Silver ions induce Ca2+ release from the SR in vitro by acting on the Ca2+ release channel and the Ca2+ pump

Abstract

Silver nitrate (AgNO3) is a sulfhydryl oxidizing agent that induces a biphasic Ca2+ release from isolated sarcoplasmic reticulum (SR) vesicles by presumably oxidizing critical sulfhydryl groups in the Ca2+ release channel (CRC), causing the channel to open. To further examine the effects of AgNO3 on the CRC and the Ca2+-ATPase, Ca2+ release was measured in muscle homogenates prepared from rat hindlimb muscle using indo 1. Cyclopiazonic acid (CPA) and ruthenium red (RR) were used to inhibit the Ca2+-ATPase and block the CRC, respectively, before inducing Ca2+ release with both AgNO3 and 4-chloro- m-cresol (4-CMC), a releasing agent specific for the CRC. With AgNO3 and CPA, the early rapid rate of release (phase 1) was increased ( P < 0.05) by 42% (314 ± 5 vs. 446 ± 39 μmol · g protein−1 · min−1), whereas the slower, more prolonged rate of release (phase 2) was decreased ( P < 0.05) by 72% (267 ± 39 vs. 74 ± 7.7 μmol · g protein−1 · min−1). RR, in combination with AgNO3, had no effect on phase 1 ( P > 0.05) (314 ± 51 vs. 334 ± 43 μmol · g protein−1 · min−1) and decreased phase 2 ( P < 0.05) by 65% (245 ± 34 vs. 105 ± 8.2 μmol · g protein−1 · min−1). With 4-CMC, CPA had no effect ( P > 0.05) on either phase 1 or 2. With addition of RR, phase 1 was reduced ( P < 0.05) by 59% (2,468 ± 279 vs. 1,004 ± 87 μmol · g protein−1 · min−1), and RR completely blocked phase 2. Both AgNO3 and 4-CMC fully inhibited Ca2+-ATPase activity measured in homogenates. These findings indicate that AgNO3, but not 4-CMC, induces Ca2+ release by acting on both the CRC and the Ca2+-ATPase.