Important aspects of urine sampling for angiotensinogen measurement: time and preservation conditions in healthy individuals.

Intrarenal renin-angiotensin system (RAS) plays an important role for the pathogenesis of renal injuries. Experimental studies have demonstrated that angiotensinogen levels in renal tissues reflect the activity of intrarenal RAS. However, dynamics of urinary angiotensinogen have not been investigated in detail. Therefore, we examined the preservation conditions of the measured values of urinary angiotensinogen concentrations and an ultradian rhythm of urinary angiotensinogen excretion in humans. Urine samples were collected from 24 healthy volunteers. The urinary concentrations of angiotensinogen were measured by using ELISA. Two different urine preservation conditions were examined. One cycle of freeze-and-thaw did not change the measured values of urinary angiotensinogen concentrations. Moreover, to keep urine samples at room temperature for 12 hours did not change the measured values of urinary angiotensinogen concentrations. Thus, preservation conditions do not change the measured values of urinary angiotensinogen concentrations. Regarding an ultradian rhythm, blood pressure and the urinary concentrations of angiotensinogen were measured at 09:00, 13:00, and 16:00. The averaged levels of blood pressure were similar over the time. The average of urinary angiotensinogen/creatinine (Cr) ratios was 8.73 ± 1.15 ng/mg Cr at 09:00, 9.53 ± 1.58 ng/mg Cr at 13:00, and 8.58 ± 1.26 ng/mg Cr at 16:00. The urinary angiotensinogen excretion in healthy volunteers does not have an ultradian change during the daytime (P = 0.482). This may be another indication that the intrarenal RAS is independent of the systemic RAS. We have to pay attention to these findings in handling urine samples for measurements of angiotensinogen.