Abstract
Segments of guinea-pig ileum were exposed to a range of concentrations of acetylcholine, carbachol, methacholine, choline, pilocarpine or histamine from 1 pM to 300 mM. Peak response, fade and desensitisation were recorded isometrically. Noradrenaline was applied to guinea-pig vas deferens and bradykinin to rat uterus and similar observations made. The development of and recovery from desensitisation was examined and observations were made on the effects of antagonists and of alterations in electrolytes in the bathing fluid. The relationship of the molar concentration of the agonist to fade, recorded as a percentage of peak response, is linear for acetylcholine, histamine and noradrenaline but not for pilocarpine. The full dose-response curve is such that with doses above the producing maximal responses, contractions decreased with increasing dose. Autodesensitisation, like peak response and fade, is a consequence of receptor stimulation but heterodensitisation is unrelated to receptor mechanism. It is peculiarly sensitive to alterations in calcium level.
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