Reduced forearm alpha1-adrenergic vasoconstriction is associated with enhanced heart rate fluctuations in humans

In the present study, we assessed whether heart rate (HR) or arterial pressure fluctuations are enhanced in healthy young humans with reduced alpha-adrenergic vasoconstrictor responses and, if so, whether this occurs for both alpha1- and alpha2-adrenergic receptor-mediated vasoconstriction. Arterial pressure (brachial artery catheter) and HR (ECG) were monitored continuously, and alpha1- and alpha2-adrenergic responsiveness was determined by assessing the effects of brachial artery infusions of phenylephrine (alpha1-adrenergic agonist) and dexmedetomidine (alpha2-adrenergic agonist), respectively, on forearm blood flow (strain gauge plethysmography). alpha1-Adrenergic responsiveness varied markedly among the subjects (n=20) and was inversely correlated with coefficient of variation for HR (R2=0.37, P<0.01), whereas the responsiveness was not correlated with the coefficient of variation for either systolic or diastolic arterial pressure. alpha1-Adrenergic responsiveness was inversely and more strongly correlated with baroreflex sensitivity (R2=0.62, P<0.0001), determined from beat-to-beat changes in HR and systolic arterial pressure, than the coefficient of variation for HR. On the other hand, alpha2-adrenergic responsiveness was not correlated with any of the parameters determined above. These results suggest that, in healthy young subjects, the enhanced HR response to changes in systolic pressure helps maintain the stability of arterial blood pressure when alpha1-adrenergic responsiveness is reduced.