Influence of calcium on the effects of okadaic acid and its interaction with caffeine and theophylline in rat myometrium

Abstract

The effects of okadaic acid (OA), a monocarboxylic acid produced by marine dinoflagellates belonging to the genera Dinophysis and Prorocentrum, and their interactions with theophylline and caffeine were studied on the rat-isolated uterus in a calcium-containing medium and a calcium-free medium in the presence of 10(-3) M EGTA. Okadaic acid (5 x 10(-6) to 5 x 10(-5) M) induced a concentration-dependent contraction of the rat-isolated uterus corresponding, with 5 x 10(-5) M, to 142.3 +/- 6.1% (n = 7) of the contraction induced by oxytocin 10(-6) M. The time to peak tension was inversely proportional to the maximum effect produced. The contraction was not sustained and was followed by a concentration-dependent decrease in tone. In a Ca(2+)-free medium containing 10(-3) M EGTA the contractile effects of OA were significantly inhibited or reduced. A 30 min pretreatment with theophylline (3 x 10(-3) M) or caffeine (2 x 10(-2) M) significantly reduced, in a Ca(2+)-containing medium, the maximum contractile effect of OA 10(-5) and/or 2 x 10(-5) M and shortened the relative time to peak tension. In a Ca(2+)-free medium containing 10(-3) M EGTA, only the second effect was observed. With a 1 min pretreatment and in a Ca(2+)-containing medium, theophylline 3 x 10(-3) M and caffeine 10(-2) M did not modify the maximum effect of OA 10(-5) M but shortened the time to peak tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Divergence in Murine Myometrium Spontaneous and Oxytocin-Stimulated Contractile Responses to Serine/Threonine Protein Phosphatase-1 Inhibition1

“…Candenas and coworkers [20] first reported that OA could inhibit and stimulate uterine contractions at low (100 nM) and high (5 M) concentrations, respectively. Okadaic acid-stimulated uterine contractions are partially inhibited when transmembrane Ca 2ϩ movement was inhibited with Ca 2ϩ -free media plus EDTA [21,22]. Administration of a cocktail of antagonists that block muscarinic, adrenergic, histaminergic, serotonergic, and opioid receptors were not found to alter significantly the stimulatory influence of 5-20 M OA [22].…”

“…Optical magnification: A1, B1, C1 ϭ ϫ100; A2, A3 ϭ ϫ400; B2, B3, C2, C3 ϭ ϫ1000. nM) inhibit contractions and higher concentrations cause a biphasic, contractile/relaxation response [21]. Effects of OA on myometrial strips would indicate that PP1 and/or PP2A are involved; however, no reports exist characterizing the PP in the myometrium.…”

“…Presently, there is a lack of agreement in regard to whether PP1 and/ or PP2A are the primary smooth muscle PP within these tissues. Likewise, OA has been shown to both inhibit and stimulate contractions of rat myometrial strips depending on concentrations used [20][21][22][23]. Low concentrations (100 Tissues were fixed, paraffin embedded, and exposed to microwave antigen retrieval prior to incubation with primary antibodies or NIRS (A1-A3) followed by biotinylated secondary antibody and colorimetric evaluation with avidin-biotin-peroxidase and DAB.…”

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

“…Hence, PP2A inhibitors are expected to cause smooth muscle contraction. Paradoxically, inhibitor-based studies have shown that Յ1-3 M OA caused relaxation in rat, guinea pig, dog, pig, and bovine vasculature, rat myometrium, and bovine trachea smooth muscles (1,10,11,18,28,29,102,109,211,235). In myometrium, only Ͼ10 M OA produced contraction or temporary contraction and then relaxation, and these contractions were attributed to PP1 inhibition (10,28,29).…”