CA2-INDEPENDENT CONTRACTION INDUCED BY HYPEROSMOLAR K+-RICH SOLUTIONSIN RAT UTERUS()

The present experiments were designed to investigate the mechanisms in volved in the contractile responses evoked by KCl, added either isoosm otically or hyperosmotically, in the rat uterus. Exposure of uterine s trips to a Ca2+-free, 3 mM EGTA-containing solution abolished the resp onses induced by isoosmotic KCl solutions. Conversely, addition of hyp erosmolar KCl induced concentration-dependent tonic responses in a Ca2 +-free, 3 mM EGTA-containing solution. The maximum increase in tension was reached with 210 mM K+. The response to hyperosmotic K+ was unaff ected by previous depletion of intracellular Ca2+ stores with oxytocin (1 mu M), by inhibition of refilling of the intracellular Ca2+ stores using cyclopiazonic acid (10 mu M) or by increasing the concentration of EGTA in the medium to 10 mM. Sucrose and mannitol (60-420 mM) indu ced concentration-dependent sustained contractions which were not repr oducible and were significantly smaller in size than those evoked by t he maximally effective concentration of hyperosmotic K+ (210 mM). The contraction induced by hyperosmotic K+ in Ca2+-free solution was not a ltered by the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphtha lenesulfonamide hydrochloride (W-7, 100 mu M), the Ca2+/calmodulin pro tein kinase II inhibitor esulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiper azine (KN-62, 10 mu M) or the tyrosine kinase inhibitor genistein (10 mu M) The protein kinase C inhibitor calphostin C (1-3 mu M) failed to modify the K+-effect curve, which was however partially inhibited in the presence of the non-selective protein kinase inhibitor 1-(5-isoqui nolinylsulphonyl)-2-methylpiperazine dihydrochloride (H-7, 3-100 mu M) . The protein kinase inhibitor staurosporine (30-300 nM) depressed the contraction induced by hyperosmolar K+ in a concentration-dependent m anner. The contraction induced by sucrose in Ca2+-free solution was un affected by W-7 (100 mu M) and KN-62 (10 mu M) but was partially reduc ed by calphostin C (1 mu M), H-7 (30 mu M), staurosporine (100 nM) and genistein (10 mu M) These results suggest that different mechanisms a re involved in the responses evoked by isoosmotic and hyperosmotic KCl in the rat uterus, A component of the contraction induced by hyperton ic KCl seems mainly independent of both external and internal Ca2+ and of hyperosmolar stress. This contraction is not mediated by protein k inase C, Ca2+/calmodulin-dependent kinases or protein tyrosine kinases but involves activation of other, at the present unknown, staurospori ne-sensitive protein kinase(s).