Reduction of endothelial NOS and bradykinin-induced extravasation of macromolecules in skeletal muscle of the fructose-fed rat model.

Abstract

Reduced capillary permeability of the skeletal muscle vascular bed has been suggested to play a role in fructose-fed rats, corroborating a long held view that insulin resistance might partially be explained by the lack of access of insulin and glucose to its target organs, mainly skeletal muscles. The goal of this study was to explore mechanisms underlying this vascular abnormality, and more specifically the role of bradykinin and nitric oxide (NO) on skeletal muscle microcirculation and the extravasation of macromolecules. For that purpose, Sprague-Dawley rats were fed with either a fructose-enriched (F) or a normal chow (N) diet and extravasation of macromolecules was assessed at 4 weeks by measuring in vivo the extravasation of Evans Blue (EB) dye in the quadriceps muscles of both groups after the intravenous injection of the potent vasodilator bradykinin (150 microg/kg). As expected, fructose-fed rats had less extravasation of EB in skeletal muscle in the basal state as compared to controls (F 17.6 +/- 4.4 vs. N 43.6 +/- 6.9 microg/g dry tissue; P<0.01). In response to bradykinin, the EB dye extravasation in skeletal muscle was 89.4% higher in rats fed the normal chow diet compared to the basal state (P<0.03). In contrast, no significant increase in vasopermeability was observed in fructose-fed animals acutely injected with BK (17.6 +/- 4.4 microg/g in the basal state versus 24.6 +/- 3.1 microg/g after the injection of BK; P=NS). To distinguish a functional from an anatomical/structural defect, hematoxylin-eosin sections as well as electron micrographs of skeletal muscle microvessels were examined in both groups of animals: no obvious abnormalities were found. However, in homogenates of skeletal muscles (quadriceps) of fructose-fed rats, there was a marked reduction of NO synthase (NOS) activity (-33.8%; P<0.001) as well as endothelial NOS immunoreactive mass (-23.4%; P<0.04) as compared to control animals. There is unresponsiveness of the skeletal muscle capillary bed to bradykinin in insulin-resistant animals most probably due to a reduction in endothelial NOS (activity and mass). Our results indicate a functional defect possibly involving responsiveness of the precapillary resistance and/or the endothelial barrier to bradykinin in skeletal muscles. Since insulin must cross the endothelial monolayer to reach its target cells on the abluminal side, it is suggested that reduced endothelial NOS and consequent reduced extravasation of macromolecules could exacerbate insulin resistance in skeletal muscles and hypertension in the fructose-fed rat.