1. We assessed the effects of changing levels of sympathetic drive on the gain of baroreflex control of the sino-atrial node, in normotensive and hypertensive subjects.

2. Autoregressive spectral and cross-spectral analysis of R—R interval and systolic arterial pressure (non-invasive Finapres method) variabilities providing an estimate of baroreflex gain through the frequency domain index α were performed on data from 63 normotensive and 78 mild hypertensive subjects. Subjects were studied at rest and during active orthostatism, which induces sympathetic predominance. Seven control subjects and 14 hypertensive subjects were also studied after chronic atenolol treatment, to attenuate β-adrenoceptor-mediated responses.

3. In both normotensive and hypertensive subjects, the index α was reduced by active standing and increased by chronic β-adrenoceptor blockade. Furthermore, at rest, the index at was correlated with R—R variance and appeared significantly reduced with age. The age-related negative correlation of the index α was no longer evident during the standing-induced increase in sympathetic drive, in both normotensive and hypertensive subjects.

4. The index α, a non-invasive frequency domain estimate of the overall gain of baroreflex control of the sino-atrial node, which appears to be reduced in essential hypertension, is negatively modulated by physiological increases in sympathetic drive, and augmented by pharmacological blockade of β-adrenoceptors.

5. In essential hypertension the enhanced sympathetic drive present already at rest, and the simultaneous reduction of the gain of baroreflex mechanisms, are the expression of a complex alteration in neural cardiovascular control.

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