title: Agonist-induced phasic and tonic responses in smooth muscle are mediated by InsP(3).
abstract: Many cellular functions are regulated by agonist-induced InsP(3)-evoked Ca2+ release from the internal store. In non-excitable cells, predominantly, the initial Ca2+ release from the store by InsP(3) is followed by a more sustained elevation in [Ca2+](i) via store-operated Ca2+ channels as a consequence of depletion of the store. Here, in smooth muscle, we report that the initial transient increase in Ca2+, from the internal store, is followed by a sustained response also as a consequence of depletion of the store (by InsP(3)), but, influx occurs via voltage-dependent Ca2+ channels. Contractions were measured in pieces of whole distal colon and membrane currents and [Ca2+](i) in single colonic myocytes. Carbachol evoked phasic and tonic contractions; only the latter were abolished in Ca2+-free solution. The tonic component was blocked by the voltage-dependent Ca2+ channel blocker nimodipine but not by the store-operated channel blocker SKF 96365. InsP(3) receptor inhibition, with 2-APB, attenuated both the phasic and tonic components. InsP(3) may regulate tonic contractions via sarcolemma Ca2+ entry. In single cells, depolarisation (to approximately -20 mV) elevated [Ca2+](i) and activated spontaneous transient outward currents (STOCs). CCh suppressed STOCs, as did caffeine and InsP(3). InsP(3) receptor blockade by 2-APB or heparin prevented CCh suppression of STOCs; protein kinase inhibition by H-7 or PKC(19-36) did not. InsP(3) suppressed STOCs by depleting a Ca2+ store accessed separately by the ryanodine receptor (RyR). Thus depletion of the store by RyR activators abolished the InsP(3)-evoked Ca2+ transient. RyR inhibition (by tetracaine) reduced only STOCs but not the InsP(3) transient. InsP(3) contributes to both phasic and tonic contractions. In the former, muscarinic receptor-evoked InsP(3) releases Ca2+ from an internal store accessed by both InsP(3) and RyR. Depletion of this store by InsP(3) alone suppresses STOCs, depolarises the sarcolemma and permits entry of Ca2+ to generate the tonic component. Therefore, by lowering the internal store Ca2+ content, InsP(3) may generate a sustained smooth muscle contraction. These results provide a mechanism to account for phasic and tonic smooth muscle contraction following receptor activation.
authors.affiliation: Neuroscience and Biomedical Systems, Institute of Biomedical and Life Sciences, West Medical Building, University of Glasgow, G12 8QQ, UK. j.mccarron@bio.gla.ac.uk
authors.forename: John G
authors.initials: JG
authors.lastname: McCarron
authors.forename: John W
authors.initials: JW
authors.lastname: Craig
authors.forename: Karen N
authors.initials: KN
authors.lastname: Bradley
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Muir
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D003106:Colon / Q000187:drug effects / Q000378:metabolism; D004594:Electrophysiology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D013748:Tetracaine / Q000494:pharmacology
chemical_list: D019837:Ryanodine Receptor Calcium Release Channel; D013748:Tetracaine; D012433:Ryanodine; D002110:Caffeine; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D011493:Protein Kinase C; D002118:Calcium