Effects of Calcium and Manganese Ions on Mechanical Properties of Intact and Skinned Muscles from the Guinea-pig Stomach

1. To investigate the mechanism of generation of contractions in tissues from the guinea-pig stomach, the effects of caffeine, procaine, acetylcholine (ACh), diltiazem or MnCl(2) on the contraction evoked from small bundles of intact or skinned muscles (50 mum in width and 250-300 mum in length) were observed.2. All these agents except for ACh blocked the spontaneously generated contraction. Diltiazem (1 x 10(-4)m) had no effect and MnCl(2) (3 mm) slightly reduced and caffeine enhanced the tonic contraction evoked in Na-free solution, whereas procaine relaxed the tissue. On the other hand, in the isotonic [K](o) solution, diltiazem, MnCl(2) and procaine relaxed the tissue, while caffeine enhanced the tonic contraction.3. Under pre-treatment with Ca-free solution (2 mm-EGTA-containing solution) after depletion of the stored Ca, application of 2.5 mm-Ca and subsequently applied 5 mm-caffeine produced contractions (Ca- and caffeine-induced contractions, respectively). In polarized (5.9 mm-K(o)) and depolarized (128 mm-K(o)) muscles, the various agents simultaneously applied with 2.5 mm-Ca modified the amplitude of the Ca-induced and the resulting caffeine-induced contractions. Thus, at least three different Ca influxes required to evoke the Ca- or caffeine-induced contraction were identified; diltiazem-sensitive Ca influx, diltiazem-insensitive but Mn-sensitive Ca influx and Mn-insensitive Ca influx.4. The Ca- and caffeine-induced contractions in Ca-free and 15.5 mm-Na-containing solutions were gradually reduced in amplitude, in proportion to the time of exposure. However, amplitude of the caffeine-induced contractions was inhibited to a greater extent and the duration of the contracts was less prolonged than the case of the Ca-induced contraction.5. In saponin-treated skinned muscles, the minimum concentration of Ca required to produce the contraction was 1 x 10(-7)m, and the maximum contraction was evoked by application of 1 x 10(-5)m-Ca. The effects of Na-free solution on the Ca accumulation and release to and from the storage site were also observed in these skinned muscles. The removal of Na from the cell seems to accelerate the Ca leakage, and depletes the stored Ca. In addition, Na-free solution inhibits to some extent the accumulation of Ca in the store site.6. In skinned muscles, Mn (over 2 x 10(-9)m) significantly enhanced the Ca-induced contraction and the pCa-tension relationship shifted to the left and upper directions. Mn seemed to possess the property of activating the contractile proteins, as determined from the pMn-tension relationship, and this agent may also inhibit leakage of Ca from the store sites. However, in relation to the latter two actions, the possible effects of Ca contaminations in the solution would have to be ruled out. Under physiological conditions, MnCl(2) may act at the level of the myoplasmic membrane and not actually penetrate the cell in this tissue.