Inositol 1,4,5-trisphosphate increases myoplasmic [Ca2+] in isolated muscle fibers. Depolarization enhances its effects.
Inositol 1,4,5-trisphosphate (InsP3) 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 ([Ca2+]i) by means of calcium selective microelectrodes in intact fibers isolated from Leptodactylus insularis microinjected with InsP3. In muscle fibers bathed in normal Ringer, the mean resting [Ca2+]i was 0.11 +/- 0.01 microM (M +/- SEM, n = 30). The microinjection of 0.3, 0.5 and 1 microM InsP3 induced transient increments in the [Ca2+]i to 0.35 +/- 0.02 microM (n = 9), to 0.53 +/- 0.03 microM (n = 11) and 0.94 +/- 0.06 microM (n = 10) respectively. Microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers incubated in low Ca2+ solution induced increments in [Ca2+]i similar to those observed in fibers bathed with normal Ringer. The microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers partially depolarized with 10 mM [K+]o induced transient enhancements of the resting [Ca2+]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 microM InsP3, the [Ca2+]i was changed to 1.45 +/- 0.14 microM (n = 20), to 3.37 +/- 0.34 microM (n = 7) and to 7.43 +/- 0.70 microM (n = 6) respectively. In all partially depolarized fibers these increments in [Ca2+]i were associated with local contraction.(ABSTRACT TRUNCATED AT 250 WORDS)
Lopez
J R
JR
Instituto Venezolano de Investigaciones Cientificas (IVIC), Centro de Biofisica y Bioquimica, Caracas, Venezuela.
Parra
L
L
D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D002463:Cell Membrane Permeability / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D015544:Inositol 1,4,5-Trisphosphate / Q000008:administration & dosage / Q000494:pharmacology*; D008564:Membrane Potentials / Q000502:physiology*; D008839:Microelectrodes; D008845:Microinjections; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000378:metabolism* / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium