Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Camp. Bioc he m . Phy sio l. Vol. zyxwvutsrqponmlkjihgfedcba 107A.No. 3, pp. 589-595, 1994 Elsevier Science Ltd Printed in Great Britain 0300-9629194 $6.00 + 0.00 Blood cell to plasma gradients of amino acids in arterial and venous blood in fed and fasted rats C. Pi& I. Llad6, A. Pons and A. Palou Lzlboratori de Biologia Molecular, Nutricib i Biotecnologia, Departamento de Biologia Fonamental i Ciencies de la Salut, Universitat de les Illes Balears, UIB Ctra Valldemossa Km 7.5. Pzlma de Mallorca, E-07071, Spain Measurement of amino acid concentrations in blood cells and plasma, and the calculated blood cell to plasma gradients (C/P) from both afferent and efferent vessels to tissues, allowed evaluation of the effect of several tissues (splanchnic bed, skeletal muscle and kidney) on blood amino acid distribution in fed and starved rats. Combined effects of tissues and erythrocyte transport capabilities determined specific C/P values for each amino acid. For amino acids related to the L-system, the high capacity of this erythrocyte transport many buffer some C/P changes as an effect of tissue metabolism. For less permeable amino acids (like Asp and Glu) plasma changes were mainly responsible for changes in C/P values, whereas for other amino acids (such as basic amino acids) blood cells became the main determinants of C/P changes, mainly in starvation. In general, the role of erythrocytes in amino acid transport was enhanced in starvation. Key words: Plasma gradient; Amino acids; Fasted rats; Erythrocyte transport. Ct’mp. Biochem. Physiol. 107A, 589-595, 1994. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Introduction Amino acids in blood cells represent about 50% of total rat blood amino acids (Gianotti et al., 1990). These amino acids are actively exchanged with tissue pools, thus contributing to the inter-organ amino acid relationships, in fed and fasted situations (Rapier et al., 1960; Ehvyn, 1966; Felig et al., 1973; Felig, 1975; Soley and Alemany, 1980; Christensen, 1982; Dsrmaun et al., 1989; Gianotti et al., 1990). Nevertheless, the in uivo pattern of amino acid exchange between erythrocytes and both plasma and tissues remains obscure. In vitro studies bade indicated slow rates of amino acid uptake by erythrocytes (minutes to hours) (Winter and Christensen, 1964; Christensen, 1982; Young et al., 1976, 1983; Felipe et al., 1990a,b; Pi& et al., 1991a, 1992, 1993) although this did no. appear to be related to the differences in -- C o rre sp o nde nc e ro : A. Palou, Bioquimica i Biologia Molecu- ar, Departamento de Biologia Fonamental i Cidncies de :a Salut, Universitat de les Illes Balears, UIB Ctra Valldemossa Km 7.5. Palma de Mallorca, E-07071, Spain. Tel.: 34-71-173454; Fax: 34-71-173184. Received 14 April 1993; accepted 20 May 1993 blood cell amino acid levels between arterial and venous blood (Elwyn, 1966; Elwyn et al., 1972). Several explanations have been put forward to account for the apparent discrepancies between the in vivo and the in vitro studies, such as the direct transfer of amino acids between erythrocytes and tissue cells, due to the similiar diameter size of the capillary vessels and the erythrocytes (Elwyn et al., 1972) or changes in erythrocyte transport velocities as an effect of substances secreted and inactivated by tissues (Elwyn, 1966). Changes in the amino acid distribution between blood cell and plasma compartments and the consequent alteration in C/P values may, in part, be the result of the continuous activity of amino acid transport systems of erythrocytes within the bloodstream, and in part dependent on the quick activity of the tissues removing (or releasing) amino acids from (or to) a concrete compartment of the blood. The relative prevalence of one or both indicated mechanisms could be dependent on the amino acid and the tissue considered. 589 CBPI \, 107,3--K