The purpose of this work was to investigate insulin receptors in growing ruminant sheep given a control diet or undergoing nitrogen restriction. Live weights ranged within 30-36 kg in both groups. Before the animals were fed (i.e. at 8 a.m.) blood insulin was not significantly different in the two groups (0.39 vs 0.53 nM in experimentals and controls, respectively). The 125I-monoiodoinsulin binding to purified liver plasma membranes was studied. Membrane recovery and purity were similar in both groups. Results showed that specific 125I-iodoinsulin binding increased with time and reached a maximum value within 60-120 min. Increasing the nonlabelled insulin level inhibited 125I-iodoinsulin binding at steadystate. Among the animals from both groups, specific insulin binding decreased significantly with increased live weight. In addition, specific insulin binding lowered with increasing blood insulin. The latter relationship partly reflected the insulin binding-live weight-blood insulin relationships. The insulin binding was similar in both groups. Furthermore Scatchard analysis indicated no significant differences between apparent affinity constants and apparent binding capacities in the two groups.

Summary. This Introduction.

Changes of free amino acids levels in the blood of ruminants were investigated for several reasons, mainly in the evaluation of feedstuffs (2, 8, 15, 16). It has been found that urea decreased the level of some amino acids in the plasma (3, 7, 10, 16), probably as a result of the deficiency of some amino acids induced by changes in rumen protein synthesis or as a result of the passage of amino acids from plasma into organs and a change of their metabolism in the tissues due to the different availability of energy sources.It is assumed that amino acids are absorbed from the jejunum continously as a function of the passage of digesta (1, 15, 17). Therefore, the changes of amino acids levels in blood result from tissue metabolic processes rather than from the fluctuation of amino acids absorption. Urea in the diet causes many changes in the metabolic processes in the tissues, chiefly because of ammonia absorption from the rumen (12).In the present experiments we tried to estimate the daily concentrations of amino acids and ammonia in the blood of sheep fed on a urea containing ration. Our previous investigations on sheep, cattle and goats (2, 3) indicated considerable variation of individual amino acid levels in blood during the day. Experimental procedureThe experiment consisted of two trials. Trial 1 was carried o u t on 4 wethers 12 months old weighing 40 kg. The daily ration containing urea (UR) included 300 g dried sugar beet pulp, 18 g urea and a mineral-vitamin mixture, all given a t 8 h, and 500 g hay given at 19 h. Urea-N in the concentrate amounted to about 70°/o and in the daily ration to about 45O/o of total N. Samples of rumen content and blood for ammonia and amino acid determinations were taken a t 7.30, 10, 12, 1 8 h.In the second period of trial 1 all animals were given a urea-free ration (UFR) consisting of 300 g concentrates (2OOio oat meal, 40°/o wheat meal, 200io soya been oil

The changes in the concentration of metabolites in the blood have been used for the evaluation of feeds for many years. This method has practical application in monogastric animals rather than in ruminants. The rate of protein and energy metabolism in the rumen influcnces the plasma cornFosition but it is not easy to establish a direct relationship between them, because many other factors (the rate of tissue metabolism, hormones) also influence the concentration of metabolites in the blood (MILLER 1973, BASSET 1974, BERGEN et al. 1973. ARMSTRONG and HUTTON (1974) and WILLIAMS and SMITH (1974) suggested however, that after obtaining a certain rhythm in the rumen processes, the digesta in the duodenum and the blood plasma contained a stabilized amino acid concentration.Since a relationship between the composition of rumen content and plasma could be found under some conditions we intended to investigate such relations by applying various feed rations. The present paper describes an experiment with bulls fed o n low-protein diet containing readily digestible carbohydrates, periodically supplemented with urea. The level of amino acids in the blood plasma and some metabolites in the blood and rumen content were estimated. Experimental procedureThe experiment was carried out on 8 bulls of an initial average body weight of 175 kg, divided into two equal groups: A and B and lasted 186 days. The animals of group A were fed o n sugar beet silage, urea-mineral concentrate and hay. The animals of group B during the first 102 days (first period) received the same ration but without urea. During the second period of 84 days the animals of group B received the same ration as group A. The composition of the rations is presented in Table 1.The rumen content and blood from all animals were analysed a t 15, 85 and 130 days of the experiment. The samples were collected before morning feeding U.S.

Studies using heterochronic blood transfer (HBT) and heterochronic plasma transfer (HPT) show that circulating factors in young blood and plasma can rejuvenate the ability of aged skeletal muscle to respond to damage. Additionally, the transfer of old blood and plasma to young mice inhibits skeletal muscle regeneration. These changes in the ability of skeletal muscle to regenerate in response to modifications in the systemic environment are largely mediated by satellite cells. However, the role of satellite cells in maintaining normal muscle function is unclear. Therefore, how the systemic environment influences skeletal muscle in the absence of extreme muscle damage is still unknown. Proteostatic maintenance, which includes protein synthesis and breakdown, is necessary for cellular stability in skeletal muscle. One measure of proteostatic maintenance is protein:DNA ratio (i.e. how much protein is synthesized for cellular maintenance verses cellular proliferation). Here we determined if altering the systemic environment of old mice with young plasma and the systemic environment of young mice with old plasma would change proteostatic maintenance of skeletal muscle. We hypothesized that compared to old mice that receive old plasma (OOp), old mice that underwent HPT from young mice (OYp) would improve proteostatic maintenance. Additionally, we hypothesized that compared to young mice that receive young plasma (YYp), young mice that underwent HPT from old mice (YOp) would decrease proteostatic maintenance. To test this, we implanted young (5‐month, n=3 YYp and 5 YOp) and old (24‐months, n=5 OOp, and 5 OYp) C57BL/6J mice with a jugular catheter. All mice underwent heterochronic plasma transfer (HPT), receiving 100μl plasma injections, every three days for 24 days. The day before HPT, mice began stable isotope labeling with a bolus injection of 99% deuterium oxide (D2O) followed by 8% D2O‐enriched drinking water. After 4 weeks, the quadriceps, gastrocnemius, and tibialis anterior muscles were harvested and analyzed for DNA and protein synthesis to calculate the protein:DNA ratios. The protein:DNA ratio for the mitochondrial fraction trended toward a higher ratio in the tibialis anterior of the OYp versus OOp (11.93 ± 1.161 and 8.928 ± 1.128, respectively: p=0.10). The protein:DNA ratio for the myofibrillar fraction trended toward being lower in the gastrocnemius of the YOp compared to YYp (8.338 ± 0.429 and 6.281 ± 0.737, respectively: p=0.06). These results suggest that manipulating the systemic environment with young or old plasma can change proteostatic maintenance of skeletal muscle so that the muscle reflects the systemic environment. Therefore, strategies to improve the systemic environment, like with exercise, may be a feasible strategy to improve proteostatic maintenance in aged skeletal muscle. Further research is needed to determine what circulating factors alter protein and DNA turnover in skeletal muscle.