title: Fast release of 45Ca2+ induced by inositol 1,4,5-trisphosphate and Ca2+ in the sarcoplasmic reticulum of rabbit skeletal muscle: evidence for two types of Ca2+ release channels.
abstract: The kinetics of Ca2+ release induced by the second messenger D-myoinositol 1,4,5 trisphosphate (IP3), by the hydrolysis-resistant analogue D-myoinositol 1,4,5 trisphosphorothioate (IPS3), and by micromolar Ca2+ were resolved on a millisecond time scale in the junctional sarcoplasmic reticulum (SR) of rabbit skeletal muscle. The total Ca2+ mobilized by IP3 and IPS3 varied with concentration and with time of exposure. Approximately 5% of the 45Ca2+ passively loaded into the SR was released by 2 microM IPS3 in 150 ms, 10% was released by 10 microM IPS3 in 100 ms, and 20% was released by 50 microM IPS3 in 20 ms. Released 45Ca2+ reached a limiting value of approximately 30% of the original load at a concentration of 10 microM IP3 or 25-50 microM IPS3. Ca(2+)-induced Ca2+ release (CICR) was studied by elevating the extravesicular Ca2+ while maintaining a constant 5-mM intravesicular 45Ca2+. An increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 55 +/- 7% of the passively loaded 45Ca2+ in 150 ms. CICR was blocked by 5 mM Mg2+ or by 10 microM ruthenium red, but was not blocked by heparin at concentrations as high as 2.5 mg/ml. In contrast, the release produced by IPS3 was not affected by Mg2+ or ruthenium red but was totally inhibited by heparin at concentrations of 2.5 mg/ml or lower. The release produced by 10 microM Ca2+ plus 25 microM IPS3 was similar to that produced by 10 microM Ca2+ alone and suggested that IP3-sensitive channels were present in SR vesicles also containing ruthenium red-sensitive Ca2+ release channels. The junctional SR of rabbit skeletal muscle may thus have two types of intracellular Ca2+ releasing channels displaying fast activation kinetics, namely, IP3-sensitive and Ca(2+)-sensitive channels.
authors.affiliation: Department of Physiology, University of Wisconsin Medical School, Madison 53706.
authors.forename: C
authors.initials: C
authors.lastname: Valdivia
authors.forename: D
authors.initials: D
authors.lastname: Vaughan
authors.forename: B V
authors.initials: BV
authors.lastname: Potter
authors.forename: R
authors.initials: R
authors.lastname: Coronado
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology* / Q000031:analogs & derivatives; D007295:Inositol Phosphates / Q000494:pharmacology; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000187:drug effects / Q000378:metabolism; D009946:Organothiophosphorus Compounds / Q000494:pharmacology; D011817:Rabbits; D011950:Receptors, Cholinergic / Q000378:metabolism; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism
chemical_list: D015220:Calcium Channels; D007295:Inositol Phosphates; D009946:Organothiophosphorus Compounds; D011950:Receptors, Cholinergic; D019837:Ryanodine Receptor Calcium Release Channel; C054737:inositol 1,4,5-triphosphorothioate; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium