Effects of External Calcium and Calcium Antagonist, Diltiazem, on Isolated Segments of Pregnant Human Myometrium

Spontaneous electrical activity and contraction of isolated segments from the isth‐mic region of pregnant human myometrium were recorded by the single sucrose‐gap method, and effects of calcium and diltiazem on the muscle were studied. The segments, obtained at cesarean section, had plateau‐ or spike‐type action potentials. When external Ca2+ was increased from 2.5 mM (normal concentration in Krebs solution) to 7 mM, the durations of both types of action potentials gradually decreased and finally disappeared. The contractions were well synchronized with action potentials, and amplitude, frequency and half‐duration of the contraction were all decreased when extracellular Ca2+ was increased. On the other hand, at a lower external Ca2+ concentration (1.25 mM) the duration of the action potentials was reduced, as was the contraction force. Diltiazem (10‐8‐10‐5 g/ml) suppressed both types of action potentials, and the accompanying contractions were reduced. However, the frequency of contraction was increased by lower doses in some muscles. Diltiazem 10‐5g/ml completely abolished the spontaneous activity. High K+ in the muscles caused a phasic contraction followed by a tonic contracture. Diltiazem 10‐5 g/ml suppressed both responses. These results suggest that spontaneous action potentials and contractions are altered by changes in external Ca2+. High Ca2+ might increase potassium conductance and stabilize the membrane. Consequently action potential duration was shortened and finally eradicated. It is also suggested that diltiazem may antagonize Ca2+ mobilization at the membrane surface including the stabilizing site, hence affecting the frequency and suppressing the spontaneous contraction and high K+ contracture.