Activation of Histamine H 3 Receptors Inhibits Carrier-Mediated Norepinephrine Release in a Human Model of Protracted Myocardial Ischemia 1 , 2

During protracted myocardial ischemia, ATP depletion promotes Na accumulation in sympathetic terminals and prevents vesicular storage of norepinephrine (NE). This forces the reversal of the neuronal uptake1 transporter, and NE is massively released (carrier-mediated release). We had shown that histamine H3 receptors (H3Rs) modulate ischemic NE release in animals. We have now used a human model of protracted myocardial ischemia to investigate whether H3Rs may control carrier-mediated NE release. Surgical specimens of human atrium were incubated in anoxic conditions. NE release increased ;7-fold within 70 min of anoxia. This release was carrier mediated because it was Ca independent and inhibited by the uptake1 inhibitor desipramine. Furthermore, the Na/H exchanger (NHE) inhibitors ethyl-isopropyl-amiloride and HOE 642, and the Na channel blocker tetrodotoxin inhibited NE release, whereas the Na channel activator aconitine potentiated it. The selective H3R agonist imetit decreased NE release, an effect that was blocked by each of the H3R antagonists thioperamide and clobenpropit. Notably, imetit acted synergistically with ethyl-isopropyl-amiloride, HOE 642 and tetrodotoxin to reduce anoxic NE release. Thus, activation of H3R appears to result in an inhibition of both NHEand voltagedependent Na channels. Most importantly, endogenous histamine was released from the anoxic human heart, and thioperamide and clobenpropit each alone increased NE release, indicating that H3R become activated in myocardial ischemia. Our findings indicate that H3Rs are likely to mitigate sympathetic overactivity in the ischemic human heart and suggest new therapeutic strategies to alleviate dysfunctions associated with myocardial ischemia. Previous work in our laboratory established the presence of heteroinhibitory H3Rs on adrenergic nerve endings in the heart of the guinea pig (Endou et al., 1994; Imamura et al., 1994), dog (Seyedi et al., 1996) and humans (Imamura et al., 1995). Once activated, these receptors attenuate NE exocytosis. The mechanism of this effect involves a Gi/Go protein and a decreased entry of Ca through N-type channels (Endou et al., 1994). A decreased PKC activity, resulting from a decreased phosphoinositide turnover (Cherifi et al., 1992) and diacylglycerol formation, may also play a role in the modulation of NE release associated with H3R activation (Imamura et al., 1995). H3R activation also inhibits NE release in animal models of acute and protracted myocardial ischemia (Imamura et al., 1994, 1996). In the acute ischemia model, NE release is exocytotic and Ca dependent. In the protracted ischemia model, NE release is Ca independent, carrier mediated and much greater than that with acute ischemia (Imamura et al., 1994, 1996). During protracted myocardial ischemia, ATP depletion promotes Na accumulation in sympathetic nerve endings and prevents the vesicular storage of NE. This forces the reversal of the neuronal uptake1 transporter from an inward to an outward direction, and NE is massively released (carrier-mediated release) (Dart and Du, 1993; Schömig, 1990). The mechanism of the H3R-induced attenuation of carrier-mediated NE release probably involves an inhibition of the NHE (Imamura et al., 1996). Such inhibition would reduce the intraneuronal accumulation of Na and therefore decrease the activity of the NE transporter in the