Cellular mechanisms involved in iso-osmotic high K+ solutions-induced contraction of the estrogen-primed rat myometrium

The aim of the present study was to investigate the mechanisms involved in the contraction evoked by iso-osmotic high K+ solutions in the estrogen-pri med rat uterus. In Ca2+-containing solution, iso-osmotic addition of KCl (3 0, 60 or 90 mM K+) induced a rapid, phasic contraction followed by a prolon ged sustained plateau (tonic component) of smaller amplitude. The KCl (60 m M)-induced contraction was unaffected by tetrodotoxin (3 mu M), omega-conot oxin MVIIC (1 mu M), GF 109203X(1 mu M) or calphostin C (3 mu M) but was ma rkedly reduced by tissue treatment with neomycin (1 mM), mepacrine (10 mu M ) or U-73122 (10 mu M). Nifedipine (0.01-0.1 mu M) was significantly more e ffective as an inhibitor of the tonic component than of the phasic componen t. After 60 min incubation in Ca2+-free solution containing 3 mM EGTA, iso- osmotic KCl did not cause any increase in tension but potentiated contracti ons evoked by oxytocin (1 mu M), sodium orthovanadate (160 mu M) or okadaic acid (20 mu M) in these experimental conditions. In freshly dispersed myom etrial cells maintained in Ca2+-containing solution and loaded with indo 1, iso-osmotic KCI (60 mM) caused a biphasic increase in the intracellular Ca 2+ concentration ([Ca2+](i)). In cells superfused for 60 min in Ca2+-free s olution containing EGTA fl mM), KCI did not increase [Ca2+](i). In Ca2+-con taining solution, KCl (60 mM) produced a 76.0 +/- 16.2% increase in total [ H-3]inositol phosphates above basal levels and increased the intracellular levels of free arachidonic acid. These results suggest that, in the estroge n-primed rat uterus, iso-osmotic high K+ solutions, in addition to their we ll known effect on Ca2+ influx, activate other cellular processes leading t o an increase in the Ca2+ sensitivity of the contractile machinery by a mec hanism independent of extracellular Ca2+.