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It was the aim of the present study to investigate the influence of Bretschneider's cardioplegia on norepinephrine (NE) release [determined by high pressure liquid chromatography (HPLC) and electrochemical detection] in isolated perfused guinea-pig hearts. The following resulted were noted. (1) Calcium-dependent exocytotic NE release evoked by electrical field stimulation (12 Hz, 1 min) was completely suppressed after only 3 min of normothermic (37.5 degrees C) Bretschneider's cardioplegia. (2) Stop-flow ischemia is associated with a substantial calcium-independent, non-exocytotic NE release, which is regarded as a sodium-dependent carrier-mediated process. Accordingly, it is inhibited by blockers of the sodium/proton-exchanger (e.g. amiloride) and the neuronal uptake1-carrier (e.g. desipramine). Compared with stop-flow ischemia alone, cardioplegia with 3 min of Bretschneider's histidine-tryptophan-ketoglutarate (HTK)-solution preceding stop-flow enhanced NE release at all stop-flow durations (10-90 min) investigated (e.g. after 30 min of normothermic Bretschneider's cardioplegia: 1070+/-41 pmol/g, n = 45, v stop-flow alone: 764+/-48 pmol/g, n = 27, P<0.05). The NE concentrations determined in the cardiac effluent upon reperfusion followed a typical first order kinetic indicating that the transmitter release had already occurred during stop-flow. Hypothermia reduced NE release in a temperature-dependent manner down to intramyocardial temperatures of 2 7.5 degrees C. NE release evoked by Bretschneider's cardioplegia still exceeded that induced by stop-flow ischemia alone by up to 60%. The NE release evoked by Bretschneider's cardioplegia and stop-flow ischemia was calcium-independent. However, it was significantly reduced by desipramine and amiloride, but both agents had a more pronounced inhibitory effect on NE release evoked by stop-flow ischemia alone. (3) This difference may be due to an intrinsic effect of Bretschneider's HTK-solution, as continuous administration of normothermic Bretschneider's HTK-solution induced a substantial NE release which was neither calcium-dependent nor inhibited by blockade of either uptake1 or sodium/proton-exchange. It is concluded that Bretschneider's cardioplegia is not neuroprotective, as it even augments the stop-flow ischemia-induced nonexocytotic NE release.
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