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 Ca(2+)-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 microM 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 microM 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 microM. A 30-min pretreatment with 1 microM TG appeared to be sufficient to induce a consistent antagonism of the ACh (0.01-10 microM) 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 microM TG. The effect of the calcium ionophore A23187 (0.01-1 microM) was reduced by the Ca(2+)-ATPase inhibitor only at the higher concentrations tested (0.3-1 microM). However, a 30-min contact time with 1 microM TG was completely ineffective in antagonizing the concentration-relaxant response curves to the two nitrovasodilators sodium nitroprusside (SNP 0.1-100 microM) and nitroglycerin (NTG 1-300 nM) and to the cyclic GMP analogue 8-Bromo-cyclic GMP (3-100 microM). The effects of the beta-adrenoceptor agonist isoprenaline (ISO 0.1-10 microM) and of the direct adenylate cyclase activator forskolin (FK 0.01-10 microM) were also completely unaffected by 1 microM 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.
Amerini
S
S
Department of Preclinical and Clinical Pharmacology, University of Florence, Italy.
Filippi
S
S
Parenti
A
A
Ziche
M
M
Ledda
F
F
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D001011:Aorta / Q000187:drug effects* / Q000502:physiology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D004347:Drug Interactions; D004730:Endothelium, Vascular / Q000187:drug effects* / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D066298:In Vitro Techniques; D008297:Male; D005996:Nitroglycerin / Q000494:pharmacology; D011817:Rabbits; D013373:Substance P / Q000494:pharmacology*; D019284:Thapsigargin / Q000494:pharmacology*; D014664:Vasodilation; D014665:Vasodilator Agents / Q000494:pharmacology*
D004791:Enzyme Inhibitors; D014665:Vasodilator Agents; D013373:Substance P; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D005996:Nitroglycerin; D000109:Acetylcholine