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▸ State the physiological utility of either excreting or reabsorbing organic solutes.
▸ State the general characteristics of the proximal tubular systems for active reabsorption or secretion of organic solutes.
▸ Describe the renal handling of glucose and state the conditions under which glycosuria is likely to occur.
▸ Describe the renal handling of proteins and small peptides.
▸ Describe the secretion of para-aminohippurate.
▸ Outline the handling of urate.
▸ Describe the secretion of organic cations.
▸ Describe how tubular pH affects the excretion and reabsorption of weak acids and bases.
▸ Describe the renal handling of urea, including the medullary recycling of urea from the collecting duct to the loop of Henle.
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As pointed out in Chapter 1, a major function of the kidneys is the excretion of organic waste, foreign chemicals and their metabolites. As the kidneys excrete these substances they also filter large amounts of organic substances that they do not excrete, such as glucose and amino acids. Therefore, the kidneys must discriminate between what to keep and what to discard. While the collective concentration of the useful organic solutes that should be kept is small in comparison with inorganic ions like sodium and chloride, the large amounts filtered means that processes must exist to reabsorb them.
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Some organic solutes handled by the kidneys are neutral molecules; most are anions or cations. As the useful metabolites are recovered from the filtrate, the waste and foreign substances are not only let go, but actively secreted. In dealing with organic solutes the kidneys perform a kind of triage. They (1) reabsorb metabolites that should not be lost, (2) eliminate waste products and unwanted foreign organic substances, and (3) partially reabsorb others. An analysis of the renal handling of every one of these organic substances would be prohibitive, so we will discuss a few key solutes and establish generalities about the others.
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One organic substance, urea, is unique in this regard. It is a waste product that must be excreted to prevent accumulation. However, it also plays a key role in renal regulation of water balance. The renal handling of urea is briefly discussed later in this chapter and again in the following chapter in the discussion of renal handling of water.
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General Properties of Organic Solute Transport
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Several generalizations apply to the handling of small organic solutes by the kidney.
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While there is a strikingly large number of organic solute species, there is a far smaller number of transport protein species, meaning that many transporters are promiscuous, accepting multiple solutes, sometimes over 100 different ones. This allows the kidneys to operate without expressing a separate transporter for each and every solute.
Most organic solutes are transported only in the proximal tubule. Those that are secreted or escape reabsorption in the proximal tubule ...