A study of the histamine H2‐receptor mediating relaxation of the parenchymal lung strip preparation of the guinea‐pig

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Abstract
1 The relaxation produced by several H2-receptor agonists and forskolin was investigated on strips of guinea-pig lung parenchyma. 2 Dimparit, 1 μM to 10 mM, 4-methyl histamine, 0.5 μM to 100 μM and impromidine, 10 nM to 1 μM, had no effect on the tone of the unstimulated strips of lung parenchyma but caused a dose-dependant relaxation of strips that were contracted by 2-pyridylethylamine (2-PEA), 15 μM. Forskolin, 10 nM to 4 μM, produced a dose-dependent relaxation of both the stimulated and unstimulated lung strips. 3 The muscarinic antagonist atropine, 1 μM, and the β2-adrenoceptor antagonist propranolol, 10 μM, had no effect on the dose-response curve for dimaprit-induced relaxation of the lung strip. 4 The dose-response curve for dimaprit was shifted to the right in a dose-dependent manner by increasing concentrations of a variety of H2-antagonists. Schild plots produced a straight line for all the H2-antagonists with slopes not significantly different from unity. The equilibrium dissociation constants for the H2-antagonists on the lung strip preparation were similar to those previously reported for inhibition of the chronotropic activity of histamine on guinea-pig right atria and inhibition of [3H]-tiotidine binding to homogenates of guinea-pig lung parenchyma.