The role of a ryanodine-sensitive Ca(2+)-store in the regulation of smooth muscle tone of the cat gastric fundus.

The smooth muscle of the gastric fundus maintains spontaneous tone, but the mechanism underlying this activity is not fully understood. The aim of the present study was to examine whether Ca2+ release from the sarcoplasmic reticulum (SR) could play a role in the maintenance of the spontaneous smooth muscle tone of the cat gastric fundus. The effects on the contractile activity of SR Ca2+ release activators ryanodine and caffeine and of the inhibitor ruthenium red were studied. The contractile activity of isolated muscle strips was recorded under isometric conditions using organ baths. Ryanodine concentration-dependently (10(-7)-3.10(-5) mol/l) increased the tone of the fundus strips. In the presence of nifedipine (10(-5) mol/l), ryanodine induced a nifedipine-resistant tonic contraction. The tonic contraction induced after inhibition of the SR Ca(2+)-pump by cyclopiazonic acid was potentiated by ryanodine (10(-5) mol/l). In strips precontracted with supramaximal concentration of acetylcholine (10(-5) mol/l), ryanodine (3.10(-5) mol/l) further potentiated the tone. Caffeine (10(-4)-10(-2) mol/l) decreased and even completely inhibited the tone, suggesting some other effects of caffeine. Ruthenium red concentration-dependently (10(-6)-10(-4) mol/l) decreased the tone. The present data provide evidence for the role of Ca2+ release from a SR ryanodine-sensitive Ca2+ store in the maintenance of the muscle tone of the cat gastric fundus.