Jf. Kuemmerle et al., AGONIST-ACTIVATED, RYANODINE-SENSITIVE, IP3-INSENSITIVE CA2+ RELEASE CHANNELS IN LONGITUDINAL MUSCLE OF INTESTINE, The American journal of physiology, 266(5), 1994, pp. 30001421-30001431
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 Ca2influx activates ryanodine-sensitive Ca2+ channels to cause Ca2+-induc
ed Ca2+ release. Ryanodine bound with high affinity to longitudinal mu
scle 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+) ef
flux [50% effective concentration (EC(50)) 1.3 +/- 0.5 nM], increase i
n 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 elic
ited Ca-45(2+) efflux in both circular and longitudinal muscle cells;
efflux in longitudinal muscle cells was abolished by Ca2+ channel bloc
kers and by pretreatment of the cells with ryanodine. Pretreatment wit
h thapsigargin abolished agonist-induced Ca-45(2+) efflux in both cell
types. We conclude that ryanodine-sensitive IP3-insensitive Ca2+ rele
ase channels with properties similar to those in cardiac muscle are pr
esent in longitudinal but not circular muscle cells of intestine and t
hat agonist-mediated Ca2+ influx activates these channels, leading to
Ca2+-induced Ca2+ release.