Abstract
We characterized thromboxane A 2/prostaglandin H 2 (TXA 2/PGH 2) receptors and histamine H 1 receptors in Guinea-pig cultured tracheal smooth-muscle cells (TSMC). [ 3H]SQ 29548 (a TXA 2 antagonist)-binding sites were saturable and a high affinity with a dissociation constant of 6.2 ± 0.60 nM (mean ± S.E.) and a receptor density of 46 ± 4.6 fmol/10 6 cells. [ 3H]SQ 29548 binding was completely inhibited by TXA 2 mimetics or antagonists. Intracellular calcium concentration ([Ca 2+] i) in TSMC was increased with U46619 stimulation and the increase was attenuated by TXA 2 antagonists, the potencies of which correlated with those inhibiting the activities of the [ 3H]SQ 29548 binding. [ 3H]Mepyramine (a H 1 antagonist)-binding sites were also present in TSMC. [ 3H]Mepyramine had a single class of low-affinity-binding sites with a dissociation constant of 2.6 ± 0.081 μM and a receptor density of 10.6 ± 0.11 nmol/mg protein. [ 3H]Mepyramine binding in TSMC membrane was inhibited by H 1 antagonists, but not by H 2 antagonists. The inhibition constants of mepyramine in TSMC were 910-times lower than those in tracheal membranes. In contrast, the histamine-induced increase in [Ca 2+] i in TSMC was inhibited in the presence of low concentrations of H 1 antagonists. All these observations provide evidence that TXA 2/PGH 2 receptors, mepyramine-binding sites and/or H 1 receptors are expressed in cultured TSMC.
Published Version
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