Effects of Cell-Permeant Calcium Chelators on Contractility in Monkey Basilar Artery

Vasospasm after traumatic or aneurysmal subarachnoid hemorrhage is associated with smooth muscle contraction, a process that results in part from increased intracellular calcium in smooth muscle cells. These experiments tested the hypothesis that chelation of intracellular calcium with the cell-permeant calcium chelator, l,2-bis-(2-aminophenoxy)ethane-N,N,N′,N′-tetracetic acid acetoxymethyl ester (BAPTA-AM), decreases smooth muscle contraction in response to agents that cause contraction by increasing intracellular calcium. Effects of BAPTA-AM on vasoconstriction induced by KCl, prostaglandin F_2α (PGF_2α), caffeine, and erythrocyte hemolysate were tested on monkey basilar artery under isometric tension. BAPTA-AM, 30 and 100 μmol/L, caused a significant decrease in resting tension in rings with and without endothelium (30 μmol/L; 8 ± 6% [n.s.] and 14 ± 5%, 100 (μmol/L; 19 ± 3% and 32 ± 6%, p < 0.05, paired t test). Contractions to caffeine were significantly decreased by 30 μmol/L BAPTA-AM and were abolished at 100 μmol/L in rings with and without endothelium (p < 0.05). BAPTA-AM, 100 μmol/L, competitively inhibited contractions to PGF_2α. BAPTA-AM, 100 μmol/L, significantly decreased the maximum contractions to KCl in rings with and without endothelium (p < 0.05). There were no significant effects of BAPTA-AM on contractions induced by hemolysate in rings with endothelium but in rings without endothelium, BAPTA-AM, 100 μmol/L, significantly inhibited contractions. In rings with endothelium contractions to hemolysate could be significantly reduced by BAPTA-AM plus indomethacin or indomethacin alone, suggesting that hemolysate releases an eicosanoid from the endothelium by a pathway that is not inhibited by BAPTA. These results suggest that the ability of BAPTA-AM to inhibit smooth muscle contractions will depend on the agonists mediating the contraction. In response to erythrocyte hemolysate, loading of endothelial cells with BAPTA-AM increases the release of a vasoconstricting eicosanoid from these cells that counteracts the decreased contraction caused by loading of smooth muscle cells with BAPTA-AM.