REUTILIZATION OF NITROGEN SECRETED IN SALIVA OF SHEEP
March 19621 NUTRITION The patient was in negative sodium bal- ance during the entire study. He had a so- dium deficit of about 26 mEq per day dur- ing the first course of spironolactone and a deficit of about 13 mEq per day during the second course. Potassium balance became positive during the first two days of spiro- nolactone therapy, with the retention being 30 and 20 mEq per 48 hour period during the two series when the aldosterone inhibitor was administered. The potassium retention is less, the investigators state, than that retained on a single dose of 4 g. of potassium chloride given orally. The serum potassium remained within the normal range during the entire study. While the primary defect in periodic paralysis is not known, sufficient evidence has accumulated to indicate that sodium -- BEVIEWS 1 1 and potassium play an important role in this disorder. The administration of potas- sium chloride by mouth can dlter the entire course of an attack. It is difficult to under- stand how aldosterone inhibitors can pre- vent the patient from having an attack and also allow an increase in muscular strength if the activity of the antagonist is only to counteract the sodium retaining and potas- sium losing effect of aldosterone. Greater changes in sodium and potassium balance have been induced in this patient by low salt diets and the administration of potas- sium chloride without inducing such pro- nounced physical changes. The possibility exists that aldosterone inhibitors have some other role in periodic paralysis than that of simply causing sodium loss and potassium retention. REUTILIZATION OF NITROGEN SECRETED IN SALIVA OF SHEEP Urea and other nitrogen compounds in the saliva of sheep can contribute significant amounts of nitrogen to the digestive tract for reutilization b y the rumen microorgair- isms. The nutrition of man and other animals, under all conditions except germ-free ones, is really the combination of the nutri- tion of the animal and his associated mi- crobial population. This is especially true for ruminants, whose nutritional needs are determined to a great extent by the syn- thetic capacities of the rumen microorgan- isms. The ability of these microorganisms to use the nitrogen of urea for protein synthesis (:Yutrition Reviews 12, 43 (1954) ) enables urea to furnish a considerable portion of the dietary nitrogen supplied to ruminants. On the basis of this information, i t might be assumed that the ruminant could utilize urea which enters the gastrointestinal tract from the salivary secretions. The utilization oi the nitrogen of the salivary urea, which is transferred from the blood into the saliva, is really a reutilization of nitrogen and may be important to the animal when the dietary nitrogen is limited. B. Schmidt-Xielsen, K. Schmidt-Nielsen, T. R. Houpt, and S. A. Jarnum 1.4m. J . Physiol. 188,477 (1957)) found that camels fed a diet low in nitrogen excreted very little urea. They also observed that when urea was injected into a camel fed a diet low in nitrogen, very little of the injected urea was excreted in the urine. In contrast, when urea was injected into a donkey which had been fed a diet low in nitrogen, nearly all of the injected urea was excreted. From these re- sults, i t was concluded that some of the urea formed from the catabolism of proteins re- enters the rumen through the saliva. The urea is then reutilized by the rumen micro- organisms to form protein available to the host animal. The ability of the sheep to use the nitro- gen present in salivary secretions has been recently investigated by M. Somers ( A w - tral. J. Exp. Biol Med. Sci. 39, 146 (1961)). Somers speculated that the secretion of saliva, which is a continuous process in 78 SUTRITIOS REVIEWS [Vol. 20, No. 3 ruminants, might be an important route for transporting blood urea and other nitroge- nous compounds back into the rumen where they could be reutilized. These studies measured the total nitrogen and the urea nitrogen secreted in 24 hours by one parotid gland i n sheep fed diets high or low in nitrogen. The effect of intravenous injec- tions of urea on the nitrogen content of the saliva and nitrogen balance was also meas- ured. Tlirec iiiature sheep were fed the experi- iiicntal diets for a t least two weeks before the collection of saliva or the nitrogen bal- ance studies were started. The high nitrogen diet, which provided 14.5 g. nitrogen per day, consisted of 350 g. oat chaff, 30 g. lucerne chaff, 100 g. oat grain, and 200 g. sheep cubes. The low nitrogen diet, which provided 6.8 g. nitrogen per day, consisted of 500 g. oat chaff, 30 g. lucerne chaff, and 150 g. oat grain. Saliva was collected from a periminent unilateral parotid fistula. An infusion of 50 g. of sodium bicarbonate was given daily through a rumen fistula to re- plenish the sodium lost through the collec- tion of the saliva. IYhcn the sheep were fed the high nitrogen diet, the average total nitrogen in the col- lected saliva was 294 mg. per day, of which 198 mg. was urea nitrogen. The average voliiiiiv of the saliva was 2,600 ml. If the saliva from both parotid glands had been availnhle to the sheep, and if the composi- tion m i l amount of saliva secreted by both p:irotiil glands tire assuniccl to lie identical, tlicti :ipprosimately 0.6 g. of total nitrogen (twiw 294 mg.) could have been returned to the riiiii(ii by the pnrotid saliva. A return of this ainount of nitrogen to the digestive tr:ict via thc s:nlivci from both parotid gl:inrls would Ii:ivc increased tlic nitrogen entering thc iligc..-tive tract by :ipprosimately 4 per cent . IYIieii tlic shecp were transferred to tlic low nitrogcn dirt, the average total nitro- gru in the collected saliva was 117 mg. per day. with 78 nig. presrnt as urea nitrogen. The total volume of saliva collected rose to 3,300 ml., :in increase attributed to the larger amount of fiber in the low nitrogen ration. If the saliva from both parotid glands had entered the digestive tract, ap- proximately 0.2 g. of total nitrogen (twice 117 nig.) would have been added to the nitrogen intake of the sheep. This return of nitrogen would have increased the nitro- gen entering the gastrointestinal tract by 3 per cent, based on a dietary intake of 6.8 g. daily. Although the secretion of nitrogen in the saliva may appear relatively small, the ability to reutilize the salivary nitrogen may be an important economy under condi- tions of limited nitrogen intake. Also, the saliva secreted by the other salivary glands would have contributed nitrogen to the in- testinal tract. During each dietary period, the sheep were also given daily for eight days an intravenous injection of a solution provid- ing 3.0 g. of urea (1.4 g. of urea nitrogen). During both periods, the injection of urea increased the salivary nitrogen, mainly by increasing the urea nitrogen. I n addition, the nitrogen balances became more positive. In the high protein period, the nitrogen hal- ance increased from +3.1 g. to +3.5 g. In the low protein period, the nitrogen balance increased frorii -0.45 g. to +0.55 g. In the latter period, the urea nitrogen in the collected saliva increased to 388 mg. per day, and the total salivary nitrogen in- creased to 479 mg. per day. If these values can be assumed to represent the output and the nitrogen content of the saliva entering the digestive tract from the non-cannulated parotid gland, then the secretion froiii this gland would have contributed approsi- mately 0.4 g. of the increase of 1.0 g. of nitrogen in the nitrogen balance. This indicntion of the reutilization of urea nitrogen, presumably secreted :is a waste product from the blood into the saliva, is anotlicr interesting example of the inutual assistance hetmeen the ruininant :ind tlic microorg:inil;iiis of the runien.