Determination of plasma and tissue levels of tyramine by radioimmunoassay

Evidence for central hypertyraminemia in hepatic encephalopathy

In mongrel dogs, the effect of end-to-side portacaval shunt on plasma, cerebrospinal fluid (CSF) and brain tyramine, tyrosine, dopamine, norepinephrine, and epinephrine were studied. It was found that the level of tyramine in plasma, CSF, and selected brain regions increased steadily after the construction of the shunts. These elevations became more pronounced when the dogs manifested symptoms of hepatic encephalopathy. In postshunted dogs with stage II and III hepatic encephalopathy, tyramine concentration in corpus striatum (1,312 +/- 371), hypothalamus (400 +/- 67.0), and midbrain (660 +/- 78.7 ng/g) was significantly (P less than 0.05) higher than the level in dogs with stage 0 and I hepatic encephalopathy and sham-operated dogs serving as controls (corpus striatum, 831 +/- 140; hypothalamus, 167 +/- 40.0; and midbrain, 132 +/- 37.4 ng/g). This was followed by a concomitant depletion of dopamine and norepinephrine in these brain regions (postshunt: dopamine 104 +/- 20.0, 3,697 +/- 977, and 105 +/- 14.1; norepinephrine 521 +/- 71.6, 81.6 +/- 13.7, and 218 +/- 31.7 ng/g; vs. sham group: dopamine 532 +/- 83.1, 8,210 +/- 1,126, and 192 +/- 35.0; norepinephrine 1,338 +/- 425, 124 +/- 21.3, and 449 +/- 89.7 ng/g) of encephalopathic dogs with portacaval shunt. Furthermore, tyramine, tyrosine, dopamine, and norepinephrine levels in plasma and CSF increased markedly as clinical features in the dogs' behavior characteristic of hepatic encephalopathy occurred, including hypersalivation, ataxia, flapping tremor, somnolence, and coma. Cerebral hypertyraminemia and a defect in sympathetic neurotransmission may contribute to the development of hepatic encephalopathy of liver disease.

Hypertyraminemia in cirrhotic patients

To evaluate the role of tyramine in hepatic disorders, we used a radioimmunoassay to study plasma concentration of tyramine in eight healthy subjects, 20 hospitalized patients without liver disease, and 13 cirrhotic patients of whom seven had hepatic encephalopathy. The effect of increasing dietary protein on tyramine level of cirrhotic patients was also assessed. No significant difference in plasma tyramine concentration was seen between normal subjects, 1.3 +/- 0.1 ng per milliliter (average +/- S.E.), hospitalized patients without hepatic disease (1.4 +/- 0.1 ng per milliliter) and cirrhotic patients without encephalopathy (2.7 +/- 0.5 ng per milliliter). However, the tyramine level in cirrhotic patients with encephalopathy, 6.4 +/- 0.1 ng per milliliter, was significantly (P less than 0.001) higher than in normal subjects or in cirrhotic patients without encephalopathy. Increasing dietary protein from 40 to 80 g per day raised fasting tyramine concentration by 30 to 70 per cent within three days in both encephalopathic and non-encephalopathic cirrhotic patients. Concentration of plasma tyramine in cirrhotic subjects was significantly correlated with that of plasma tyrosine (P less than 0.001).