Studies on the Ca2 Release from Intracellular Stores in Permeabilized Smooth Muscle

STUDIES ON THE Ca2+ RELEASE FROM INTRACELLULAR STORES IN PERMEABILIZED SMOOTH MUSCLE  

Smooth muscle preparations permeabilized with saponin, p-escin, or a-toxin have significantly contributed to the understanding of Ca2+ release from intra- 

In general, the stores are loaded with Ca + buffered with millimolar concentrations of EGTA (Endo et al., 1977 Saida and Nonomura, 1978 Itoh et al., 1982a,b Saida, 1982). The stores are fully loaded within 3 to 5 min at 1 (jlM Ca2+ (Saida, 1982), whereby the Ca2+ uptake depends on the Ca + concentration (Saida, 1982 Yamamoto and van Breemen, 1986). At Ca + concentrations greater than 1 jlM, a Ca +-induced Ca release was observed (Itoh et al., 1981 Saida, 1982). The deposition of Ca2+ in the SR of saponin-permeabilized smooth muscle was demonstrated by electron probe X-ray microanalysis (Kowasaki et al., 

The stored Ca + can be released by either caffeine orlPg. Many studies (e.g., Endo eta/., 1977 Saida and Nonomura, 1978 Itoh et al, 1982a,b Saida, 1982) used the transient tension response elicited by the released Ca2+ as a measure of Ca + uptake into the SR. In order to observe a contractile effect, a solution with a weak Ca + buffering capacity (0.1 mM EGTA) has to be used so that Ca + released from the store, instead of being bound by the buffer system, is made available to the contractile elements. The limitations of this index of Ca + uptake have been discussed previously (Meisheri et al, 1985 Yamamoto and van Breemen, 

The most serious limitation probably is that the experimental procedures may not only affect Ca + release but also the Ca + sensitivity of the myofilaments. Therefore, some authors investigated the properties of the Ca + stores with sCa flux measurements (Stout and Diecke, 1983 Yamamoto and van Breemen, 1986 Missiaen et al., 1991, 1993). 

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