Simultaneous changes in intracellular calcium and tension induced by endothelin-1 and sarafotoxin S6c in guinea pig isolated gallbladder: influence of indomethacin

The relationships between changes in intracellular Ca2+ and smooth muscle tension triggered by endothelin-1 and the selective endothelin ETB receptor agonist sarafotoxin S6c, as well as their susceptibility to modification by the nonselective cyclooxygenase blocker indomethacin, were assessed in guinea pig isolated gallbladder strips. Cumulative additions of either agonist (1, 10, and 100 nM) induced simultaneous graded, strongly correlated, slowly developing, and sustained changes in tension and intracellular Ca+2 (Fura-2 technique). Sarafotoxin S6c was more effective than endothelin-1 in raising intracellular Ca2+ at 1 or 10 nM, but their abilities to cause contractions were similar at all concentrations. Indomethacin (5.6 µM) markedly inhibited the changes in both intracellular Ca2+ and tension caused by all concentrations of sarafotoxin S6c (in response to 100 nM, increases in Ca+2 fluorescence intensity and tension were inhibited from 7.7 ± 0.7 to 4.0 ± 0.4% and from 460 ± 100 to 160 ± 40 mg, respectively) but only reduced the contraction triggered by 100 nM endothelin-1 (from 560 ± 100 to 230 ± 70 mg). Endothelin-1 caused greater prostacyclin release from gallbladder than sarafotoxin S6c (at 100 nM, 6-keto-PGF1α levels in the medium rose 4.8- and 2.8-fold, respectively; P < 0.05) and slightly increased thromboxane A2 release (1.6-fold; P < 0.05). Thus, gallbladder contractions triggered by combined ETA/ETB or selective ETB receptor stimulation (with endothelin-1 or sarafotoxin S6c, respectively) are strongly correlated with increases in intracellular Ca2+ but differentially affected by indomethacin. It remains to be assessed if this difference is because endothelin-1 triggers greater prostacyclin release than sarafotoxin S6c and (or) is due to the coupling of ETA and ETB receptors to distinct patterns of generation of cyclooxygenase-derived eicosanoids.Key words: endothelin, gallbladder, prostacyclin, indomethacin, calcium.