Acta Physiologica 82. (1994)

4. szám - Z. Kukor-M. Tóth: Ca2+-dependent and Ca2+-independent NO-synthesizing activities of human primordial placenta

Acta Physiologica Hungarica, Volume 82 (4), pp. 313 - 319 (1994) Ca2+-DEPENDENT AND Ca2+-INDEPENDENT NO-SYNTHESIZING ACTIVITIES OF HUMAN PRIMORDIAL PLACENTA Z. Kukor, M. Tóth 1ST INSTITUTE OF BIOCHEMISTRY, SEMMELWEIS MEDICAL UNIVERSITY, BUDAPEST, HUNGARY Received April 30, 1994 Accepted June 14, 1994 In order to localize the site of production of nitrogen monoxide (NO) in first trimester human pregnancy, the cytosol and microsome fractions prepared from homogenized primordial placentas were tested for NO-synthase (NOS) activities by measuring the NADPH- dependent conversion of [3H]arginine to [3H]citrulline. Our results demonstrate that Ca2+dependent enzyme activities are present in both fractions, whereas microsomes exhibit significant Ca2+-independent enzyme activity too. The highest specific activity is measurable in the presence of Ca2+ with microsomes, this activity is about 2-fold higher than the Ca2+dependent specific activity of the cytosol. The Ca2+-independent specific NOS activity is about 30% of the Ca2 +-dependent microsomal activity. The microsomal Ca2+-dependent NOS activity is inhibited by 50% in the presence of 0.5 mM aminoguanidine (AG), whereas the Ca2 +-independent activity does not respond to this concentration of AG, suggesting that it is not the inducible isoform of NOS. Our results indicate that (I) NOS activity is present from an early phase of placental development, (II) the NOS activity is of trophoblastic origin, since the primordial placenta is avascular and (III) NO-production by the primordial placenta can proceed in the absence of any Ca2+-mobilizing agonist. Keywords: NO-synthase, microsomal NO-synthase, cytosolic NO-synthase, Ca2+sensitivity of NO-synthase, aminoguanidine, primordial placenta (human) Accumulating data strongly suggest that nitrogen monoxide (NO) is a key regulator in mammalian pregnancy. NO is envisaged as an agonist with dual effect on uterine smooth muscle contractility: (I) it inhibits myométrial contractions through increased rate of formation of cGMP [6, 16] and (II) it can influence contractility by stimulating the synthesis of various eicosanoids including prostaglandin E2 (PGE2), Correspondence should be addressed to Miklós Tóth Semmelweis Medical University, 1st Institute of Biochemistry H -1088 Budapest, Puskin u. 9, Hungary 0231-424X/94/S 4.00 © 1994 Akadémiai Kiadó, Budapest