Metabolism of [U-¹⁴C]-l-threonine and [U-¹⁴C]-l-phenylalanine by the isolated perfused udder

Abstract

A lactating mammary gland of a goat was perfused for 9 h in the presence of [U-14 C]-L-threonine and received adequate quantities of glucose, acetate and amino acids. Two lactating sheep udders were likewise perfused in the presence of [U-14 C]-Lphenylalanine: the plasma levels of phenylalanine in the first of these experiments were 4 times higher than in the second.In the P 4 C]threonine experiment, 4 % of the casein and 0-4 % of the expired CO 2 were derived from threonine; 85 % of the threonine and 1-6 % of the glycine residues in casein originated from plasma threonine. Small 14 C levels were found in glutamic acid, aspartic acid and serine of casein. The relative specific activities amongst the casein amino acids and the appearance of appreciable labelling in plasma glycine are consistent with the view that threonine is split by threonine aldolase.In the [ 14 C]phenylalanine experiments virtually no radioactivity was detected in CO 2 , lactose or citric acid, indicating that this substrate is not broken down by mammary tissue. In the second experiment, 96 % of the phenylalanine and 0-3 % of the tyrosine of casein originated from plasma phenylalanine. In the first experiment, a 30-fold higher 14 C incorporation into casein tyrosine relative to phenylalanine was observed. The possible significance of the phenylalanine concentration in the plasma on the degree of conversion of phenylalanine into tyrosine within the mammary gland is discussed.Most milk proteins are synthesized in the mammary gland from free amino acids. Studies on the uptake of free plasma amino acids by the lactating udder have been carried out by the technique of arterio-venous differences across the udder on cows (Verbeke & Peeters, 1965) and goats (Mepham & Linzell, 1966). In both species an important uptake of phenylalanine, tyrosine and threonine by the udder was observed. In the experiments on cows the mammary blood flow was not measured, but calculations were made on the basis of the differences observed, using average values from the literature for the blood flow through the udder. It was calculated that in the cow the uptake of these amino-acids was probably large enough to provide all the corresponding amino-acid residues in milk proteins. Mepham & Linzell (1966) measured arterio-venous differences in lactating goats, simultaneously estimating mammary blood flow by a direct method. These authors calculated that there was a good agreement between the uptake of phenylalanine, tyrosine and threonine and output of these amino acids in the milk.

Extracorporeal perfusion of isolated organs of large animals – Bridging the gap between in vitro and in vivo studies

“…For specific recipes, compounds and additives to optimize the fluid's performance, refer to the respective publications. Cowie et al, 1951;Verbeke et al, 1959;Lauryssens et al, 1959Lauryssens et al, , 1960Lauryssens et al, , 1961Wood et al, 1965 Caprine Physiology Hebb and Linzell, 1951;Hardwick and Linzell, 1960;Hardwick et al, 1961Hardwick et al, , 1963Hardwick, 1965Hardwick, , 1966Linzell et al, 1967;Verbeke et al, 1957Verbeke et al, , 1968Verbeke et al, , 1972Roets et al, 1974Roets et al, , 1979a Udder Bovine Pharmacology Kietzmann et al, 1993Kietzmann et al, , 1995Kietzmann et al, , 2008Kietzmann et al, , 2010Forster et al, 1999;Kietzmann, 1999, 2001;Ehinger and Kietzmann, 2000a,b;Ehinger et al, 2006;Schumacher et al, 2011 One of the advantages associated with the use of cell free solutions is their relatively easy handling in a perfusion set-up compared with blood. The solution is readily available, even in large volumes that might be needed if an open system is the set-up of choice and variability in perfusate composition is not a concern.…”

“…As cell free solutions are associated with low oncotic pressure, resulting in edema formation and weight gain, adding plasma expanders such as plasma proteins (Verbeke et al, 1968;Roets et al, 1974;Patan et al, 2009) and purified albumin (Rehfeld et al, 1982;Barthel et al, 1989;Riviere et al, 1989;Brunicardi et al, 2001) perfused specimens thus reflects the degree of ischemia/reperfusion injury (Petrasek et al, 1994;Adham et al, 1997) and the integrity of the microvasculature (Müller et al, 2013). Excessive edema formation results in rapidly progressive deterioration of organ function, which renders it unsuitable for transplantation and research alike (Verbeke et al, 1972). Next to reperfusion injury, edema formation is also promoted by inadequate oncotic pressure, perfusion pressure, or flow rate.…”

“…Nevertheless most secretory cells appeared normal in histology upon completion of the experiment (Hardwick and Linzell, 1960). Addition of an artificial kidney to the perfusion system allowed maintenance of high flow rates for a longer perfusion time (Hardwick et al, 1961(Hardwick et al, , 1963Hardwick, 1965Hardwick, , 1966Linzell et al, 1967;Verbeke et al, 1968Verbeke et al, , 1972Roets et al, 1974Roets et al, , 1979a.…”

Amino Acid Uptake by the Mammary Gland of the Lactating Ewe

“…The interpretation of the pattern of amino acid uptake is substantiated by experiments carried out in vitro with the perfused udder (see Mepham 1971;Verbeke et al 1972;Roets et al 1974). There is little evidence to suggest that under normal circumstances the residues of methionine, phenylalanine and threonine undergo catabolism or transformation to any significant extent, i.e.…”

“…Amino acid metabolism by the lactating udder of the sheep has been investigated in a number of in vitro perfusion experiments (see Mepham 1971;Verbeke et al 1972;Roets et al 1974). Thus it is of interest to compare amino acid metabolism of the sheep udder in vitro with the pattern of amino acid uptake in vivo.…”