Manganese Ions Induce Tonic Contraction after Relaxation in a High-K+ Medium in Ileal Longitudinal Smooth Muscle of Guinea-pig

In ileal longitudinal muscle 5 mM Mn2+ inhibited completely the K+ (60 mM)-induced tonic tension to the base line; however, the tension progressively increased to above the level of original tonic response evoked by K+ after 3 h in the presence of Mn2+. Tetrodotoxin 5 x 10(-5) M) had no influence on the tension development in the presence of Mn2+ in the high-K+ medium. Mn2+ also increased the tension in a high-K+, Ca(2+)-free medium. The Ca2+ antagonist, gallopamil (10(-6) M) inhibited the development of tension in the presence of Mn2+ in the high-K+ medium. The 45Ca uptake determined by the lanthanum method remained unchanged from control levels after 3 h of the 5 mM Mn2+ application in the high-K+ medium in spite of the development of the tension. The manganese uptake in the high-K+ medium, increased in accordance with the increase of duration of 5 mM Mn2+ application. Gallopamil inhibited manganese uptake in the high-K+ medium. These results suggest that Mn2+ firstly reduces K(+)-induced tension by inhibition of Ca2+ influx, subsequently, Mn2+ ions accumulate in the intracellular compartments through voltage-operated Ca2+ channels and may activate contractile proteins in the ileal muscle.