In vivo assessment of microvascular nitric oxide production and its relation with blood flow

To assess the hypothesis that microvascular nitric oxide (NO) is critical t o maintain blood flow and solute exchange, we quantified NO production in t he hamster cheek pouch in vivo, correlating it with vascular dynamics. Hams ters (100-120 g) were anesthetized and prepared for measurement of microves sel diameters by intravital microscopy, of plasma flow by isotopic sodium c learance, and of NO production by chemiluminescence. Analysis of endothelia l NO synthase (eNOS) location by immunocytochemistry and subcellular fracti onation revealed that eNOS was present in arterioles and venules and was 67 +/- 7% membrane bound. Basal NO release was 60.1 +/- 5.1 pM/min (n = 35), and plasma flow was 2.95 +/- 0.27 mul/min (n = 29). Local NO synthase inhib ition with 30 muM N-omega-nitro-L-arginine reduced NO production to 8.6 +/- 2.6 pmol/min (-83 +/- 5%, n = 9) and plasma flow to 1.95 +/- 0.15 mul/min (-28 +/- 12%, n = 17) within 30-45 min, in parallel with constriction of ar terioles (9-14%) and venules (19-25%). The effects of N-omega-nitro-L-argin ine (10-30 muM) were proportional to basal microvascular conductance (r = 0 .7, P< 0.05) and fully prevented by 1 mM L-arginine. We conclude that in th is tissue, NO production contributes to 35-50% of resting microvascular con ductance and plasma-tissue exchange.