Nitric oxide synthase in the rat fallopian tube is regulated during the oestrous cycle

Nitric oxide produced from L-arginine by nitric oxide synthase (NOS) acts in a variety of biological processes via the stimulation of guanylyl cyclase and subsequent elevation of cGMP. Constitutive, calcium-dependent isoforms of NOS are found in endothelial cells (eNOS) and neurones (nNOS), while macrophages express an inducible, calcium-independent isoform (iNOS) in response to the action of certain cytokines or bacterial endotoxin. While the regulation of NOS by exogenous glucocorticoids and steroid hormones is well documented, the effects of endogenous steroid hormones on NOS activity, such as those released during the oestrous cycle, is unknown. Here we demonstrate, using specific antibodies for eNOS, nNOS and iNOS, the presence of NOS in the epithelium of rat fallopian tubes at pro-oestrus, late pro-oestrus, oestrus, metoestrus and dioestrus. Western blot analysis of rat fallopian tube homogenates revealed a protein band at approximately 125 kDa which was recognised by antibodies to different isoforms of NOS, but no bands at the expected molecular weights (eNOS, 140 kDa; nNOS, 160 kDa; iNOS, 135 kDa). NOS activity in fallopian tubes was measured by the conversion of L-[3H]arginine to L-[3H]citrulline. Both calcium-dependent and -independent NOS activities were present. However, in late pro-oestrus when circulating oestrogens are low, NOS activity was reduced in comparison to all other stages of the oestrous cycle. Thus we show that NOS is present in the epithelial lining of the fallopian tube and is recognised at a previously undescribed molecular weight.(ABSTRACT TRUNCATED AT 250 WORDS)