Effects of tetraphenylboron-induced increase in inner surface charge on Ca2+ release from sarcoplasmic reticulum

We studied the relationship between surface charge and release of Ca2+ in the heavy sarcoplasmic reticulum (SR) of skeletal muscle. The inner and outer surface potentials and charge densities of the membrane treated with a lipophilic anion, tetraphenylboron (TPB-), were measured using 1-anilino-8-naphthalene-sulfonate fluorescence. Ca2+ was loaded passively or actively by the SR. Ca2+ release was estimated by the fluorescence of chlortetracycline, and protein conformational change was monitored by use of the sulfhydryl group fluorescent probe, N-(7-dimethylamino-4-methyl-3-coumarinyl) maleimide (DACM). Treatment of Ca2(+)-loaded SR vesicles with micromolar TPB- dose-dependently increased the local fixed negative charge on the inner surface, and changed the DACM fluorescence intensity in parallel with the Ca2+ release. The changes in surface charge and in DACM fluorescence intensity did not originate from the Ca2+ flux. A lipophilic cation, tetraphenylarsonium (TPA+), screened the negative inner surface charge which was increased by TPB-, and inhibited both TPB(-)-induced change in DACM fluorescence intensity and Ca2+ release. Millimolar Mg2+ reduced degrees of TPB(-)-induced Ca2+ release from the SR and of TPB(-)-induced contraction in mechanically skinned fibers. Mg2+ did not inhibit the increase in the negative inner surface charge and DACM fluorescence intensity produced by TPB-. Thus, the local increase in negative charge on the SR inner membrane leaflet seems to be causally related to the Ca2+ release. Mg2+ and TPA+ are suggested to inhibit TPB(-)-induced Ca2+ release by different mechanisms.