Measurement of changes in glomerular filtration rate induced by atrial natriuretic peptide in the rat kidney

This study was undertaken to improve the measurement of glomerular filtrati on rate (GFR) during the acute diuretic phase induced by atrial natriuretic peptide (ANP), which may indeed alter the renal clearance of inulin (GFR(C L)) due to dead space error. A technique to measure GFR without urine colle ctions was therefore developed in anaesthetized rats prepared as for microp uncture. To do so, arterial blood was periodically collected and renal veno us blood was withdrawn simultaneously from a catheter inserted into the lef t suprarenal vein to determine the renal extraction coefficient of inulin ( CEIN). In addition, renal Mood flow (RBE) was continuously measured with an electromagnetic flow transducer fitted around the left renal artery to est imate renal plasma flow (RPF). GFR (GFR(CE)) was then calculated as the pro duct of RPF and CEIN. To study the effects of ANP on GFR, rats were injecte d I.V. with 10 mu l of saline without (n = 6; vehicle) or with 1 mu g ANP ( n = 6; ANP) and GFR(CE) and GFR(CL) were compared before and after each tre atment. They did not differ significantly during baseline measurements in e ach experimental group and were not modified after vehicle. Similarly, RBF remained constant. In contrast, RBF and GFR(CE) increased rapidly and simul taneously 90 s after ANP, from 9.07 +/- 0.25 to 10.07 +/- 0.35 (12%) and fr om 1.209 +/- 0.188 to 1.715 +/- 0.190 ml min(-1) (42%), respectively (P < 0 .05). GFR(CL) increased to an even greater extent (88%). Moreover, the peak enhancement of GFR,, was delayed and occurred 180 s after ANP. The renal c learance of inulin was thus unduly elevated due to sudden changes in the de ad space induced by the diuretic effect of ANP. In conclusion, determinatio n of glomerular filtration rate by the method of renal extraction of inulin provided more reliable results than those achieved using the classical met hod of renal clearance of inulin. Moreover, it was sufficiently sensitive t o detect small and transient changes in GFR induced by the injection of 1 m u g ANP.