Binding and Clearance of Radioactive Adrenaline and Noradrenaline in Sheep Blood

Abstract

An understanding of the conditions influencing protein binding of catecholamines (CAs) is important in studying their metabolic effects. Unfortunately, reports on plasma protein binding of CAs are scarce, conflicting and mainly performed in vitro. The aim of our in vivo and in vitro studies was to investigate binding and clearance of radioactive adrenaline (epinephrine) ((3)H-A), noradrenaline (norepinephrine) ((3)H-NA) and their metabolites in sheep blood. The time course of the radioactivity in the blood after intravenous injection of (3)H-A and (3)H-NA (3.7 MBq each) in 4 sheep (2 of each sex; total of 8 administrations) was determined. Blood samples were taken from the jugular vein. The highest radioactivity was observed in the first sample (5 min) following injection. Radioactivity showed a biphasic disappearance. An initial stage, in which radioactivity decreased rapidly (within 1 h) after the injection, was followed by a slow stage, lasting for up to 1 month, until background levels were reached. In vitro results indicated that NA and A were present not only in plasma (70%) but also in the erythrocytes (30%; mainly bound to haemoglobin). Sephadex G-25 gel filtration revealed that from the plasma fraction about 15% was strongly bound to proteins (mainly albumin). These results demonstrate that previous experiments in this field have overestimated the percentage of CAs bound to plasma proteins, because binding to haemoglobin was previously not known. In the future, efforts should be made to characterize the adduct products of CAs and establish an assay to determine them in vivo. If this could be achieved, it would yield a valuable tool for measuring the stress experienced for a longer period.

Use of inotropes and vasopressor agents in critically ill patients

“…Sixty per cent of circulating adrenaline and noradrenaline remains free within the plasma. The remainder is bound covalently to either plasma proteins (11%) or to haemoglobin in erythrocytes (El‐Bahr et al ., 2006). There is a biphasic decay in plasma catecholamine levels, the first phase lasting around 5 min (uptake‐2), and the second occurring over a month (plasma protein decay) (El‐Bahr et al ., 2006).…”

“…The remainder is bound covalently to either plasma proteins (11%) or to haemoglobin in erythrocytes (El‐Bahr et al ., 2006). There is a biphasic decay in plasma catecholamine levels, the first phase lasting around 5 min (uptake‐2), and the second occurring over a month (plasma protein decay) (El‐Bahr et al ., 2006). Catecholamine metabolism occurs slowly in erythrocytes, which also act as a storage pool (Azoui et al ., 1996).…”

Excretion of catecholamines in rats, mice and chicken

“…Adduct formation in vitro has been reported not only for rats (Powis 1975a ), but for several other species including humans (Powis 1975b ; Sager et al 1987 ), dogs (Teixeira et al 1979 ), sheep (El-Bahr et al 2006 ) and domestic fowl (Powis 1975a ). The uptake and/or binding of CA in blood cells offers further explanations for delayed excretion of 3 H–CA and was demonstrated in humans (Altman et al 1988 ; Azoui et al 1994 , 1996a ; Ratge et al 1991 ; Born et al 1967 ), sheep (El-Bahr et al 2006 ), rabbits (Blakeley and Nicol 1978 ; Friedgen et al 1993 ) and rats (Azoui et al 1996b ; Alexander et al 1984 ; Bouvier et al 1987 ; Yoneda et al 1984 ). The significantly lower recovery in female rats in the NE trial may also be attributed to these mechanisms.…”