Activation of histamine H3-receptors inhibits carrier-mediated norepinephrine release during protracted myocardial ischemia. Comparison with adenosine A1-receptors and alpha2-adrenoceptors.
We previously showed that prejunctional histamine H3-receptors downregulate norepinephrine exocytosis, which is markedly enhanced in early myocardial ischemia. In the present study, we investigated whether H3-receptors modulate nonexocytotic norepinephrine release during protracted myocardial ischemia. In this setting, decreased pH(i) in sympathetic nerve endings sequentially leads to a compensatory activation of the Na+-H+ antiporter (NHE), accumulation of intracellular Na+, reversal of the neuronal uptake of norepinephrine, and thus carrier-mediated release of norepinephrine. Accordingly, norepinephrine overflow from isolated guinea pig hearts undergoing 20-minute global ischemia and 45-minute reperfusion was attenuated approximately 80% by desipramine (10 nmol/L) and 70% by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 micromol/L), inhibitors of norepinephrine uptake and NHE, respectively. The H3-receptor agonist imetit (0.1 micromol/L) decreased carrier-mediated norepinephrine release by approximately 50%. This effect was blocked by the H3-receptor antagonist thioperamide (0.3 micromol/L), indicating that H-receptor activation inhibits carrier-mediated norepinephrine release. At lower concentrations, imetit (10 nmol/L) or EIPA (3 micromol/L) did not inhibit carrier-mediated norepinephrine release. However, a 25% inhibition occurred with imetit (10 nmol/L) and EIPA (3 micromol/L) combined. This synergism suggests an association between H-receptors and NHE. Conceivably, activation of H-receptors may lead to inhibition of NHE. In fact, alpha2-adrenoceptor activation, which is known to stimulate NHE, enhanced norepinephrine release, whereas alpha2-adrenoceptor blockade attenuated it. Furthermore, activation of adenosine A1-receptors markedly attenuated norepinephrine release, whereas their inhibition potentiated it. Because norepinephrine directly correlated with the severity of reperfusion arrhythmia and imetit reduced the incidence of ventricular fibrillation by 50%, our findings with H-receptor agonists may further the development of novel pharmacological means to reduce reperfusion arrhythmias in the clinical setting.
Imamura
M
M
Department of Pharmacology, Cornell University Medical College, New York, NY, USA.
Lander
H M
HM
Levi
R
R
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000209:etiology / Q000378:metabolism; D006168:Guinea Pigs; D006632:Histamine / Q000378:metabolism; D017442:Histamine Agonists / Q000494:pharmacology; D066298:In Vitro Techniques; D008024:Ligands; D008297:Male; D017202:Myocardial Ischemia / Q000378:metabolism*; D015428:Myocardial Reperfusion Injury / Q000150:complications / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D014179:Neurotransmitter Uptake Inhibitors / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D058906:Purinergic P1 Receptor Agonists; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D018100:Receptors, Histamine H3 / Q000378:metabolism*; D018047:Receptors, Purinergic P1 / Q000378:metabolism*
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D017442:Histamine Agonists; D008024:Ligands; D014179:Neurotransmitter Uptake Inhibitors; D058906:Purinergic P1 Receptor Agonists; D018341:Receptors, Adrenergic, alpha-2; D018100:Receptors, Histamine H3; D018047:Receptors, Purinergic P1; D006632:Histamine; D009638:Norepinephrine