Effects of nitric oxide synthase inhibitors on bone formation in rats

NO is synthesized from L-arginine by at least three isoforms of nitric oxide synthase (NOS) and is known to function as a vasodilator and neurotransmitter. NO is produced by bone cells but its function in bone biology is, as yet, unclear. We hypothesized that NOS mediates bone formation in remodeling regions of the skeleton. We studied the effects of two NOS inhibitors: N(G)-nitro-L-arginine methyl ester (L-NAME), which is a general inhibitor of NOS activity and is known to inhibit the vasodilatory effects of the endothelial NOS (eNOS) isoform; and aminoguanidine, which is a selective inhibitor of the inducible NOS (iNOS) isoform. Our hypothesis was tested by treating rats with NOS inhibitors and measuring bone formation rates in the tibial epiphysis and diaphysis. Bone formation indices were measured using standard bone histomorphometry. L-NAME treatment significantly raised mean arterial blood pressure (MAP). This effect was partially reversed by addition of L-arginine. Aminoguanidine had no significant effect on MAP, indicating that it did not block eNOS. The treatments also had substantial effects on bone formation in remodeling trabecular bone. L-NAME did not significantly change trabecular bone formation rate, whereas aminoguanidine reduced bone formation rate in the tibial epiphysis by 79% compared with control. This reduction was completely reversed by L-arginine, suggesting that bone formation during remodeling is, in part, mediated through L-arginine metabolism. No effect of aminoguanidine on bone formation was seen in the tibial diaphysis, a site that undergoes minimal bone remodeling. This finding suggests that the L-arginine-NO pathway is important in bone remodeling.