Knepper M A
Am J Physiol. 1983 Jun;244(6):F622-7. doi: 10.1152/ajprenal.1983.244.6.F622.
To evaluate possible routes of urea delivery to the renal medulla, urea transport was studied in cortical thick ascending limbs and proximal straight tubules dissected from inner cortical medullary rays of rabbit kidneys. Urea was measured colorimetrically in the perfused, collected, and bath fluids. No evidence for active transport of urea was found in either segment. With imposed urea concentration differences between perfusion and bath fluids, there were significant passive fluxes of urea in both segments. The magnitude of the flux was independent of the direction of the concentration difference. Apparent permeability coefficients (X10(-5) cm/s) for urea were 2.0 for the cortical thick ascending limbs and 1.5 for the proximal straight tubules. Based on the measured permeability in the cortical thick ascending limb, substantial passive absorption of urea is predicted in vivo. This will contribute to the dilution of tubular fluid in this segment. The results in proximal straight tubules are compatible with passive urea secretion but not with a significant degree of active secretion.
为评估尿素输送至肾髓质的可能途径，我们对取自兔肾内皮质髓放线的皮质厚升支和近端直小管中的尿素转运进行了研究。通过比色法测定灌注液、收集液和浴液中的尿素含量。在这两个节段中均未发现尿素主动转运的证据。当灌注液和浴液之间存在人为设定的尿素浓度差异时，两个节段中均存在显著的尿素被动通量。通量的大小与浓度差的方向无关。皮质厚升支的尿素表观渗透系数（×10⁻⁵ cm/s）为2.0，近端直小管为1.5。基于在皮质厚升支中测得的通透性，预计在体内会有大量尿素被动重吸收。这将有助于该节段肾小管液的稀释。近端直小管的结果与尿素被动分泌相符，但与显著程度的主动分泌不符。
