from rat and toad were exposed to the permeabilizing agents beta-escin and saponin. The effects of these agents on the sealed transverse tubular system (t-system) and sarcoplasmic reticulum (SR) were examined by looking at changes in the magnitude of the force responses to t-system depolarization, the time course of the fluorescence of fura-2 trapped in the sealed t-system, and changes in the magnitude of caffeine-induced contractures following SR loading with Ca2+ under defined conditions. In the presence of 2 microg ml-1 beta-escin and saponin, the response to t-system depolarization was not completely abolished, decreasing to a plateau, and a large proportion of fura-2 remained in the sealed t-system. At 10 microg ml-1, both agents abolished the ability of both rat and toad preparations to respond to t-system depolarization after 3 min of exposure, but a significant amount of fura-2 remained in sealed t-tubules even after exposure to 100 microg ml-1 beta-escin and saponin for 10 min. beta-Escin took longer than saponin to reduce the t-system depolarizations and fura-2 content of the sealed t-system to a similar level. The ability of the SR to load Ca2+ was reduced to a lower level after treatment with beta-escin than saponin. This direct effect on the SR occurred at much lower concentrations for rat (2 microg ml-1 beta-escin and 10 microg ml-1 saponin) than toad (10 microg ml-1 beta-escin and 150 microg ml-1 saponin). The reverse order in sensitivities to beta-escin and saponin of t-system and SR membranes indicates that the mechanisms of action of beta-escin and saponin are different in the two types of membrane. In conclusion, this study shows that: (1) beta-escin has a milder action on the surface membrane than saponin; (2) beta-escin is a more potent modifier of SR function; (3) simple permeabilization of membranes is not sufficient to explain the effects of beta-escin and saponin on muscle membranes; and (4) the t-system network within muscle fibres is not a homogeneous compartment.
Launikonis
B S
BS
Department of Zoology, La Trobe University, Plenty Rd, Bundoora 3083, Victoria, Australia. zoobl@zoo.latrobe.edu.au
Stephenson
D G
DG
D000818:Animals; D002024:Bufo marinus; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D004928:Escin / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000187:drug effects*; D007700:Kinetics; D008297:Male; D008870:Microtubules / Q000187:drug effects*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000648:ultrastructure*; D051381:Rats; D020318:Rats, Long-Evans; D012503:Saponins / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000648:ultrastructure*
D012503:Saponins; D002110:Caffeine; D004928:Escin; D002118:Calcium