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 increase in intracellular Ca2+ as a result of Ca2+ release from intracellular stores. This study examined the role of the Ca2+-induced Ca2+ release from the ryanodine-sensitive intracellular Ca2+ stores in shaping the smooth muscle contractions of guinea pig stomach. The contractile activity of isolated muscle strips of the fundus, corpus and antrum region of the stomach was recorded under isometric conditions. Ryanodine, an activator of Ca2+-induced Ca2+ release, concentration dependently (10(-7)-3x10(-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)-3x10(-5) M). Nifedipine (10(-5) M) completely inhibited the ryanodine (10(-6) M)-induced phasic contractions and only partly the ryanodine (3x10(-5) M)-induced tonic contractions. In the presence of 10(-5) M cyclopiazonic acid, a specific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, ryanodine (3x10(-5) M) further increased the tone of the corpus and fundus strips. Ryanodine (3x10(-5) M) induced tonic contractions in the fundus and corpus precontracted by acetylcholine (10(-5) M), and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the antrum and corpus. Ruthenium red, an inhibitor of Ca2+-induced Ca2+ release, concentration dependently (10(-6)-10(-4) M) decreased the tone and amplitude of the phasic contractions. The data obtained provide evidence for the participation of a sarcoplasmic reticulum Ca2+-induced Ca2+ release mechanism in shaping the tonic and phasic contractions of guinea pig stomach, and highlight important tissue differences.