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 a gents on the sealed transverse tubular system (t-system) and sarcoplasmic r eticulum (SR) were examined by looking at changes in the magnitude of the f orce responses to t-system depolarization, the time course of the fluoresce nce of fura-2 trapped in the sealed t-system, and changes in the magnitude of caffeine-induced contractures following SR loading with Ca2+ under defin ed conditions. In the presence of 2 mu g ml(-1) beta-escin and saponin, the response to t-system depolarization was not completely abolished, decreasi ng to a plateau, and a large proportion of fura-2 remained in the sealed t- system. At 10 mu g ml(-1), both agents abolished the ability of both rat an d toad preparations to respond to t-system depolarization after 3 min of ex posure, but a significant amount of fura-2 remained in sealed t-tubules eve n after exposure to 100 mu g 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 th e SR to load Ca2+ was reduced to a lower level after treatment with beta-es cin than saponin. This direct effect on the SR occurred at much lower conce ntrations for rat (2 mu g ml(-1) beta-escin and 10 mu g ml(-1) saponin) tha n toad (10 mu g ml(-1) beta-escin and 150 mu g ml(-1) saponin). The reverse order in sensitivities to beta-escin and saponin of t-system and SR membra nes indicates that the mechanisms of action of beta-escin and saponin are d ifferent 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 permeab ilization of membranes is not sufficient to explain the effects of beta-esc in and saponin on muscle membranes; and (4) the t-system network within mus cle fibres is not a homogeneous compartment.