Agonist-activated, ryanodine-sensitive, IP3-insensitive Ca2+ release channels in longitudinal muscle of intestine.
We have previously shown that Ca2+ mobilization in longitudinal muscle is not mediated by inositol 1,4,5-trisphosphate (IP3) and depends on an obligatory influx of Ca2+. The present study examined whether Ca2+ influx activates ryanodine-sensitive Ca2+ channels to cause Ca(2+)-induced Ca2+ release. Ryanodine bound with high affinity to longitudinal muscle cells [dissociation constant (Kd) 7.3 +/- 0.3 nM] and microsomes (Kd 7.5 +/- 0.4 nM) and induced concentration-dependent 45Ca2+ efflux [50% effective concentration (EC50) 1.3 +/- 0.5 nM], increase in cytosolic free Ca2+ (EC50 2.0 +/- 0.7 nM), and contraction (EC50 0.9 +/- 0.2 nM) but had no effect in circular muscle cells. Ryanodine binding and ryanodine-induced Ca2+ release were enhanced by caffeine and inhibited by dantrolene and ruthenium red but were not affected by IP3 or heparin. Changes in Ca2+ concentration (50-500 nM) caused Ca2+ release from permeabilized longitudinal but not circular muscle cells loaded with 45Ca2+. The contractile agonist cholecystokinin-8 elicited 45Ca2+ efflux in both circular and longitudinal muscle cells; efflux in longitudinal muscle cells was abolished by Ca2+ channel blockers and by pretreatment of the cells with ryanodine. Pretreatment with thapsigargin abolished agonist-induced 45Ca2+ efflux in both cell types. We conclude that ryanodine-sensitive IP3-insensitive Ca2+ release channels with properties similar to those in cardiac muscle are present in longitudinal but not circular muscle cells of intestine and that agonist-mediated Ca2+ influx activates these channels, leading to Ca(2+)-induced Ca2+ release.
Kuemmerle
J F
JF
Department of Medicine, Medical College of Virginia, Richmond 23298-0711.
Murthy
K S
KS
Makhlouf
G M
GM
D000818:Animals; D001667:Binding, Competitive; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D003620:Dantrolene / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism* / Q000494:pharmacology*; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism*; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009124:Muscle Proteins / Q000378:metabolism*; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011817:Rabbits; D012430:Ruthenium Red / Q000494:pharmacology; D012433:Ryanodine / Q000378:metabolism* / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D013997:Time Factors
D015220:Calcium Channels; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D013729:Terpenes; D012430:Ruthenium Red; D012433:Ryanodine; D002110:Caffeine; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D006493:Heparin; D000252:Calcium-Transporting ATPases; D003620:Dantrolene; D002118:Calcium