Plasma noradrenaline levels and thermogenic responses to injected noradrenaline in the conscious rat.

The changes in metabolic rate (MR) and plasma noradrenaline (NA) were studied in rats injected with NA either subcutaneously (S.C.) or intravenously (I.V.). Injection of NA (400 micrograms kg-1 S.C.) raised the MR by 54.0 +/- 10.2% above baseline and the levels of plasma NA were higher in right atrial venous plasma (147.0 +/- 6.6 nmol l-1) than in arterial plasma (54.4 +/- 6.6 nmol l-1) 90 min after the injection. There was no significant increase in MR in response to the infusion of 0.4 microgram NA min-1 kg-1 but significant responses occurred (P less than 0.01) with infusion of 1.5, 3.0, 4.0 and 8.0 micrograms min-1 kg-1. The maximum increase in MR was 90.3 +/- 7.4% during the 3.0 micrograms min-1 kg-1 infusion. The arterial plasma NA levels were directly proportional to the NA infusion rate, 18.0 +/- 8.3, 72.4 +/- 14.3, 159.5 +/- 28.4, 222.3 +/- 62.1 and 590.0 +/- 172.1 nmol l-1 for the 0.4, 1.5, 3.0, 4.0 and 8.0 micrograms min-1 kg-1 infusion respectively. This study shows that high plasma levels of NA are needed to induce thermogenesis when NA is administered either S.C. or I.V.