Sparing action

We shall consider these possibilities in detail, but it may be helpful if we state briefly at the outset what we believe are the most likely conclusions that can be drawn from existing evidence. 

We believe it to be likely that each of the mechanisms ennumerated functions in different situations. For example, a reduction in the metabolic need undoubtedly occurs when animals are fed diets free from carbohydrate, and are then able to survive for considerable periods without dietary thiamine (Banerji and Yudkin, 1942 Yudkin, 1951). However, we know of no example of the reverse situation, where the addition of carbohydrate or carbohydrate-like substances to the diet will reduce the metabolic needs. 

Increased absorption is almost certainly the cause of the raising of the blood and tissue levels of vitamin B12 by sorbitol and some other substances. Intestinal synthesis is responsible for the far more dramatic sparing of other B vitamins by sorbitol, ascorbic acid, and several other substances, so that animals become completely independent of dietary sources of these vitamins. Finally, the ability of ascorbic acid to prolong the life of animals deficient in folic acid is very probably due to the promotion of the synthesis of this vitamin in the tissues. 

If substances spare the B vitamins by increasing vitamin synthesis, or by reducing metabolic requirements, they should act equally well when given parenterally. Ekman and Strcimbeck (1949) reported that rats survived a very short while longer on riboflavin-deficient diets when they were injected with small doses of ascorbic acid, though there was no difference in the appearance of the signs of deficiency. If these results are confirmed, they would imply that ascorbic acid can indeed reduce the metabolic need for riboflavin, but only to a very limited extent. 

The only report in which the effect of injection was compared with that of oral administration is that of Hundley and Ing (1953), who showed that 

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Sparing action on nutrient requirement in animals on incomplete diets. 

The K+-sparing action of spironolactone, triamterene, and amiloride serves as the basis for their occasional use in the therapy of primary hypertension. The drugs can be employed in conjunction with other types of diuretics to help alleviate the K+ loss caused by them. Under these conditions, K+ balance is improved while natriuresis is maintained. 

The protein-anabolic and protein-sparing actions of GH require the metabolic effects of insulin, glucagon, cortisol, and thyroid hormone in the unstressed individual. These actions depend on fine-tuned control of GH release, which is achieved mainly by substrate feedback to the hypothalamus (Figure 31-5). 

Normally, 65-80% of filtered calcium and 85-90% of filtered phosphate are reabsorbed, mainly in the proximal tubule. The daily loss of 700-800 mg of phosphate is balanced by dietary intake. Fine-tuning of calcium excretion is accomplished by PTH in the distal convoluted tubules and collecting ducts. Phosphate excretion is regulated by PTH in the proximal tubules. Elevation of the PTH level increases reabsorption of calcium and decreases reabsorption of phosphate from the tubules. This phos-phaturic action opposes the phosphate-sparing action of 1,25-(0H)2D. 

A more marked protein-sparing action has been reported for fructose as compared to lactose, glucose, sucrose, or dextrins (2, UU) While both glucose and fructose speed alcohol metabolism, fructose has the greater effect (13). 

Jones, H. M. The protein sparing action of utilisable carbohydrates in cultures of certain sugar fermenting organisms. Jour. Infectious Diseases, Vol. 19,... 

The Vitamin-Sparing Action of Sorbitol, Sugars, and Related Substances. 

With this device, Morgan and Yudkin (1959) and Cremer and Hotzcl (1959) showed that the prevention of coprophagy abolished the thiamine-sparing action of sorbitol. Similarly, Morgan (1960a,b) has shown that starch refection occurred in all rats kept on sawdust, in only about half of the rats kept on wire-mesh screens, and in none of the rats fitted with aiiticoprophagy devices. 

The sparing action of vitamin E on vitamin A, discovered by Dr. Moore, and studied in great detail by Dr. Harris, is one of the most well-characterized nutritional effects of vitamin E. The consensus of opinion seems to be that vitamin E protects vitamin A and carotene from oxidation by acting as an antioxidant. 

Amino acids arising from the mobilisation of body protein are utilised with the same efficiency as absorbed amino acids, and liveweight loss has a sparing action on metabolisable protein requirement equal to its protein content, i.e. 138 g/kg. An example of the calculation of the metabolisable protein requirement of a lactating cow is given in Box 16.4. 

Even today, after the development of many powerful and useful new drugs, the organomercurials are unique among these potent diuretics, in spite of their several disadvantages. The mercurials cause little or minimal potassium loss and, in fact, under certain conditions, may exhibit a potassium-sparing action. They are not known to disturb carbohydrate metabolism. They do not cause uric acid retention and certain of them have been shown to have a uricosuric action in man. Whether these desirable properties are a consequence of their structure and mode of action or result from the intermittent or spaced manner of administration is not clear. But these properties of the mercurials continue to be a challenge to the medicinal chemist and present objectives for the design of new structures. 

Pyrimidines - The leukocytes of patients responding to l- -D-arabino-furanosylcytosine (ara-C) exhibited reduced DNA and elevated RNA and protein contents, whereas the normal bone marrow cells of these patients exhibited increased DNA, RNA, and protein contents. Despite extensive intracellular conversion of ara-C to nucleotides, only limited incorporation of the compounds into nucleic acids occurred. 2-Deoxy-cytidine reverses the effects of ara-C by inhibiting its phosphorylation, an anabolic step that must occur before it can inhibit DNA polymerase and, consequently, cell proliferation. Uridine exerts a sparing action on ara-C, and the effective level of this drug can be prolonged by coadministration of tetrahydrouridine, a potent inhibitor of deoxycytidyl-ate aminohydrolase. ... 

Experiments performed on the nicotinic acid-ricinine relationship in sterile cultures of R, communis established clearly that (1) the relationship exists, and (2) the metabolism of ricinine can be spared by the presence of higher concentrations of nicotinic acid than normally found in the tissue (Waller and Nakazawa, 1963). This sparing action of nicotinic acid on ricinine utilization suggests a vitamin-alkaloid metabolic relationship not previously found in a plant system. 

In the presence of the antibiotic the amount of vitamin B12 required is often reduced, the antibiotic appearing to exert a sparing action. This does not obviate the need of a dietary source of this factor for normal growth in young animals or for reproduction. Nor does the feeding of vitamin Bi2 and antibiotics at optimal levels appear to alter the need for other members of the B complex. 

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