NK 1 receptor-mediated endothelium-dependent relaxation and contraction with different sensitivity to post-receptor signaling in pulmonary arteries

Abstract

In rabbit intrapulmonary arteries, substance P (SP) has been reported to induce endothelium-dependent relaxation (EDR) and endothelium-dependent contraction (EDC) via tachykinin NK 1 receptors, and endothelium-independent contraction (EIC) via tachykinin NK 2 receptors. The present study pharmacologically examined whether these opposite responses (EDR and EDC) are mediated by the same NK 1 receptor. Five tachykinin agonists, including septide, a reportedly atypical NK 1 agonist, caused concentration-dependent EDR in the presence of NK 2 antagonist (SR-48968) + TXA 2 synthetase inhibitor (ozagrel), which blocked EIC and EDC, in pre-contracted arteries, and concentration-dependent EDC in the presence of NK 2 antagonist (SR-48968) + nitric oxide synthase inhibitor ( l-N G-nitro-arginine methyl ester), which blocked EIC and EDR, in non-contracted arteries. The EC 50 values of these agonists for EDR were smaller than those for EDC, indicating that the affinities of NK 1 agonists to NK 1 receptors are different between EDR and EDC. However, the rank order of their potency for EDR and EDC was the same: SP = septide > SP methyl ester (SPME) > neurokinin A > neurokinin B. [Ala 5, β-Ala 8]-α-neurokinin fragment 4–10 (NK 2 agonist) and senktide (NK 3 agonist) caused no responses. Two structurally different NK 1 antagonists, CP-99994 and SR-140333, shifted the concentration–EDC and –EDR curves of SPME, a selective NK 1 agonist, and septide rightward and suppressed their maximal responses in a similar concentration-dependent manner, indicating that the affinities of NK 1 antagonists to NK 1 receptors are similar between EDR and EDC. U-73122, a phospholipase C inhibitor, and thapsigargin, 2,5-di-tert-butylhydroquinone, and ruthenium red, all intracellular Ca 2+ release blockers, inhibited SP-induced EDR and EDC. Effective concentrations of ionomycin (Ca 2+ ionophore) causing EDR were also lower than those causing EDC. Taken together, SP-induced EDR and EDC are mediated by activation of the same NK 1 receptor followed by an increase in intracellular Ca 2+, and sensitivity to Ca 2+ may be higher in the EDR than EDC pathway.