Control of the phasic and tonic contractions of guinea pig stomach by a ryanodine-sensitive Ca2+ store

In some smooth muscle cells, the rise in intracellular Ca2+ as a result of a Ca2+ influx via plasma membrane Ca2+ channels can activate a further incr ease in intracellular Ca2+ as a result of Ca2+ release from intracellular s tores. This study examined the role of the Ca2+-induced Ca2+ release from t he ryanodine-sensitive intracellular Ca2+ stores in shaping the smooth musc le contractions of guinea pig stomach. The contractile activity of isolated muscle strips of the fundus, corpus and antrum region of the stomach was r ecorded under isometric conditions. Ryanodine, an activator of Ca2+-induced Ca2+ release, concentration dependently (10(-7)-3.10(-5) M) increased the tone of fundus and corpus strips. Ryanodine had a dual action on the phasic contractions of the antrum and corpus: increase by the low concentrations (up to 10(-6) M) and inhibition by the high concentrations (10(-6)-3.10(-5) M). Nifedipine (10(-5) M) completely inhibited the ryanodine (10(-6) M)-in duced phasic contractions and only partly the ryanodine (3.10(-5) M)-induce d tonic contractions. In the presence of 10(-5) M cyclopiazonic acid, a spe cific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, ryanodine (3.10(-5) M) further increased the tone of the corpus and fundus strips. Ryanodine (3 .10(-5) M) induced tonic contractions in the fundus and corpus precontracte d by acetylcholine(10(-5) M), and inhibited the acetylcholine(10(-6) M)-ind uced phasic contractions in the antrum and corpus. Ruthenium red, an inhibi tor of Ca2+-induced Ca2+ release, concentration dependently (10(-6)-10(-4) M) decreased the tone and amplitude of the phasic contractions. The data ob tained provide evidence for the participation of a sarcoplasmic reticulum C a2+-induced Ca2+ release mechanism in shaping the tonic and phasic contract ions of guinea pig stomach, and highlight important tissue differences. (C) 1999 Elsevier Science B.V. All rights reserved.