Background: The aim of our study was to evaluate the diurnal variation in glomerular filtration rate (GFR), and the potential mechanisms responsible for such variations in GFR and albuminuria in diabetic nephropathy.
Methods: In three 24-hour urine samples, divided into a night- and daytime portion, diurnal variation in albuminuria (ELISA) was assessed. Furthermore, during recumbency changes in albuminuria, GFR (51Cr-EDTA plasma clearance) and arterial blood pressure (TM2420) from nighttime (00:00 to 05:00 hours) to subsequent daytime (08:00 to 13:00 hours) were examined in 20 type 1 diabetic patients with diabetic nephropathy.
Results: The 24-hour urine collections showed an average rise in albuminuria from night- to daytime of 51% (95% CI; 16 to 95; P < 0.01). During recumbency a non-significant rise was recorded from night- to daytime in albuminuria (22%, -8 to 61, P=0.15), simultaneously with an increase in GFR of 9.0% (3.4 to 14.5, P < 0.005) and mean arterial blood pressure (MABP) of 8.0% (4.3 to 11.7, P < 0.0001). No diurnal variation in fractional clearance of albumin was found. Significant associations between MABP and albuminuria were demonstrated during night- (R2=0.50; P < 0.001) and daytime (R2=0.48; P < 0.005). A linear regression analysis between diurnal variations in MABP and GFR showed that an increase in MABP (of 10%) from night- to daytime was associated with a significant increase in GFR (of 8.0%, 0.2 to 4.1, P < 0.02).
Conclusions: Our study revealed diurnal variations in GFR, albuminuria and MABP in diabetic nephropathy, with lowest values during sleep at night. The observed diurnal variation in albuminuria seems to be explained partly by mechanisms related to orthostasis, and partly by the diurnal variation in GFR and serum albumin concentration. The diurnal variation of blood pressure seems to play a role for the diurnal changes in GFR and albuminuria.