Thapsigargin-induced tone and capacitative calcium influx in mouse anococcygeus smooth muscle cells

The sarcoplasmic reticulum Ca-ATPase inhibitor thapsigargin (Tg; 0.4-100 nM) produced concentration-related, strong and sustained contractions of the mouse-isolated anococcygeus muscle; these contractions were dependent on extracellular calcium but were only partially reduced (by about 50%) in the presence of verapamil (10 and 100 microM). The verapamil-resistant component of the Tg-induced contraction was relaxed by the general calcium entry blockers SKF96365 (0.4-40 microM) and cadmium (50-300 microM), and by the tyrosine kinase inhibitor genistein (10-180 microM). In single smooth muscle cells loaded with Fura-2, addition of Tg (100 nM) to calcium-free medium produced a small, transient increase in fluorescence; subsequent addition of calcium (2.5 mM) produced a larger and sustained increase which was abolished on return to calcium-free conditions, but was only partially reduced by verapamil (10 microM; by about 30%). Manganese quenching of Fura-2 was enhanced in cells treated with Tg. The verapamil-resistant calcium influx was reduced by SKF96365 (20 microM) and to a lesser extent by genistein (40 microM); cadmium (200 microM) produced an initial decrease in fluorescence followed by a marked increase. These results demonstrate that, in the mouse anococcygeus, Tg can cause sustained contractions and elevations of calcium influx in the presence of verapamil; the time-course, calcium dependence and, although to a lesser extent, pharmacology of these effects generally support the proposal that excitation-contraction coupling in this tonic smooth muscle involves sustained capacitative calcium influx.