Functionally separate intracellular Ca2+ stores in smooth muscle.
In smooth muscle, release via the inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)R) and ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) controls oscillatory and steady-state cytosolic Ca(2+) concentrations ([Ca(2+)](c)). The interplay between the two receptors, itself determined by their organization on the SR, establishes the time course and spatial arrangement of the Ca(2+) signal. Whether or not the receptors are co-localized or distanced from each other on the same store or whether they exist on separate stores will significantly affect the Ca(2+) signal produced by the SR. To date these matters remain unresolved. The functional arrangement of the RyR and Ins(1,4,5)P(3)R on the SR has now been examined in isolated single voltage-clamped colonic myocytes. Depletion of the ryanodine-sensitive store, by repeated application of caffeine, in the presence of ryanodine, abolished the response to Ins(1,4,5)P(3), suggesting that Ins(1,4,5)P(3)R and RyR share a common Ca(2+) store. Ca(2+) release from the Ins(1,4,5)P(3)R did not activate Ca(2+)-induced Ca(2+) release at the RyR. Depletion of the Ins(1,4,5)P(3)-sensitive store, by the removal of external Ca(2+), on the other hand, caused only a small decrease ( approximately 26%) in caffeine-evoked Ca(2+) transients, suggesting that not all RyR exist on the common store shared with Ins(1,4,5)P(3)R. Dependence of the stores on external Ca(2+) for replenishment also differed; removal of external Ca(2+) depleted the Ins(1,4,5)P(3)-sensitive store but caused only a slight reduction in caffeine-evoked transients mediated at RyR. Different mechanisms are presumably responsible for the refilling of each store. Refilling of both Ins(1,4,5)P(3)-sensitive and caffeine-sensitive Ca(2+) stores was inhibited by each of the SR Ca(2+) ATPase inhibitors thapsigargin and cyclopiazonic acid. These results may be explained by the existence of two functionally distinct Ca(2+) stores; the first expressing only RyR and refilled from [Ca(2+)](c), the second expressing both Ins(1,4,5)P(3)R and RyR and dependent upon external Ca(2+) for refilling.
Flynn
E R
ER
Neuroscience and Biomedical Systems, Institute of Biomedical and Life Sciences, West Medical Bldg., University of Glasgow, Glasgow G12 8QQ, United Kingdom.
Bradley
K N
KN
Muir
T C
TC
McCarron
J G
JG
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000737:chemistry; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D006168:Guinea Pigs; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008564:Membrane Potentials; D009130:Muscle, Smooth / Q000378:metabolism*; D009132:Muscles / Q000166:cytology; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000737:chemistry / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D013997:Time Factors
D015220:Calcium Channels; D000697:Central Nervous System Stimulants; D002614:Chelating Agents; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002110:Caffeine; D004533:Egtazic Acid; D019284:Thapsigargin; D000242:Cyclic AMP; D002118:Calcium