Alpha 2-adrenoceptor activity exerts dual control of coronary blood flow in canine coronary artery

Alterations of alpha-adrenergic modulations of coronary microvascular tone in dogs with heart failure

BACKGROUND

It remains unclear whether coronary microvascular response to alpha-adrenergic activation alters in chronic heart failure (CHF).

METHODS AND RESULTS

We investigated the alpha-adrenergic receptor-mediated effects on coronary pressure-flow relationship (CPFR) in a tachycardia-induced canine heart failure model. The dogs studied were male (29 of 31) and the drugs were given intracoronary. The slope of CPFR during long diastole was evaluated as an index of coronary vascular resistance, during alpha1- or alpha2-adrenergic stimulation or inhibition under anesthesia in the baseline and failing state after 3 weeks of rapid ventricular pacing. Resting coronary blood flow and CPFR did not change in the failing state from the baseline state. Neither alpha1 nor alpha2 stimulation changed the slope of CPFR in the baseline state. However, in the failing state, alpha1 stimulation decreased the slope of CPFR by 23 +/- 5% (P < .05), whereas alpha2 stimulation increased it by 73 +/- 10% (P < .05), which was nearly abolished by pretreatment with NG-nitro-L-arginine methyl ester.

CONCLUSION

Alpha2-mediated vasodilatory action, presumably via endothelium-derived nitric oxide release, would be enhanced in the coronary microvascular bed, which may antagonize enhanced alpha1-induced vasoconstriction in CHF.

Adenosine and cardioprotection in the diseased heart

Biological and mechanical stressors such as ischemia, hypoxia, cellular ATP depletion, Ca2+ overload, free radicals, pressure and volume overload, catecholamines, cytokines, and renin-angiotensin may independently cause reversible and/or irreversible cardiac dysfunction. As a defense against these forms of stress, several endogenous self-protective mechanisms are exerted to avoid cellular injury. Adenosine, a degradative substance of ATP, may act as an endogenous cardioprotective substance in pathophysiological conditions of the heart, such as myocardial ischemia and chronic heart failure. For example, when brief periods of myocardial ischemia precede sustained ischemia, infarct size is markedly limited, a phenomenon known as ischemic preconditioning. We found that ischemic preconditioning activates the enzyme responsible for adenosine release, ie, ecto-5'-nucleotidase. Furthermore, the inhibitor of ecto-5'-nucleotidase reduced the infarct size-limiting effect of ischemic preconditioning, which establishes the cause-effect relationship between activation of ecto-5'-nucleotidase and the infarct size-limiting effect. We also found that protein kinase C is responsible for the activation of ecto-5'-nucleotidase. Protein kinase C phosphorylated the serine and threonine residues of ecto-5'-nucleotidase. Therefore, we suggest that adenosine produced via ecto-5'-nucleotidase gives cardioprotection against ischemia and reperfusion injury. Also, we found that plasma adenosine levels are increased in patients with chronic heart failure. Ecto-5'-nucleotidase activity increased in the blood and the myocardium in patients with chronic heart failure, which may explain the increases in adenosine levels in the plasma and the myocardium. In addition, we found that further elevation of plasma adenosine levels due to either dipyridamole or dilazep reduces the severity of chronic heart failure. Thus, we suggest that endogenous adenosine is also beneficial in chronic heart failure. We propose potential mechanisms for cardioprotection attributable to adenosine in pathophysiological states in heart diseases. The establishment of adenosine therapy may be useful for the treatment of either ischemic heart diseases or chronic heart failure.

Coronary vascular effects of dexmedetomidine during reactive hyperemia in the anesthetized dog

OBJECTIVE

The central sympatholytic effects of alpha2-adrenergic agonists are believed to be beneficial during myocardial ischemia, but the peripheral vasoconstrictive effects are controversial. The aim of this study was to investigate the coronary vascular effects of dexmedetomidine (DM) during reactive hyperemia.

DESIGN

The study had a prospective, randomized, open-comparative design.

SETTING

University animal laboratory.

PARTICIPANTS

Nine mongrel dogs.

INTERVENTIONS

Coronary artery occlusions lasting 2 minutes were induced five times at 40-minute intervals. DM, 0.1, 1, and 10 micrograms/kg was administered 15 minutes before the second, third, and fourth coronary occlusion, respectively. The alpha2-antagonist atipamezole was administered before the fifth coronary occlusion.

MEASUREMENTS AND MAIN RESULTS

DM, 1 microgram/kg, significantly decreased heart rate (from 128 +/- 13 to 96 +/- 21 beats/min); 10 micrograms/kg of DM also significantly decreased cardiac output (from 3.4 +/- 1.1 to 1.4 +/- 0.4 L/min). DM decreased myocardial blood flow in all layers of normally perfused myocardium. In hyperemic myocardium, DM significantly decreased epicardial blood flow (from 3.30 +/- 1.43 to 1.44 +/- 0.49 mL/min/g after DM 10 micrograms/kg), whereas endocardial blood flow did not change, hereby significantly increasing the endo/epi blood flow ratio (from 0.99 +/- 0.54 to 2.28 +/- 0.78).

CONCLUSIONS

In the postischemic hyperemic subendocardial layer, coronary blood flow was preserved after DM. DM reduced primary determinants of myocardial oxygen demand. These effects of DM may be beneficial in conditions of temporary coronary artery occlusion and subsequent reperfusion.

Coronary microcirculation: autoregulation and metabolic control

The majority of studies examining the regulation of coronary blood flow and vascular resistance have considered the coronary circulation as being composed of large conduit vessels and resistance vessels. Recently, it has become apparent that regulation of coronary microvascular resistance is not distributed uniformly, but varies across different segments or microdomains of the vasculature. Generally, small arterioles, those less than 100 microns in diameter, respond differently than larger arterioles and small arteries. There are major differences in the level of autoregulatory control, myogenic control, endothelial modulation and control by metabolic factors across these various microvascular domains. There are also transmural variations which may account for some of the differences in coronary blood observed between epicardial and endocardial regions. In addition, interactions between these various regulatory mechanisms further complicate the understanding of coronary microvascular regulation. Importantly however, it may be these complex interactions and heterogeneous regulatory mechanisms which allow for adequate perfusion of the myocardium under an extreme range of metabolic conditions. This segmental distribution of regulation suggests an integrative hypothesis of regulation whereby a variety of mechanisms play a role in the overall response.