3H-inulin and electrolyte concentrations in Bowman's capsule in rat kidney. Comparison with artificial ultrafiltration.

Micropuncture experiments were performed on Bowman's capsules in male and female non-diuretic Munich rats. 55 samples were collected and analysed for Na, Cl, K, Ca, P, Mg and 3H-inulin contents. Their electrolyte concentrations were compared to the corresponding concentrations obtained from plasma artificial ultrafiltrates. Compared to normal Wistar rats, our Munich rats had several special characteristics: high arterial pressure (115-155 mm Hg), high concentrating ability (phi equals 2684 mOsm/1) and high NaCl reabsorption capacity. Whole kidney GFR was low in males (0.361 ml/min/kidney/100 g B.W.). 3H-Inulin concentration was the same in plasma and glomerular ultrafiltrates (GF). The corresponding ratio (GF/P) in equals 1.03 plus or minus 0.01 (N equals 55) confirmed the lack of sieving effect for inulin. GF electrolyte concentrations were, for Na, Cl, K, Ca, P, Mg respectively 139 plus or minus 0.05, 1.46 plus or minus 0.07 and 0.52 plus or minus 0.04 ml/1 (N equals 23) in females and 141 plus or minus 2, 120 plus or minus 2, 3.84 plus or minus 0.07, 1.31 plus or minus 0.03, 1.78 plus or minus 0.05 and 0.48 plus or minus 0.01 mM/1 (N equals 32) in males. Comparison of ultrafiltration trough a cuprophan membrane and glomerular ultrafiltration led to the following conclusions: For Na, Cl and P artificial and glomerular ultrafiltration produced identical results. On the other hand, for Ca and to a lesser extent for Mg and K ions, artificial ultrafiltration did not accurately reflect the true glomerular ultrafiltrate composition. The reasons for these differences are discussed below.