DISAPPEARANCE OF MYOFILAMENTS DURING ISOMETRIC POTASSIUM CONTRACTION OF SMOOTH MUSCLE

Abstract

Structural changes during K⁺-induced isometric contraction of guinea pig taenia coli were investigated under the electron microscope. A marked change was observed in the myofilaments of the smooth muscle cells after the tissue was contracted for more than 30 min. Almost all thick filaments disappeared and thin filaments came to be located around the submembraneous area. Thin filaments became uniformly distributed after the tissue was relaxed by the addition of diltiazem, a calcium antagonist, during prolonged exposure to high K⁺ solution; however, no thick filaments were restored. When the tissue was incubated in a hypotonic solution, thick filaments disappeared and thin filaments became uniformly distributed. Under the hypotonic condition, carbachol produced strong contraction with the same features as prolonged K⁺-induced contraction. However, when the solution was hypertonic, thick filaments remained intact and moved to the central part of the cell after prolonged K⁺-induced contraction. Therefore, two independent phenomena proceed during K⁺-induced contraction in the smooth muscle cell. One is the disappearance of thick filaments, which is produced by the influx of water into the cell and by the decrease of free magnesium concentration. The other is the rearrangement of the filaments, which occurs during the prolonged isometric contraction. These findings indicate that the smooth muscle contracts even without observable thick filaments.