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1 Department of Internal Medicine I and 2 Experimental Cardiology, Thoraxcenter, Cardiovascular Research Institute COEUR, Erasmus University Rotterdam, 3015 GD Rotterdam, The Netherlands
Experimental
findings suggest a pronounced concentration gradient of norepinephrine
(NE) between the intravascular and interstitial compartments of the
heart, compatible with an active neuronal reuptake (U1) and/or an
endothelial barrier. Using the microdialysis technique in eight
anesthetized pigs, we investigated this NE gradient, both under
baseline conditions and during increments in either systemic or
myocardial interstitial fluid (MIF) NE concentration. At steady state,
baseline MIF NE (0.9 ± 0.1 nmol/l) was higher than arterial NE (0.3 ± 0.1 nmol/l) but was not different from coronary venous NE (1.5 ± 0.3 nmol/l). Local U1 inhibition raised MIF NE concentration to
6.5 ± 0.9 nmol/l. During intravenous NE infusions (0.6 and 1.8 nmol · kg
1 · min
1),
the fractional removal of NE by the myocardium was 79 ± 4% to 69 ± 3%, depending on the infusion rate. Despite this extensive removal, the quotient of changes in MIF and arterial concentration (
MIF/
A ratio) for NE were only 0.10 ± 0.02 for the lower
infusion rate and 0.11 ± 0.01 for the higher infusion rate, whereas
U1 blockade caused the
MIF/
A ratio to rise to 0.21 ± 0.03 and
0.36 ± 0.05, respectively. From the differences in
MIF/
A
ratios with and without U1 inhibition, we calculated that 67 ± 5%
of MIF NE is removed by U1. Intracoronary infusion of tyramine (154 nmol · kg
1 · min
1)
caused a 15-fold increase in MIF NE concentration. This
pronounced increase was paralleled by a comparable increase of NE in
the coronary vein. We conclude that U1 and extraneuronal uptake, and not an endothelial barrier, are the principal mechanisms underlying the
concentration gradient of NE between the interstitial and intravascular
compartments in the porcine heart.
norepinephrine; spillover; pig; myocardial interstitium
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