Effects of calcium and manganese ions on mechanical properties of intact and skinned muscles from the guinea-pig stomach.

Abstract

To investigate the mechanism of generation of contractions in tissues from the guinea-pig stomach, the effects of caffeine procaine, acetylcholine(ACh) diltiazem or MnCl2 on the contraction evoked from small bundles of intact or skinned muscles were observed. The antrum possesses multiple Ca channels for induction of the Ca influx. In physiological solution, Mn and diltiazem inhibited the spontaneously generated contraction presumably due to suppression of the voltage-dependent Ca-influx. In the Ca-free polarized and depolarized muscles after depletion of stored Ca, the influx of Ca by addition of Ca may relate to at least three different channels. Furthermore, some of the Ca influx directly increases the amount of free Ca, while a part of Ca is sequestered into the store site and may activate the Ca-induced Ca-release mechanism, thus increasing the amount of free Ca. These results indicate that the mode of Ca in the muscle cell of the antrum is not the same as that in the mesenteric artery. Reduction in [Na]o transiently enhanced the amplitude and prolonged the duration of the Ca- and caffeine-induced contraction in Ca-free solution. Presumably these effects may relate to accelerated leakages of Ca from the store sites, as estimated from skinned muscles and inhibition of the Ca extrusion from the cell. MnCl2 acts on the surface membrane and suppresses most of the Ca influxes, except for some fractions directly sequestered into the store site. If MnCl2 penetrates the cell membrane, such as occurs in Ca-free solution, the interaction of Ca and its receptors of contractile proteins is accelerated, and the contractile protein is activated.