Limited excitation spread and regional contractions in spontaneously active, isolated bundles of smooth muscle of frog stomach.

Abstract

The spread of excitation in isolated bundles of smooth muscle from frog stomach has been measured with movable external electrodes. Amplitudes of the compound action potentials vary along the bundle. They are maximal in pacemaker regions and decrease as a function of distance from the pacemaker. Excitation spread in the bundle is limited and does not exceed 6–7 mm on both sides of a pacemaker region. This distance is independent of the diameter of the bundles. Bathing the bundles in saline with double concentration of potassium (5 mMol/l) does not influence conduction distance. If the bundle is stretched conduction distance increases in proportion to the increase in length. Duration and conduction velocity of the compound action potential remain constant along the region which is activated by one pacemaker. Surface marking with carbon particles revealed that also the degree of shortening varies along the bundle. In the vicinity of a pacemaker region, where compound action potentials have their maximum amplitude, shortening is maximal. Electrical and mechanical activity decay in a similar way along the bundle. A spontaneous isometric contraction was found to consist of a regional shortening while non-active or less active regions were stretched.