The effect of Mn on the electrical and mechanical activities in the guinea-pig stomach.

The effects of Mn on electrical and mechanical activities in the circular muscle of the guinea-pig stomach were investigated. A low concentration of Mn (0.1 mM) in Krebs solution caused a prolongation of the first component, resulting in an increase in frequency of the second component. However, a high concentration of Mn (more than 1 mM) had a similar effect on the electrical activities in the early period, but subsequently inhibited them. The membrane was depolarized by Mn from 69 (S.D., +/- 6) to 63 (S.D., +/- 6) mV at 0.1 mM and to 42 (S.D., +/- 5) mV at 2 mM in the Krebs solution. The membrane resistance slightly decreased with 0.1 mM Mn, but slightly increased with 2 mM Mn. A low concentration of Mn (0.1 mM) also increased resting tone and phasic contraction. These contractile responses were completely inhibited when Ca was removed from the bathing solution. The enhanced, later phasic contraction was reduced by verapamil. All these electrical and mechanical responses for Mn were not related to the nervous system. Excess Ca (15 mM) inhibited the actions of Mn on the spontaneous slow waves, the resting potential, and the duration of the second component. Thus, it is concluded that Mn displaces or releases Ca from its stored sites, which are loosely-bound pools, and then induces membrane depolarization, resulting in subsequent stimulation of the Ca-channel, which is blocked by a high concentration of Mn.