Thapsigargin Inhibits the Response to Acetylcholine and Substance P But Does Not Interfere with the Responses to Endothelium-Independent Agents

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Abstract
We investigated the influence of the Ca2+-ATPase inhibitor thapsigargin (TG) on the vasorelaxant response to different endothelium-dependent and endothelium-independent relaxing agents in an isolated thoracic aorta preparation of the rabbit, precontracted by norepinephrine (NE). Pretreatment with 100 μM L-arginine methyl ester (L-NAME) an inhibitor of nitric oxide (NO) synthesis, completely prevented acetylcholine (ACh)-induced relaxation; the inactive stereoisomer D-NAME did not modify the effect of ACh. The exposure of the preparations to 1 μM TG induced a slowly developing slight increase in the basal tension during 30-min contact. The same concentration of TG also slightly reduced the response to the subsequent administration of NE. The antagonist effect of TG on the ACh response was concentration dependent in the range between 0.1 and 10 μM. A 30-min pretreatment with 1 μM TG appeared to be sufficient to induce a consistent antagonism of the ACh (0.01-10 μM) concentration-relaxant effect curve, since an increase to 60 min did not produce a further significant increment in the degree of the antagonist effect. The concentration-dependent relaxation induced by substance P (SP 0.1-3 nM) was also significantly antagonized by 1 μM TG. The effect of the calcium ionophore A23187 (0.01-1 μM) was reduced by the Ca2+-ATPase inhibitor only at the higher concentrations tested (0.3-1 μM). However, a 30-min contact time with 1 μM TG was completely ineffective in antagonizing the concentration-relaxant response curves to the two nitrovasodilators sodium nitroprusside (SNP 0.1-100 μM) and nitroglycerin (NTG 1-300 nM) and to the cyclic GMP analogue 8-Bromo-cyclic GMP (3-100 μM). The effects of the β-adrenoceptor agonist isoprenaline (ISO 0.1-10 μM) and of the direct adenylate cyclase activator forskolin (FK 0.01-10 μM) were also completely unaffected by 1 μM TG. These results demonstrate that TG affects the response to agents that induce an endothelium-dependent relaxation through receptor-dependent calcium mobilization. However, they do not support the hypothesis that sarcoplasmic pump activity is essential for the development of a vasorelaxant response to endothelium-independent agents.