Histamine Develops Homologous Desensitization under Ca2+-free Conditions with Increase in Basal Tone in Smooth Muscle of Guinea Pig Taenia Caeci

Histamine regulates a variety of physiological or pathophysiological processes via the activation of G(q/11) protein-coupled and Ca2+ -mobilizing histamine H, receptors, including smooth muscle contraction. We have found that histamine induces progression from heterologous to homologous desensitization of contraction under normal physiological conditions in smooth muscle of guinea pig taenia caeci. In this study, we characterized the development of histamine-induced desensitization under Ca2+ -free conditions and we found that histamine developed only a homologous phase of desensitization to histamine with an increase in EC50 values for histamine and basal tone. In contrast, histamine treatment reduced EC50 values for a muscarinic agonist, carbachol, and depolarizing high K+. These results suggest that the failure of excess histamine to induce a normal Ca2+ response under Ca2+ -free conditions may lead to homologous desensitization to histamine with apparent hyper-reactivity of smooth muscles to cholinergic and depolarizing stimuli. We estimate that this characteristic of histamine to change smooth muscle contractility may be potentially involved in its physiological and pathophysiological aspects, including histamine-induced allergic conditions, depending on cellular circumstances.