THAPSIGARGIN INHIBITS THE RESPONSE TO ACETYLCHOLINE AND SUBSTANCE-P BUT DOES NOT INTERFERE WITH THE RESPONSES TO ENDOTHELIUM-INDEPENDENT AGENTS

We investigated the influence of the Ca2+-ATPase inhibitor thapsigargi n (TG) on the vasorelaxant response to different endothelium-dependent and endothelium-independent relaxing agents in an isolated thoracic a orta preparation of the rabbit, precontracted by norepinephrine (NE). Pretreatment with 100 mu M L-arginine methyl ester (L-NAME) an inhibit or of nitric oxide (NO) synthesis, completely prevented acetylcholine (ACh)-induced relaxation; the inactive stereoisomer D-NAME did not mod ify the effect of ACh. The exposure of the preparations to 1 mu M TG i nduced a slowly developing slight increase in the basal tension during 30-min contact. The same concentration of TG also slightly reduced th e response to the subsequent administration of NE. The antagonist effe ct of TG on the ACh response was concentration dependent in the range between 0.1 and 10 mu M. A 30-min pretreatment with 1 mu M TG appeared to be sufficient to induce a consistent antagonism of the ACh (0.01-1 0 mu M) concentration-relaxant effect curve, since an increase to 60 m in did not produce a further significant increment in the degree of th e antagonist effect. The concentration-dependent relaxation induced by substance P (SP 0.1-3 nM) was also significantly antagonized by 1 mu M TG. The effect of the calcium ionophore A23187 (0.01-1 mu M was redu ced by the Ca2+-ATPase inhibitor only at the higher concentrations tes ted (0.3-1 mu M) However, a 30-min contact time with 1 mu M TG was com pletely ineffective in antagonizing the concentration-relaxant respons e curves to the two nitrovasodilators sodium nitroprusside (SNP 0.1-10 0 mu M) and nitroglycerin (NTG 1-300 nM) and to the cyclic GMP analogu e 8-Bromo-cyclic GMP (3-100 mu M). The effects of the beta-adrenocepto r agonist isoprenaline (ISO 0.1-10 mu M) and of the direct adenylate c yclase activator forskolin (FK 0.01-10 mu M) were also completely unaf fected by 1 mu M TG. These results demonstrate that TG affects the res ponse to agents that induce an endothelium-dependent relaxation throug h receptor-dependent calcium mobilization. However, they do not suppor t the hypothesis that sarcoplasmic pump activity is essential for the development of a vasorelaxant response to endothelium-independent agen ts.