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 (IPB) and depends on an obligatory influx of Ca2+. The present study examined whether Ca2+ influx activates ryanodine-sensitive Ca2+ channels to cause Ca2+-induced Ca2+ release. Ryanodine bound with high affinity to longitudinal muscle cells [dissociation constant (K-d) 7.3 +/- 0.3 nM] and microsomes (K-d 7.5 +/- 0.4 nM) and induced concentration-dependent Ca-45(2+) efflux [50% effective concentration (EC(50)) 1.3 +/- 0.5 nM], increase in cytosolic free Ca2+ (EC(50) 2.0 +/- 0.7 nM), and contraction (EC(50) 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 IPB or heparin. Changes in Ca2+ concentration (50-500 nM) caused Ca2+ release from permeabilized longitudinal but not circular muscle cells loaded with Ca-45(2+). The contractile agonist cholecystokinin-8 elicited Ca-45(2+) 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 Ca-45(2+) 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 Ca2+-induced Ca2+ release.