CALCIUM-DEPENDENT NITRIC-OXIDE SYNTHESIS IS POTENTLY STIMULATED BY TETRAHYDROBIOPTERIN IN HUMAN PRIMORDIAL PLACENTA

Homogenized first trimester human placenta exhibits both Ca2+-dependen t (90-95 per cent) and Ca2+-independent (5-10 per cent) nitric oxide ( NO)-synthesizing activities. Addition of tetrahydrobiopterin (BH4) to homogenates containing Ca2+ in maximally activating concentrations (>0 .5 mu M) results in a further 2-2.5-fold activation of NO synthesis, w ith half-maximal stimulation observed ar 26+/-8.2 mu m BH4 (mean+/-SEM , n=4). Chelation of Ca2+ in the medium abolishes the stimulator) effe ct, indicating that only a Ca2+-dependent NO-synthase (NOS) isoform is activated by BH4. Based on our previous findings, we suggest that thi s isoform is the endothelial or Type III NOS. Importantly, BH4 has no significant effect on the Ca2+-dependency of NOS activity, the apparen t K-m values for Ca2+ are comparable in the absence (1.8+/-0.4 mu M, m ean+/-SEM, n=6) or presence (2.5+/-0.6 mu M, mean+/-SEM, n=6) of 50 mu M BH4. The BH4 content of these placentae is 207.4+/-86.7 pmol/g wet tissue (mean +/-s.d., n=9), therefore, BH4 added to the homogenate doe s not simply restore the concentrations that occur endogenously. The r esults provide the first evidence that in the early human placenta, a constitutively expressed Ca2+-dependent NOS isoform is stimulated by e xogenous BH4, raising the possibility that BH4 is an important regulat or of NOS activity in this tissue. This novel aspect of the NO-generat ing pathway may have implications in the aetiology and treatment of pr egnancy-induced hypertension and pre-eclampsia. (C) 1996 W. B. Saunder s Company Ltd