Dopamine beta-hydroxylase (DBH) and noradrenaline were determined both in the venous effluent (perfusate) and in the transmyocardial fluid (TMF) collected from the apex of the rabbit isolated heart which was prepared according to De Deckere and Ten Hoor (1977) and perfused with Tyrode's solution at 20 ml/min. 2. Perfusion for 2 min with the nicotinic drug, p-aminophenethyl-trimethylammonium (PAPETA), both in the absence of presence of atropine evoked noradrenaline overflow into the perfusate and TMF that was maximal in the 0-2 min sample and declined from maximum with a t 1/2 of 0.6 min. DBH was released into TMF with the maximum from 2-4 min and a t 1/2 of decline of 5.6 min. 3. High K-low Na solution containing 54 mM KCl was perfused for 4 min. The maximum outputs of noradrenaline into the perfusate and TMF occurred between 2 and 4 min and that of DBH into TMF 2 min later. The t 1/2 of decline from the maxima were similar to those after PAPETA but the output ratio DBH/noradrenaline was 5 times that after PAPETA. 4. Both chemical stimuli were not observed to increase the DBH content of the perfusates. The noradrenaline concentration in the TMF was 3 times that in the perfusates. 5. It is concluded that TMF reflects concentration changes in the synaptic region more truly than the venous effluent does. TMF is particularly suitable for determination of DBH output of the heart; the DBH concentration is higher than in the perfusate and its washout faster than from the conventional Langendorff preparation.
