The effects of depletion of intracellular Ca2+ stores on muscle tension and the intracellular Ca2+ concentration ([Ca2+])i were studied in fura-2 loaded longitudinal smooth muscle cells of the rat ileum. 2. After exposure to a Ca(2+)-free solution, application of Ca2+ caused a small contraction and a rise in [Ca2+]i, both of which were potentiated when the muscle was challenged with carbachol or caffeine before the addition of Ca2+. 3. Cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, dose-dependently decreased tension development and the rises in [Ca2+]i induced by carbachol and caffeine in the Ca(2+)-free solution, but conversely increased the Ca(2+)-induced responses even in the presence of the voltage-dependent Ca2+ channel blockers, methoxyverapamil and nifedipine. 4. The contraction and rise in [Ca2+]i evoked by Ca2+ gradually declined with time after removal of CPA, while the reverse was the case for the responses to carbachol and caffeine. 5. The Ca(2+)-induced contraction and rise in [Ca2+]i in the presence of CPA were inhibited by the replacement of Na+ with K+ or Cs+, and by the addition of Cd2+, Ba2+, Ni2+ or La3+. 6. The influx of Mn2+ was much greater in extent in the presence of CPA than in its absence. 7. These results suggest that the emptying of intracellular Ca2+ stores may activate Ca2+ influx not associated with voltage-dependent Ca2+ channels in the rat ileal smooth muscle.
