Analysis of the compartments involved in the extraneuronal storage and metabolism of isoprenaline in the perfused heart
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Rat hearts were perfused with various concentrations of 3H-(±)-isoprenaline, and initial rates were determined for the removal of the amine from the perfusion fluid and for its O-methylation. Both removal and O-methylation obeyed Michaelis-Menten kinetics, K m and Vmax being 21 μM and 38 nmoles · g−1 · min−1 for the former, and 2.9 μM and 1.7 nmoles · g−1 · min−1 for the latter. After block of COMT the kinetic constants for removal (which equals accumulation under these conditions) were about the same as before. The kinetics of O-methylation seem to differ strikingly from those of accumulation of unchanged amine.
Corticosterone and 3-O-methylisoprenaline were about equipotent in antagonizing the accumulation and O-methylation of isoprenaline in the rat heart during perfusion with 3H-isoprenaline.
U-0521 (dihydroxy-2-methyl propiophenone; 100 μM) was used as a blocker of COMT. In addition it was found to be a weak inhibitor of the extraneuronal uptake of isoprenaline (K i =230 μM).
After block of COMT and subsequent to perfusion of the heart with 0.95 μM 3H-isoprenaline, efflux curves were determined during wash out with amine-free solution. Four compartments were detected (in order of increasing half time of efflux): I represented the fluid in dead space, cardiac cavities and large vessels; II equalled the extracellular space; III and IV represented extraneuronal storage sites. Corticosterone impaired the filling of compartments III and IV when present during filling. Both corticosterone and 3-O-methylisoprenaline (OMI) delayed the efflux from compartment III when present in the wash out solution only.
Experiments with guinea-pig hearts showed qualitative similarities between these and rat hearts. However, the storage and the O-methylating capacity of the guinea-pig heart was considerably smaller than that of the rat heart.
Rat ventricle slices (exposed to 0.95 μM 3H-(±)-isoprenaline for 30 min) were compared with perfused hearts. While the accumulation of 3H-isoprenaline was about 1/4, the total formation of 3H-OMI was only 1/50 of that determined for the perfused heart. This low rate of formation of 3H-OMI was also observed for slices of aorta, vas deferens and spleen, while slices of salivary glands had a high O-methylating capacity. Apparently, perfusion of the heart provides optimal access to the O-methylating compartment which may be located in vascular smooth muscle.
Key wordsIsoprenaline Extraneuronal COMT Uptake2 Corticosterone Extraneuronal Compartments
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