High levels of sympathetic drive in several
cardiovascular diseases including postmyocardial
infarction, chronic
congestive heart failure and
hypertension are reinforced through dysregulation of afferent input and central integration of autonomic balance. However, recent evidence suggests that a significant component of sympathetic hyperactivity may also reside peripherally at the level of the postganglionic neuron. This has been studied in depth using the spontaneously hypertensive rat, an animal model of genetic
essential hypertension, where larger neuronal
calcium transients, increased release and impaired reuptake of
norepinephrine in neurons of the stellate ganglia lead to a significant
tachycardia even before
hypertension has developed. The release of additional sympathetic cotransmitters during high levels of sympathetic drive can also have deleterious consequences for peripheral cardiac parasympathetic neurotransmission even in the presence of β-
adrenergic blockade. Stimulation of the cardiac vagus reduces heart rate, lowers myocardial
oxygen demand, improves coronary blood flow, and independently raises
ventricular fibrillation threshold. Recent data demonstrates a direct action of the sympathetic cotransmitters
neuropeptide Y (NPY) and
galanin on the ability of the vagus to release
acetylcholine and control heart rate. Moreover, there is as a strong correlation between plasma NPY levels and coronary microvascular function in patients with
ST-elevation myocardial infarction being treated with primary
percutaneous coronary intervention. Antagonists of the NPY receptors Y1 and Y2 may be therapeutically beneficial both acutely during
myocardial infarction and also during chronic
heart failure and
hypertension. Such medications would be expected to act synergistically with β-blockers and implantable vagus nerve stimulators to improve patient outcome.