Effects of β-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 β-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 µg ml–1β-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 µg 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 µg ml–1β-escin and saponin for 10 min. β-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 β-escin than saponin. This direct effect on the SR occurred at much lower concentrations for rat (2 µg ml–1β-escin and 10 µg ml–1 saponin) than toad (10 µg ml–1β-escin and 150 µg ml–1 saponin). The reverse order in sensitivities to β-escin and saponin of t-system and SR membranes indicates that the mechanisms of action of β-escin and saponin are different in the two types of membrane. In conclusion, this study shows that: (1) β-escin has a milder action on the surface membrane than saponin; (2) β-escin is a more potent modifier of SR function; (3) simple permeabilization of membranes is not sufficient to explain the effects of β-escin and saponin on muscle membranes; and (4) the t-system network within muscle fibres is not a homogeneous compartment.