The collagens of the extracellular matrix are the most abundant structural proteins in the mammalian body. In tissue remodeling and in the invasive growth of malignant tumors, collagens constitute an important barrier, and consequently, the turnover of collagen is a rate-limiting process in these events. A recently discovered turnover route with importance for tumor growth involves intracellular collagen degradation and is governed by the collagen receptor, urokinase plasminogen activator receptor-associated protein (uPARAP or Endo180). The interplay between this mechanism and extracellular collagenolysis is not known. In this report, we demonstrate the existence of a new, composite collagen breakdown pathway. Thus, fibroblast-mediated collagen degradation proceeds preferentially as a sequential mechanism in which extracellular collagenolysis is followed by uPARAP/Endo180-mediated endocytosis of large collagen fragments. First, we show that collagen that has been pre-cleaved by a mammalian collagenase is taken up much more efficiently than intact, native collagen by uPARAP/Endo180-positive cells. Second, we demonstrate that this preference is governed by the acquisition of a gelatin-like structure by the collagen, occurring upon collagenase-mediated cleavage under native conditions. Third, we demonstrate that the growth of uPARAP/Endo180-deficient fibroblasts on a native collagen matrix leads to substantial extracellular accumulation of well defined collagen fragments, whereas, wild-type fibroblasts possess the ability to direct an organized and complete degradation sequence comprising both the initial cleavage, the endocytic uptake, and the intracellular breakdown of collagen.

Intracellular mechanisms regulating fat oxidation were investigated in human skeletal muscle during exercise. Eight young, healthy, moderately trained men performed bicycle exercise (60 min, 65% peak O₂consumption) on two occasions, where they ingested either 1) a high-carbohydrate diet (H-CHO) or 2) a low-carbohydrate diet (L-CHO) before exercise to alter muscle glycogen content as well as to induce, respectively, low and high rates of fat oxidation. Leg fat oxidation was 122% higher during exercise in L-CHO than in H-CHO ( P < 0.001). In keeping with this, the activity of α₂-AMP-activated protein kinase (α₂-AMPK) was increased twice as much in L-CHO as in H-CHO ( P < 0.01) at 60 min of exercise. However, acetyl-CoA carboxylase (ACC)β Ser²²¹phosphorylation was increased to the same extent (6-fold) under the two conditions. The concentration of malonyl-CoA was reduced 13% by exercise in both conditions ( P < 0.05). Pyruvate dehydrogenase activity was higher during exercise in H-CHO than in L-CHO ( P < 0.01). In H-CHO only, the concentrations of acetyl-CoA and acetylcarnitine were increased ( P < 0.001), and the concentration of free carnitine was decreased ( P < 0.01), by exercise. The data suggest that a decrease in the concentration of malonyl-CoA, secondary to α₂-AMPK activation and ACC inhibition (by phosphorylation), contributes to the increase in fat oxidation observed at the onset of exercise regardless of muscle glycogen levels. They also suggest that, with high muscle glycogen, the availability of free carnitine may limit fat oxidation during exercise, due to its increased use for acetylcarnitine formation.

Previous studies show that exercise‐induced hyperaemia is unaffected by systemic inhibition of nitric oxide synthase (NOS) and it has been proposed that this may be due to compensation by other vasodilators. We studied the involvement of cytochrome P450 2C9 (CYP 2C9) in the regulation of skeletal muscle blood flow in humans and the interaction between CYP 2C9 and NOS. Seven males performed knee extensor exercise. Blood flow was measured by thermodilution and blood samples were drawn frequently from the femoral artery and vein at rest, during exercise and in recovery. The protocol was repeated three times on the same day. The first and the third protocols were controls, and in the second protocol either the CYP 2C9 inhibitor sulfaphenazole alone, or sulfaphenazole in combination with the NOS inhibitor N^ω‐monomethyl‐l‐arginine (l‐NMMA) were infused. Compared with control there was no difference in blood flow at any time with sulfaphenazole infusion (P > 0.05) whereas with infusion of sulfaphenazole and l‐NMMA, blood flow during exercise was 16 ± 4 % lower than in control (9 min: 3.67 ± 0.31 vs. 4.29 ± 0.20 l min⁻¹; P < 0.05). Oxygen uptake during exercise was 12 ± 3 % lower (9 min: 525 ± 46 vs. 594 ± 24 ml min⁻¹; P < 0.05) with co‐infusion of sulfaphenazole and l‐NMMA, whereas oxygen uptake during sulfaphenazole infusion alone was not different from that of control (P > 0.05). The results demonstrate that CYP 2C9 plays an important role in the regulation of hyperaemia and oxygen uptake during exercise. Since inhibition of neither NOS nor CYP 2C9 alone affect skeletal muscle blood flow, an interaction between CYP 2C9 and NOS appears to exist so that a CYP‐dependent vasodilator mechanism takes over when NO production is compromised.