H. Izumi et al., SOME MECHANICAL-PROPERTIES OF SKINNED FIBERS OF PREGNANT HUMAN MYOMETRIUM, European journal of obstetrics, gynecology, and reproductive biology, 56(1), 1994, pp. 55-62
The properties of contractile elements and intracellular Ca2+ storage
sites of pregnant human myometrium were studied by recording the mecha
nical responses in skinned (saponin-treated and membrane-permeable) fi
bres. Calmodulin increased the amplitude of contractions induced by Ca
2+ and the Ca2+ sensitivity for contractile elements in small myometri
um strips, but PGF(2 alpha), PGE2, oxytocin, or cyclic AMP failed to p
roduce similar effects. After accumulation of Ca2+ in intracellular Ca
2+ storage sites, 10 mu mol/l PGF(2 alpha), 10 mu mol/l PGE(2), 30 mmo
l/l caffeine, and 20 mu mol/l InsP3 (inositol-trisphosphate) produced
contractions by releasing Ca2+ from storage sites. However, 20 nmol/l
oxytocin had no effects under the same conditions. The InsP3 sensitive
Ca2+ store was much larger than those of PGs or caffeine. These resul
ts suggest that pregnant human myometrium contracts with low Ca2+ by a
calmodulin sensitive system. The data also indicate that direct appli
cation of PGF(2 alpha) or PGE(2) into the cells discharges Ca2+ from C
a2+ storage sites and that oxytocin extricates Ca2+ via a pathway invo
lving InsP3 by activation of phosphoinositide turnover. We suggest tha
t these agents induce added contractile responses due to a Ca release
mechanism from store sites in addition to the influx of Ca2+ from the
extracellular space.