A distinct advantage of MRI over the usual means of assessing renal anatomy, gross dissection, in the process of renal phenotyping is the ability to image changes in individual mice non-invasively over time. Indeed, in the present study, there was a high correlation between the MRI images and the renal morphology shown on renal dissection. Important factors in the application of MRI to the mouse kidney are high spatial resolution, respiratory gating to remove respiratory motion artifacts in the abdomen, and use of a spin echo technique. We used a 7.0T MR system, ECG gating to remove arterial motion blurring, and respiratory gating. Both single-echo spin echo techniques and respiratory gating prolong imaging time compared to ungated or faster repetition-time imaging techniques, but produce superior images in this application. High contrast visualization of the renal pelvic space results from concentration of the contrast agent in urine as it is excreted through the kidneys from the bloodstream. We used subcutaneous injection of the contrast agent as opposed to the standard intravenous route. This allowed at least a 30-minute period of highly enhanced fluid in the kidney. This was a sufficient imaging period to obtain a multislice, respiratory gated spin echo data set through the kidney.
Contrast-enhanced MRI at 7.0T yielded high-contrast images of mouse kidneys in UT-A1/3-/- mice, differentiating three different morphological appearances. These three appearances, confirmed by direct observation of resected kidneys, were: 1) normal kidneys with no accumulation of contrast agent in the renal pelvis; 2) frank unilateral hydronephrosis, with marked dilation of the pelvic space accompanied by atrophy of the renal parenchyma; and 3) a pattern of contrast accumulation consistent with pelvic reflux. The latter was associated with the presence of contrast agent over the inner medulla with no deformation of the renal parenchyma.
The MRI imaging method allowed assessment of the effects of time and diet on kidney morphology in serial images of kidneys from UT-A1/3-/- mice over several days (). The reflux pattern was usually not present or relatively mild at the earliest time point (11 weeks) but appeared or progressed at longer time points. Low protein diet did not eliminate the progression of the reflux pattern, and overall the results indicate that dietary protein restriction did not significantly reduce the extent of reflux or hydronephrosis, suggesting that the degree of polyuria in later stages of progression is not a significant factor. MRI results also indicate that although the reflux pattern may advance over time, it does not convert to hydronephrosis in the 16 week time frame. We observed that in those mice that have hydronephrosis, it is unilateral in the right kidney, while reflux pattern was observed either unilaterally or bilaterally. When observed, the hydronephrosis was far advanced at 11 weeks, and did not appreciably change by 16 weeks. Thus, hydronephrosis and reflux appear to be separate, independent processes.
UT-A1/3 knockout mice also manifested mild hypertension and accelerated excretion of a saline/urea load administered by gastric gavage. Hypertension has been seen previously in rodents with hydronephrosis [19
], although it is not clear whether the either renal pelvic abnormality is causally related to the hypertension observed in the present experiments. Based on the present findings, the hypertension in UT-A1/3-/-
mice is unlikely to be due to abnormalities in the renin-angiotensin-aldosterone system. Furthermore, we previously observed that UT-A1/3-/-
mice exhibit more than double the rate of NO product excretion than wild-type mice [2
]. Thus, the hypertension in UT-A1/3-/-
mice does not appear to be due to NO deficiency. Further studies are required to determine the mechanism of hypertension in these mice.
Could the urine concentrating defect seen in the previous studies with UT-A1/3-/- mice [1
] be attributable to hydronephrosis? The answer is no, because the kidneys in the previous studies were examined visually and the occasional mouse that manifested hydronephrosis was not included in the analysis. It is likely, however, that reflux was present in many of the mice studied previously, since the presence of reflux is not associated with obvious structural abnormalities of the renal medullary parenchyma and would not have been detected by visual inspection. Whether reflux is truly an abnormality or a physiological variation of normal is presently unclear. Schmidt-Nielsen [18
] demonstrated that pelvic reflux is seen in normal rodents in states of rising or high urine flow as may be present in our mice. The physiological and pathological significance of pelvic reflux remains to be evaluated and high resolution MRI provides a tool that could be exploited in the pursuit of this question.