Impact of Extracellular Volume on Transcutaneous GFR Measurement Accuracy in Male and Female rats

Introduction: With the introduction of transcutaneous devices for measuring glomerular filtration rate (GFR), it has become possible to conveniently measure GFR in rats and mice. However, concerns about its accuracy have been increasingly raised. This study aims to assess and improve the accuracy of GFR measurements by addressing issues with the current method, particularly those caused by growth-related changes in body composition. Variations in the ratio of fat and muscle to body weight (BW) affect extracellular volume (ECV), and accounting for these changes is essential for more accurate GFR assessments. Methods: Inbred Dahl salt-sensitive (SS) rats (male & female; 8 wk age; fed 0.4% salt diet (LS)) were examined. Two inferior vena cava (IVC) catheters inserted via femoral veins were placed under isoflurane anesthesia into the IVC together with a carotid artery radiotelemetry catheter for blood pressure measurement. After one week of recovery, the rat was lightly anesthetized with isoflurane and the transcutaneous GFR measurement device (Medibeacon, inc.) was attached to the cutaneous region of the lateral abdomen. Following rapid recovery from the isoflurane, FITC sinistrin (Medibeacon, inc.) was injected through one of the catheters followed by 0.5 mL of saline to flush the catheter. FITC fluorescence intensity was measured transcutaneously every 1 second and ~70 μL of blood was collected from the other catheter at 2, 5, 10, 20, 40, and 90 min after the FITC sinistrin administration. The FITC fluorescence half-life intensity determined from the transcutaneous signal (TCT 1/2 , min) was calculated using software provided by Medibeacon. To determine the half-life (T 1/2 ) of FITC sinistrin from the sampled blood, following centrifugation of the samples, the FITC fluorescence intensity of the plasma (PlT 1/2 , min) was determined using a fluorescence intensity meter (SPECTRA Fluor Plus), and the PlT 1/2 was calculated to compare to the TCT 1/2 . Results: A strong correlation was observed between TCT 1/2 and PlT 1/2 (TCT 1/2 = 0.89 x PlT 1/2 + 0.90, n = 125, r = 0.9, p < 0.01), confirming that transcutaneous device can accurately measure the PlT 1/2 . During the observation period from weeks 9 to 15, BW of male rats grew from 270 g to 420 g, while female rats grew from 200 g to 260 g and showed little increase after reaching 250 g. The GFR calculated based on the plasma FITC sinistrin decay curve under LS conditions was 1.15 ± 0.03 for males and 1.19 ± 0.04 mL/min/100g for females, with no correlation with BW. In males, the value of GFR x PlT 1/2 was negatively correlated with BW up to 370 g, after which it stabilized at 20.0 (the developer's reference value is 21.33) (GFR x PlT 1/2 = -0.065 x BW + 45.6, n=42, r = -0.61, p < 0.01). In females, the value continued to decrease until reaching the maximum body weight of 260 g (GFR x PlT 1/2 = -0.099 x BW + 48.9, n=20, r=-0.79, p<0.01) Conclusion: For improved accuracy, transcutaneous measurements of GFR need to be adjusted as determined by sex, BW, and related differences in ECV. GFR in male rats under 370 g or female rats under 250 g, can be adjusted using the conversion factor derived in this study. Studies are ongoing to determine the effects of various factors that are known to effect ECV/BW ratios such as high-salt loading and obesity. R01 HL151587, AHA 23POST1008714, AHA 24CDA1273496 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.