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D)epartmnct ni Phi sinology Sc/nool onf Vctcrinlarv 'Icdicine, Bisc/hofs/liol/r Danitni 15, D-3000 Hannover 1, (MANIXSC RIPT RECLIN'DED 6 AtL (GUST 1991 2NCC FPTEI) 16 SEPTEMBFR 1991i SUMMARYThe cefccts of short-chain fatty acids (SCFA) on ammonia net absorption from the sheep rumen in rino and on ammonia transport across rumen wall mucosa in ritro were studied. Ammonia net absorption was directly, though in a non-lincar manner, correlated with the SCFA concentration in the artificial rumen fluid. Almost 70% of total ammonia absorption was dependent upon the presencc of SCFA when 12 mmol 1-1 ammonia and 67-5 mmol I SCFIA were present. Lactic acid was ineffective. Incubation experiments showed that mucosal disappearance and serosal appearance of ammonia were reduccd by 38 % and 32 %, respectively, when SCFA (63 mmol 1 i) were replaced by lactic acid. The SCFA effect was independent of thc type of SCFA used. In part of the experiments up to 540% of the ammonia taken up by the tissue was not recovered in the serosal incubation solution and must have been nmetabolized in the mucosa. INTRO1)UCT IONAlthough it is generally accepted that ammonia absorption through the rumen wall occurs mainly by simple diffusion of undissociated NH3 across the membrane there are experimental observations by several authors (Nolan & Stachiw, 1979; Siddons, Nolan, Beever & MacRae, 1985; B6deker, Winkler & H6ller, 1990) which show that this process is influenced by factors other than the intraruminal ammonia concentration and the pH value. Such factors may include, amongst others, short-chain fatty acids (Hogan, 1961) though to date their effects have not been fully examined.The present both in rilo and iM vitro experiments were therefore carried out to study in detail the effects of short-chain fatty acids (SCFA) on ammonia passage across the rumen wall. METHODS A417iinale feedingFour castrated male sheep (Dcutsches Schwarzkoepfiges Fleischschaf) wcighing 50-60 kg and fitted with an 8 cm fistula in the dorsal part of the rumeni were used. They were fed hay ad lib. and on aserage each consumed approximately I kg dry matter and 12 g nitrogen per day and per animal. (40°C) were instilled into the isolated reticulo-rumen and continuously gassed with CO2. Samples of 10 ml were taken before and 3 and 23 min after the solution was placed into the reticulo-rumen. Immediately after sampling the pH values were determined, then the samples were acidified with formic acid (90 %) and stored at -18°C for later analysis. All buffer solutions used had an osmolarity of about 300 mosM, were adjusted to pH 6-8 and contained Cr-EDTA (0 8 mM) as a fluid volume marker.In the first set of experiments, the effect of a SCFA mixture on ammonia net absorption was studied. Six different concentrations of total SCFA were applied, and lactic acid was added to the buffer solutions to obtain a total acid concentration of 67 5 mm. Ammonia concentration was always 12 mm. The solutions were instilled into the rumen in random order. The composition of the buffe...