Uptake kinetics and metabolism of 7–³H‐dopamine in the isolated perfused rat heart

Abstract

Summary1. The isolated, perfused rat heart accumulates dopamine by two distinct uptake mechanisms characterized by different kinetic constants and different patterns of metabolite production (Uptake 1: Km 0-69 x 10-6M and Vmax(1-45 x 10-9 mol/g)/min; Uptake 2: Km 5 9 x 10-4M and VYnax (0-14 x 10-C mol/g)/min). 2. The metabolic fate of dopamine taken up by the isolated, perfused rat heart depends on the concentration of dopamine in the perfusion medium. At a very low perfusion concentration (0 047 x 10-1°mol/ml) most of the radio--activity is stored as unchanged dopamine and the main metabolite is noradrenaline. With increasing perfusion concentrations deamination becomes the main metabolic pathway, deaminated metabolites accounting for more than 50% of the total radioactivity after perfusion with 2,6144 x 101o mol/ml for 16 minutes. The 0-methylated, and the 0-methylated deaminated metabolites are of minor importance at all perfusion concentrations. 3. The resistance to wash out of the dopamine taken up by the isolated, perfused rat heart is dependent on the perfusion concentration used. At a concentration of 66-9 x 10`1 mol/ml, 50% of the total activity is washed out during an 8 min wash period. Within the same time interval there is no wash out when a perfusion concentration of 0 042 x 10-10 mol/ml is used. 4. It is concluded that the metabolic fate of dopamine taken up at various perfusion concentrations reflects the distribution of dopamine within intraand extraneuronal compartments in the hearts.

Uptake Processes for Biogenic Amines

“…Uptake2 was also a saturable process (Table 14) and the kinetic constants indicated that in contrast to UptakeJ, which is a high-affinity, low-capacity system, U ptake2 had a much lower affinity for the catecholamine substrates, but also a very much higher capacity (Vrnax). Subsequent studies showed that (±)-isoproterenol had a higher affinity than either epinephrine or NE for Uptake2 (Callingham and Burgen, 1966), whereas dopamine was accumulated with only low affinity by Uptake2 (Hellmann et al, 1971). The effects of various drugs on Uptake2 gave an indication of the structural specificity of this process (Burgen and Iversen, 1965) (Table 15).…”

Effect of pretreatment with 6‐hydroxydopamine on the uptake and metabolism of catecholamines by the isolated perfused rat heart

“…The uptake was decreased by only 15% in the chemically sympathectomized hearts. Using the kinetic data given by Hellmann et al (1970), one can calculate that with a perfusion concentration of 327 45 x 10-10 mol/ml DA the initial rate of Uptake 1 is about 20% of the initial rate of Uptake 2.…”

“…Perfusion technique of rat hearts The perfusion technique is described in detail by Hellmann et al (1970). There was no difference in the flow rates (8-10 ml/min) between the two groups.…”

“…The stock solutions were prepared from 7-3H-DA (NENCO 1-5 Ci/mmol) or (±)-7-3H-NA (NENCO 9-71 Ci/mmol) respectively, and the appropriate amounts of unlabelled DA or (± )-NA. The The procedures for the isolation and determination of the 3H-DA and 3H-DA metabolites are modifications of the isolation procedures described by Kopin, Axelrod & Gordon (1961) and Bertler, Carlsson & Rosengren (1958), as used by Hellmann et al (1970). The method of Kopin et al (1961) was used without modification for the isolation of the 3H-NA and its metabolites.…”

Proceedings of the Physiological Society, 22‐23 July 1988, Cambridge Meeting: Communications

“…Department ofPhysiology, King's College London, Campden Hill Road, London W8 7AH, UK Dopamine uptake and metabolism has been studied in the isolated rat heart, and at tracer concentrations it is largely removed by neuronal reuptake mechanisms (Hellmann et al 1970). In the present study dopamine uptake was examined in the isolated perfused rabbit heart using multiple tracer dilution techniques (Martin de Julian & Yudilevich, 1964).…”