INDUCTION OF NITRIC-OXIDE SYNTHASE IN THE NEO-VASCULATURE OF EXPERIMENTAL-TUMORS IN MICE

Maintenance of blood flow is an important factor in sustaining tumour growth. Functional studies have previously demonstrated a reduction in tumour blood flow with selective inhibitors of nitric oxide (NO) synt hesis, L-NAME (N(G)-nitro-L-arginine-methylester) and L-NMMA (N(G)-mon omethyl-L-arginine), when administered locally to tumours derived from murine colon 26 adenocarcinoma and B16 melanoma cells. The type of NO synthase which might be responsible for this locally-derived NO and t he site of synthesis was not described. Here we have investigated the distribution of immunoreactivity and the biochemical characteristics o f the enzymes synthesizing NO in the same murine model. Adenocarcinoma (colon 26) or melanoma (B16) cells were introduced into a sponge matr ix implanted subcutaneously in mice. After 7, 12, and 14 days, the imp lants were removed and frozen sections were immunostained with rabbit antisera to constitutive and inducible isoforms of NO synthase. Immuno reactivity with antisera to inducible NO synthase was detected in the vasculature of neoplastic implants, with and without the sponge, at 12 and 14 days. The enzyme was not evident in 7-day-old tumours, in non- neoplastic implants, in areas of tissue outside the tumour, or in aden ocarcinoma or melanoma cells. Enzyme activity was measurable in homoge nates of neoplastic implants removed at day 7 and was found to be Ca2/calmodulin-independent. Immunoreactivity with antisera to inducible N O synthase was seen principally in the endothelium of newly-formed cap illaries, identified by immunostaining for von Willebrand factor in se rial sections. Immunoreactivity with antiserum to constitutive NO synt hase was not evident in either neoplastic or non-neoplastic implants. These results suggest that neoplasms can modulate the synthesis of NO in their vasculature by inducing the generation of NO synthase and hen ce the production of NO which could be very important in maintaining t he tumour blood flow.