Inositol 1,4,5-trisphosphate increases myoplasmic [Ca 2+] in isolated muscle fibers. Depolarization enhances its effects

Abstract

Inositol 1,4,5-trisphosphate (InsP 3) has been proposed as an intracellular messenger which mobilizes calcium from the sarcoplasmic reticulum, during excitation-contraction coupling in skeletal muscle. We have measured the myoplasmic free calcium concentration ([Ca 2+] i) by means of calcium selective microelectrodes in intact fibers isolated from Leptodactylus insularis microinjected with InsP 3. In muscle fibers bathed in normal Ringer, the mean resting [Ca 2+] i was 0.11 ± 0.01 μM (M ± SEM, n = 30). The microinjection of 0.3, 0.5 and 1 μM InsP 3 induced transient increments in the [Ca 2+] i to 0.35 ± 0.02 μM (n = 9), to 0.53 ± 0.03 μM (n = 11) and 0.94 ± 0.06 μM (n = 10) respectively. Microinjection of 0.3, 0.5 and 1 μM InsP 3 in muscle fibers incubated in low Ca 2+ solution induced increments in [Ca 2+] i similar to those observed in fibers bathed with normal Ringer. The microinjection of 0.3, 0.5 and 1 μM InsP 3 in muscle fibers partially depolarized with 10 mM [K +] o induced transient enhancements of the resting [Ca 2+] i that were greater than the transients observed in the normally polarized muscle. In partially depolarized fibers microinjected with 0.3, 0.5 and 1 μM InsP 3, the [Ca 2+] i was changed to 1.45 ± 0.14 μM (n = 20), to 3.37 ± 0.34 μM (n = 7) and to 7.43 ± 0.70 μM (n = 6) respectively. In all partially depolarized fibers these increments in [Ca 2+] i were associated with local contraction. These data support the hypothesis that InsP 3 could play an important role as an intracellular messenger involved in the calcium release process in skeletal muscle, and that the InsP 3 effect on Ca 2+ release might be regulated by the membrane potential.