COMPARATIVE RENAL DISPOSITION OF CREATINE, AND TECHNETIUM DIAGNOSTIC CONTRAST AGENTS IN THE PIGEON (COLUMBA LIVIA)

Clinical assessment of renal function in avian species often involves the measurement of plasma uric acid and blood urea nitrogen, relatively insensitive markers of renal dysfunction and dehydration. In mammals, endogenous creatinine is widely used as an indicator of renal glomerular dysfunction. However, avian species produce primarily creatine. Here, renal creatine, (99m)Tc99-DTPA (diethylenepentaacetic acid, DTPA) and 99mTc-MAG3 (mercaptoacetyl triglycine, MAG3) renal clearances are characterized in the pigeon avian model by infusing DTPA with inulin and creatine with each tracer and examining the slope of their blood disappearance curves. Clearance curves for inulin and DTPA were parallel, suggesting DTPA is cleared by renal filtration. MAG3 clearance (slope: -2.74 x 10(5), r(2) = 0.97) had a slope almost 10-fold steeper than for DTPA (slope: -6.29 x 10(4), r(2) = 0.90), and orders of magnitude steeper than for creatine (slope: -1.4, r(2) = 1.0). These results suggest that DTPA is cleared by glomerular filtration like inulin, while MAG3 is filtered and actively excreted in a manner similar to mammals. In contrast, creatine is filtered and resorbed, has a larger volume of distribution (Vd), or exhibits a greater blood protein binding, making it more complex as a renal marker, when compared with creatinine handling in mammals. The two radiotracers can be readily adapted for use in birds, inviting both qualitative and semiquantitative functional evaluation of avian renal function for research and clinical purposes. The elimination of creatine appears to be more complex requiring further study.