Thapsigargin inhibits repletion of phenylephrine-sensitive intracellular Ca++ pool in vascular smooth muscles

Abstract

Thapsigargin (TSG), a putative selective Ca++-ATPase inhibitor, has been used to study Ca++ mobilization in many non-excitable cell types. This study aims to determine whether TSG is effective as a selective microsomal Ca++ uptake inhibitor by studying its ability to affect repletion of the phenylephrine (PE)-sensitive Ca++ pool in rat aorta and dog mesenteric artery evaluated by contractility studies. TSG caused a concentration-dependent contraction that was dependent on the concentration of extracellular Ca++. Ca++ influx promoted by TSG was found to occur mostly through L-type Ca++ channels in the dog mesenteric artery but not in the rat aorta. When arterial rings, depleted to their PE-sensitive internal store, were allowed to replete their stores in normal Krebs' solution or in the presence of elevated K+ levels, it was found that repletion was significantly enhanced in the presence of elevated K+. In TSG-treated rings, however, repletion was significantly inhibited under both conditions as indicated by the subsequent PE-induced contraction in Ca++-free medium. While the rate of contraction induced by elevated K+ levels was slow immediately after pool depletion in controls, it was rapid in TSG-treated arterial rings. The slow onset of K+ contraction may reflect Ca++ uptake in the pool which was absent in TSG-treated arteries. Differences in the behavior of the two arteries were noted and these may reflect differences in the size of their Ca++ store and their coupling to the extracellular space. Single cells isolated from the dog mesenteric artery were also found to shorten in response to TSG to an amount comparable with that obtained from whole tissue experiments. In conclusion, the findings of this study showed that TSG was effective in preventing the repletion of a PE-sensitive internal Ca++ store. These results are consistent with its proposed inhibitory action on the microsomal Ca++-transport ATPase activity.