Micropuncture and renal clearance techniques have been used to investigate the effect of molecular size on glomerular permeability in the river lamprey, Lampetra fluviatilis, and to investigate tubular absorption of native and foreign macromolecules.

Polyvinyl pyrrolidone (PVP) molecules of 16 Å in molecular radius were freely filterable but there was significant retention of 37 Å molecules. The estimated sieving coefficients for PVP of 160 000 and 360 000 in molecular weight (molecular radii > 100 Å) were 0.06 and 0.01 respectively, indicating that the glomerular barrier is relatively leaky.

Up to 50% of microinjected protein was absorbed by the nephron and the results with bovine serum albumin demonstrated that protein uptake occurs chiefly in the proximal brush border segment. The relative rates of uptake for different proteins and different concentrations of injected solutions indicated that the uptake mechanism is easily saturated and unable to distinguish between native and foreign proteins of high molecular weight.

Most lamprey plasma proteins are > 100 000 in molecular weight. This, together with other factors such as molecular charge and tubular reabsorption, could account for the low urinary protein concentration of 0.05 mg/ml.

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