Electrophysiological and mechanical characteristics of histamine receptors in smooth muscle cells of the guinea-pig ileum

To elucidate the role of the histamine receptor in functions related to intestinal motility, we investigated the effects of histamine and its antagonists on electrical and mechanical activities in longitudinal and circular layers of the terminal region of the guinea-pig ileum. Histamine hyperpolarized the membrane in the circular smooth muscle cells by increasing the permeability of K+ and it transiently inhibited generation of the spike while resting tone was elevated. Cimetidine (CIM) inhibited the hyperpolarization and relaxation induced by histamine while mepyramine (MEP) inhibited the contraction but did not affect the histamine-induced hyperpolarization. In the presence of CIM, histamine did not depolarize the membrane but did lower the threshold potential required for generation of the spike potential, increased the appearance of the spike and enhanced the phasic contraction. Histamine, in 20 mM K+ solution, hyperpolarized the membrane and produced a biphasic response, an initial relaxation and a subsequently generated contraction, in a concentration-dependent manner. In the longitudinal smooth muscle cells, histamine depolarized the membrane, and enhanced both generation of the spike and the contraction. MEP (0.1 microM) but not CIM (1 microM) blocked the histamine-induced responses. CIM at a higher concentration (10 microM) enhanced the histamine-induced contraction, while histamine did not relax the tissue precontracted by 20 mM K+. These results indicate that the circular muscle cells have both H1 and H2 receptors while the longitudinal muscle cells have the H1 receptor. The excitatory responses induced by activation of the H1 receptor in smooth muscle cells differ in these layers.