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-primed rat uterus. In Ca2+-containing solution, iso-osmotic addition of KCl (30, 60 or 90 mM K+) induced a rapid, phasic contraction followed by a prolonged sustained plateau (tonic component) of smaller amplitude. The KCl (60 mM)-induced contraction was unaffected by tetrodotoxin (3 mu M), omega-conotoxin MVIIC (1 mu M), GF 109203X(1 mu M) or calphostin C (3 mu M) but was markedly 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 effective as an inhibitor of the tonic component than of the phasic component. After 60 min incubation in Ca2+-free solution containing 3 mM EGTA, iso-osmotic KCl did not cause any increase in tension but potentiated contractions evoked by oxytocin (1 mu M), sodium orthovanadate (160 mu M) or okadaic acid (20 mu M) in these experimental conditions. In freshly dispersed myometrial cells maintained in Ca2+-containing solution and loaded with indo 1, iso-osmotic KCI (60 mM) caused a biphasic increase in the intracellular Ca2+ concentration ([Ca2+](i)). In cells superfused for 60 min in Ca2+-free solution containing EGTA fl mM), KCI did not increase [Ca2+](i). In Ca2+-containing 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 estrogen-primed rat uterus, iso-osmotic high K+ solutions, in addition to their well known effect on Ca2+ influx, activate other cellular processes leading to an increase in the Ca2+ sensitivity of the contractile machinery by a mechanism independent of extracellular Ca2+.