Urinary excretion of renin and its biochemical properties in dogs

The amount and biochemical properties of renin excreted by anesthetized dogs were investigated to elucidate the significance of urinary excretion in the metabolism of renin. Mean arterial blood pressure was 127 +/- 4 mm Hg, renal blood flow was 170 +/- 8 ml/min, glomerular filtration rate, 38.6 +/- 2.3 ml/min, and urine flow rate, 0.37 +/- 0.09 ml/min (n = 11). Urinary renin concentration (URC) was 9.2 +/- 2.1 ng angiotensin I (ANG I)/ml X hr (n = 11), as determined by radioimmunoassay for ANG I generated by incubation with semipurified homologous renin substrate. The ANG I-producing activity was inhibited by more than 90% by a specific antibody to dog kidney renin. The renin secretion rate from the left kidney into the renal vein was 76.4 +/- 13.3 ng ANG I/ml X hr per min (n = 11), and the simultaneous urinary excretion rate of renin was 2.3 +/- 0.4 ng ANG I/ml X hr per min (n = 11). Molecular weight of the urinary renin was 40,000 daltons. The pH dependent curves of the angiotensin-forming capacity of renin showed an optimum between pH 5.5 to 6.0, and the estimated Michaelis constant was 0.42 microM. These biochemical parameters were similar to the findings in the case of renin in the plasma and the kidney. Moreover, neither acid nor trypsin treatment altered the renin activity in the urine. Thus, the active form of renin with a molecular weight 40,000 was excreted into the urine in dogs. Urinary excretion of renin was a small percentage of the renin secretion rate, thereby indicating the minor role of urinary excretion in the metabolism of renin.