Nitric oxide synthase isoenzymes during in vitro development of rat neuronal and human non-neuronal cells

We studied the expression of neuronal (n), endothelial (e) and inducible (i) nitric oxide synthase (NOS) in cell cultures of rat mesencephalic neurons (embryonic day 14), human keratinocytes from juvenile epidermis, human endothelial cells from juvenile coronary arteries, and human osteoblasts. All cell types were cultured for 5, 10 or 15 days. During proliferation (round cells without processes), the intracellular distribution and the intracellular amount of the calcium-dependent NOS isoforms (n- and e-NOS) did not change whereas the calcium-independent i-NOS changed from a cytosolic distribution pattern to compartmentalized distribution. A striking decrease of i-NOS immunoreactivity was measured by means of image analysis. Our results support the opinion that i-NOS acts as a switch between proliferation and differentiation of cells.