Calcium reversal: Intracellular localization of site of inhibition by a submicromolar concentration of Ca ion in uterine smooth muscle shown by studies with EGTA

Abstract

1. 1. The effect of EGTA on the decay of oxytocin-induced contraction of rat uterine smooth muscle in Ca-free solution was studied to clarify the mechanism of inhibition by low concentration of Ca ion (10 −8−10 −7M) of the contraction, i.e. “ Ca reversal”. 2. 2. The decay was significantly greater in the absence than in the presence of EGTA (0.2 mM). But the effect was not rapid. The concentration of contaminating Ca ion was ca. 10 −5 M and the concentration of free Ca ion was ca. 10 −7 M in the presence of EGTA (0.2 mM). 3. 3. The difference in the decay rate in the absence and presence of EGTA was found to be due to the difference of concentration of free Ca ion, the contaminating Ca ion causing inhibition ( Ca reversal). The slow appearance of inhibition in the absence of EGTA was due to the slow influx of Ca ion into the cell. 4. 4. It is concluded that the “ Site of Ca reversal”, the site of this inhibition, is intracellular and has very high affinity for Ca ion.