Origin of urinary fibronectin

Fibronectin (FN) is a major component of the glomerular extracellular matrix and it is also abundant in plasma. In physiologic conditions, FN fragments are excreted into urine. Increased urinary FN excretion has been observed in renal diseases raising the question whether urinary FN could reflect changes in the renal extracellular matrix. This study examines whether urinary FN is filtered from plasma or derived from the kidney and it attempts to specify the potential renal source of urinary FN.

(a) To investigate whether urinary FN is filtered from plasma, labeled, biotinylated FN (b-FN) was injected into rats and urine samples were specifically analyzed for b-FN by an immunoblot procedure. (b) To specify the renal source of urinary FN and to evaluate possible alterations of this protein during passage into final urine, tubular fluid was collected from distinct localizations of the nephron by micropuncture techniques. The samples were analyzed for endogenous FN by a highly sensitive immunoblot system, and the pattern was compared with that normally found in final urine.

(a) Urine samples collected over a period of 5 days after injection of b-FN contained no labeled FN. Control experiments in rats with highly elevated glomerular permeability confirmed that the plasma levels of injected b-FN were sufficiently high since b-FN was detected in urine in this condition. (b) The FN fragment pattern, with two protein bands at 75 and 45 kilodaltons, that is normally found in final urine, was already present in samples taken from the early proximal tubule and the distal tubule.

The fibronectin fragments normally found in urine originate from the kidney and are not derived from plasma. Since these fragments are already observed in early proximal tubular fluid, the glomerulus is the probable source of urinary FN. The FN fragment pattern of early proximal tubular fluid is not substantially altered during passage into final urine, thus reflecting glomerular FN release in urine. It is suggested that examination of urinary FN excretion could be helpful in the assessment of altered glomerular extracellular matrix in pathologic conditions.