Chemical modification of sarcoplasmic reticulum. Stimulation of Ca2+ release

Abstract

Pretreatment of sarcoplasmic membranes with acetic or maleic anhydrides, which interact principally with amino groups, resulted in an inhibition of Ca2+ accumulation and ATPase activity. The presence of ATP, ADP or adenosine 5'-[beta, gamma-imido]triphosphate in the modification medium selectively protected against the inactivation of ATPase activity by the anhydride but did not protect against the inhibition of Ca2+ accumulation. Acetic anhydride modification in the presence of ATP appeared to increase specifically the permeability of the sarcoplasmic reticulum membrane to Ca2+ but not to sucrose, Tris, Na+ or Pi. The chemical modification stimulated a rapid release of Ca2+ from sarcoplasmic reticulum vesicles passively or actively loaded with calcium, from liposomes reconstituted with the partially purified ATPase fraction but not from those reconstituted with the purified ATPase. The inactivation of Ca2+ accumulation by acetic anhydride (in the presence of ATP) was rapid and strongly pH-dependent with an estimated pK value above 8.3 for the reactive group(s). The negatively charged reagents pyridoxal 5-phosphate and trinitrobenzene-sulphonate, which also interact with amino groups, did not stimulate Ca2+ release. Since these reagents do not penetrate the sarcoplasmic reticulum membranes, it is proposed that Ca2+ release is promoted by modification of internally located, positively charged amino group(s).

Activation of Ca2+ release in isolated sarcoplasmic reticulum

“…Ca 2+ release from preloaded sarcoplasmic reticulure vesicles could be induced by TPB-but not vesicles were prepared as described in Materials and Methods and acetic anhydride-modified membranes were prepared as described by Shoshan-Barmatz (1986 a Ca2+ phosphate-loaded vesicles and acetic anhydride-modified membranes were prepared as described in Fig. 1.…”

“…Recent studies indicate that there is more than one type of Ca 2+ release system in the sarcoplasmic reticulum which operates via two or more types of divalent cation channels (Miyamoto & Racker, 1982;Palade, Mitchell & Fleischer, 1983;Martonosi, 1984;Taguchi & Kasai, 1984;Shoshan-Barmatz, 1986;Smith, Coronado & Meissner, 1985, 1986Suarez-Islea et al, 1986). These channels differ in: their divalent cation conductivity and specificity, their distribution across the different structural regions of the sarcoplasmic reticulum membrane and also in their activation mechanism.…”

High‐affinity Ca2+‐binding site inhibiting Ca2+ release from sarcoplasmic reticulum

“…We have studied Ca 2÷ release in unfractionated native SR membranes and have developed several different conditions for activation of Ca 2÷ release [4][5][6][7]19]. This Ca2+-release system is present in all structural regions of the SR membranes, and apparently operates via a channel different from the junctional Ca2+-release channel.…”

“…Recent studies indicate that there is more than one Ca2+-release system in the sarcoplasmic reticulum, involving two or more types of divalent cation channels [6,7,[9][10][11][12][13][14][15][16]. These channels differ …”

Change in surface charge of sarcoplasmic reticulum membrane may elicit conformational change in sulfhydryl groups of membrane proteins to release calcium.

“…The mechanism by which TPB -induces the release of CaZ + from the SR is not clear. A localized alteration of surface charge may be important to the opening and closing of the channel, as suggested by SHOSHAN et al, (1983) and SHOSHAN-BARMATZ (1986). If so, TPB -would bind to a fixed positive charge which blocks access of luminal CaZ + to the CaZ + channel.…”