Evidence of alpha-adrenoceptor-mediated chronotropic action in children.

We investigated alpha-adrenoceptor-mediated positive chronotropic action in children using a pharmacological autonomic function test. Under cardiac vagal and sympathetic blockade, chronotropic responses to intravenous infusion of an alpha-agonist, phenylephrine, were determined in 129 patients with suspected autonomic dysfunction. Heart rate changes to phenylephrine infusion (0.4 microg/kg/min) for 5 minutes ranged from -2 to 50 beats/min (mean +/- SD, 13.8 +/- 9.7 beats/min). alpha-Adrenoceptor-mediated chronotropic (APC) action more than 5 beats/min was observed in 88 (68%) subjects. APC was negatively correlated with age. Multivariate analysis showed that APC was correlated negatively with beta-sympathetic tone and positively correlated with intrinsic heart rate. These results suggest that the alpha-adrenoceptor-mediated chronotropic action is a developmental phenomenon and may play an important role in controlling heart rate in response to cardiac sympathetic activation.