Tetraphenylboron causes Ca2+ release in isolated sarcoplasmic reticulum and in skinned muscle fibers

Abstract

The lipophilic anion tetraphenylboron (TPB-) but not the lipophilic cation tetraphenylarsonium (TPA+) inhibited ATP-dependent Ca2+ accumulation by isolated sarcoplasmic reticulum. TPB- did not inhibit ATP hydrolysis but did induce Ca2+ release from preloaded vesicles. It did not appear to disrupt lipid bilayers or to act as a Ca2+ ionophore since it had no effect on the Ca2+ content of phospholipid vesicles. TPB- also induced Ca2+ release from sarcoplasmic reticulum in chemically skinned muscle fibers causing tension development. In contrast to other Ca2+-releasing agents such as caffeine, proton ionophores, or quercetin, the rise to peak tension was slow and tension was sustained, suggesting that Ca2+ release channels, once opened by TPB-, were held open as long as the compound was present in the membrane. Ca2+ uptake was re-established upon removal of TPB- or addition of TPA+. TPB- or TPA+ would probably distribute within the membrane, altering surface charges on both sides of the membrane. The fact that only a negatively charged ion brought about opening of Ca2+ release channels suggests that specific surface charges control Ca2+ release channels in sarcoplasmic reticulum. Although we have not been able to prove that TPB- acts exclusively on physiologically relevant Ca2+ release channels, we have shown that TPB- does not release Ca2+ from proteoliposomes reconstituted with the Ca2+ + Mg2+ ATPase. Thus TPB- does not induce Ca2+ release through channels formed by the ATPase molecule.