Vol. 282, Issue 1, 201-207, 1997

An Analysis of the Mechanisms Involved in the Okadaic Acid-Induced Contraction of the Estrogen-Primed Rat Uterus¹

Elena Arteche, Giuseppe Strippoli, Gervaise Loirand, Pierre Pacaud, Luz Candenas, Juan-Carlos Moltó, Luisa Souto, Javier Fernandez, Manuel Norte, Julio D. Martín and Jean-Pierre Savineau

Department of Physiology, University of Bordeaux II, Bordeaux, France (G.L., P.P., J.P.S.), Institute of Chemical Research, Scientific Research Center Isla de La Cartuja, Sevilla, Spain (E.A., G.S., L.C., J.D.M.), Department of Nutrition, University of Valencia, Spain (J.C.M.), and Institute of Bio-Organic research, Center of Natural Products Antonio Gonzalez, University of la Laguna, Tenerife, Spain (L.S., J.F., M.N.)

The contractile effect of okadaic acid (OA) and its derivatives was investigated in the rat uterus. OA (20 µM) induced a transient contraction which, after plateauing, slowly decreased. The structurally related compound okadanol (20 µM) failed to induce any significant contraction. Conversely, the synthetic compound methyl okadaate (20 µM) and the naturally occurring ester 7-hydroxy-4-methyl-2-methylen-hept-4(E)-enyl okadaate (20 µM) were as active as the free acid. The OA-induced contraction was unaffected in the presence of neomycin (5 mM), mepacrine (30 µM), 1-[N,O-bis(1,5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (10 µM), calphostin C (3 µM) and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (30 µM). The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (100 µM) did not modify the amplitude of the OA-induced contraction but significantly increased the rate of tension decay. The myosin light chain kinase inhibitor 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (1 mM) significantly reduced the peak amplitude of the contraction. Staurosporine (0.03-0.1 µM) did not modify the contractile component of the OA-induced response but inhibited the subsequent decrease in tension. In freshly dispersed myometrial cells loaded with the fluorescent Ca⁺⁺ indicator indo 1, OA did not produce any significant increase in [Ca⁺⁺]_i. OA (5- to 90-min contact) also failed to modify the intracellular levels of arachidonic acid, compared with basal values. These data suggest that in the rat uterus 1) the contractile effect of OA (20 µM) is specifically mediated by inhibition of protein phosphatases type 1 and/or 2A and is related to a direct interaction with the contractile machinery; 2) the decreasing phase of the OA-induced mechanical response could be mediated by a staurosporine-sensitive protein kinase different from protein kinase C.