A variety of critical renal processes exhibit flow dependence, which include glomerular filtration and glomerulotubular balance in various nephron segments and the mechanisms can be classified as generative and/or dissipative in nature. Flow-dependent examples include glomerular filtration, reabsorption in the proximal tubule and thick ascending limb, and K+ secretion in the collecting duct. For generative flow dependence, the driving force for reabsorption is generated within the transporting segment. With regard to the dissipative mechanism, the driving force for transport is generated upstream from the site of transport. The transport process itself then tends to dissipate its driving force and higher flow rates act to sustain this driving force. Various examples of these processes will be analyzed conceptually and then applied specifically to data relating to the tubuloglomerular feedback system. Transport in the loop of Henle and specifically the thick ascending limb and macula densa demonstrates a high degree of flow dependence. Na-2Cl-K symporter activity in the thick ascending limb and macula densa exhibits dissipative flow dependence related to the effects of flow upon luminal sodium concentration. However, more recently, it has become apparent that luminal potassium concentration within the thick ascending limb exhibits flow dependence which may contribute to the flow dependence of transport process. In this sense, it is both generative and dissipative. The shape of the tubuloglomerular feedback curve is characterized by saturation of vasoconstrictor responses at high flows which may relate to saturation of transport at the sensing transport elements. Data on intracellular sodium concentration or some other consequence of NaCl uptake may be required to fully understand the characteristics of the feedback system.
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