Effects of ions and drugs on the negative afterpotential in the longitudinal muscle of pregnant rat myometrium.

Effect of Na, Ca anions and drugs (isoprenaline, TEA, procaine) on the negative afterpotential in the longitudinal muscle of pregnant rat myometrium were investigated using the double sucrose gap method. The negative afterpotential having a peak amplitude of about 15 mV decayed initially rapidly then slowly, the total duration being about 1 sec. Raising the external Ca concentration up to 10 mM or reducing Na to 54 mM increased the amplitude of the negative afterpotential. Replacement of Cl with Br enhanced the negative afterpotential and and replacement with benzene sulphonate or isethionate depressed it. The application of 10 mM TEA increased, while 10 mM procaine reduced the amplitude of both spike and negative afterpotential. 10(-10)-107(-8)g/ml isoprenaline depressed the negative afterpotential dosedependently, while leaving the spike potential nearly unaffected. It was concluded that the negative afterpotential was composed of a least two ionic mechanisms, one of Cl origin, the other of Ca origin.