Experiments were carried out in vivo and in vitro to study the flux of Ca2+ across the sheep rumen wall. Buffer solutions with five different Ca2+ concentrations in the range 0.2-3.6 mmol l-1 were introduced into the isolated, emptied and washed reticulo-rumen for 1 h to determine Ca2+ net flux. Phosphate concentrations were either kept constant at 8 mmol l-1, or varied so as to maintain a constant phosphate/Ca2+ ratio of 2:1. Under both conditions there was a Ca2+ net secretion into the rumen at Ca2+ concentrations below approximately 1 mmol l-1 whereas Ca2+ net absorption occurred at higher Ca2+ concentrations and linearly increased with increasing Ca2+ concentrations. When phosphate concentrations were reduced to 4.3 and 0.6 mmol l-1 (Ca2+, 3 mmol l-1) Ca2+ net absorption was reduced by about 50% and completely abolished, respectively. The unidirectional (mucosal-to-serosal or serosal-to-mucosal) Ca fluxes in vitro, JCams and JCasm, were measured in Ussing chambers with computer-controlled voltage clamp. Under short-circuit current conditions (Ca2+ and phosphate, 2 mmol l-1) there was a significant net flux of Ca2+ from the mucosal to the serosal side. This net flux of Ca2+ was abolished when ouabain was added to the serosal solution, indicating that the unidirectional Ca2+ flux JCams and, hence, the net flux of Ca2+ was dependent upon active Na+ transport accomplished by the Na, K-ATPase system. Under voltage clamp conditions, the net flux of Ca2+ was greatly increased at a potential difference PD = -25 mV (serosa negative) but disappeared at PD = +25 mV (serosa positive). Analysis of the unidirectional Ca2+ fluxes JCams and JCasm under varying potential differences indicated that both Ca2+ fluxes consisted of a PD-dependent and a PD-independent component. The results show that the reticulo-rumen participates in Ca2+ absorption in sheep.

Experiments were carried out to study the effects of dietary P depletion on plasma concentrations of inorganic P (P|), calcium, l,25-(OH) 2 -Vit.D 3 and alkaline phosphatase, and to investigate the effects of P depletion, in comparison with P repletion, on intestinal flow and net disappearance of Ca and P in sheep. The animals were adapted to an experimental diet of pellets and chopped straw providing between 0-91 and 1-04 g P/day for depletion. They were repleted by single infusions of phosphate into the duodenum raising total P supply to about 4-1 g/day. During P depletion plasma P, concentrations decreased significantly whereas those of Ca increased. The development of hypercalcaemia is discussed with respect to intestinal absorption and bone accretion and resorption. Plasma l,25-(OH) 2 -Vit.D 3 and alkaline phosphatase were not affected by P depletion.In P depletion the flow through the gastrointestinal tract of total P contained in the digesta and faecal P excretion were significantly reduced and the P balance was slightly negative. Calcium net absorption from the gut was also reduced. Daily flow of total P and P, contained in the particle-free intestinal fluid was markedly lower in P depletion as compared with P repletion. The percentage of net disappearance from the small intestines of total P and P, in the particle-free fluid was 57 and 70 % in the depleted state and 79 and 93% in the repleted state.

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...

SUMMARYExperiments were done in vivo and in vitro to study the flux of inorganic phosphate (Pi) across the rumen wall in sheep. The technique of the temporarily isolated and washed reticulo-rumen was applied to measure Pi net flux at nine different Pi concentrations in the buffer solutions introduced into the reticulo-rumen for 1 h. Six sheep were either P replete (4-25 g P d-l) or in a P-depleted state (1-05 g P d-1). Under both dietary treatments Pi net secretion into the reticulorumen was found at Pi concentrations between 0 1 and 2-2 mmol 1-1 whereas Pi net absorption was observed at P, concentrations between 4-1 and 15 8 mmol 1-1. There

SUMMARYIn fistulated sheep (50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60) kg live weight) the absorption of ammonia from the reticulo-rumen in vivo was studied applying the technique of the temporarily isolated and washed reticulorumen. It was found that, at ammonia concentrations between 3 and 18 mm, ammonia efflux and ammonia net absorption were linearly related to the ammonia concentration in the artificial rumen fluid, whereas influx of ammonia nitrogen from endogenous sources remained almost constant. When the concentration of unionized NH3 was changed at the ratio 1: 10:76 by varying the pH from 5-8 to 6-8 and 77, ammonia net absorption did not reflect the concentration ratio of unionized NH3, indicating either flux of NH4+ ions or titration of NH4 + to NH3 at the absorptive surface. In the experiments with buffer solutions without ammonium salts and extended over 2 h, ammonia concentrations in the artificial rumen fluid increased due to endogenous nitrogen influx and reached levels far beyond the expected plateau concentration of about 2 mm. Labelling of the N pool in the isolated organ by 15N showed that ammonia efflux had almost ceased in these experiments. It is argued that as yet unidentified changes have taken place in the artificial rumen fluid during the experiment, but there is some reason to believe that volatile fatty acid (VFA) absorption was affecting ammonia absorption.