Properties of intracellular calcium stores in pregnant rat myometrium

1. The properties of the Ca2+ stores in myometrium of 21-day pregnant rats were studied by recording the contractile responses of saponin-treated skinned muscles. 2. After accumulation of Ca2+ into the stores in the presence of 5 mM NaN3, inositol 1,4,5-trisphosphate (InsP3) at concentrations exceeding 3 microM produced a contraction. The amplitude of this contraction was maximal at about 20 microM. A second application of 20 microM InsP3 produced a smaller contraction than the first one. However after reloading the stores with Ca2+, 20 microM InsP3 produced a contraction of the same amplitude as the initial one. 3. After application of 20 microM InsP3, 1 microM A23187 still evoked a large contraction. If A23187 was applied first, the subsequent application of InsP3 or A23187 no longer induced a contraction, even after Ca2+ loading. 4. Guanosine triphosphate (GTP) or arachidonic acid, both 100 microM neither evoked a contraction nor enhanced the subsequent contraction elicited by 20 microM InsP3. 5. Caffeine 25 mM did not induce a contraction nor did it affect the contraction elicited by 20 microM InsP3. 6. The results indicate that in pregnant rat myometrium InsP3 releases Ca2+ from intracellular stores as has been proposed in vascular smooth muscles.