1. The carotid baroreceptors were stimulated for 2 min by neck suction at -30 and -60 mmHg in 19 normotensive subjects and 12 patients with moderate essential hypertension. 2. Blood pressure was measured with a mercury sphygmomanometer and heart rate was derived from beat-to-beat analysis of the electrocardiogram. Blood flow was measured simultaneously at calf and finger with venous occlusion plethysmography and the vascular resistance was calculated. 3. During neck suction at -30 and -60 mmHg there was a significant decrease in arterial blood pressure and heart rate. There was a transient vasodilatation of the calf blood vessels, while there was a sustained vasoconstriction of the finger blood vessels. These results were qualitatively similar in both groups; however, there were quantitative differences. 4. These experiments show that there is a selective autonomic control of the different peripheral vascular beds by the carotid baroreceptors in both normotension and mild essential hypertension.

The present paper reviews our recent studies set up to define the role of sympathetic and vagal nervous influences on blood pressure variability. Blood pressure and its variability was measured in men by Arteriosonde or by Portometer (the latter recording 12 hours ambulatory blood pressure) and in the dog by intraarterial recordings. It was shown that sympathetic nerves do not influence variability as no change was seen with drugs acting on either beta (atenolol, metoprolol, propranolol) or alpha adrenergic receptors (prazosin, phentolamine, guanfacine). Also no correlation was found with plasma catecholamines or sympathetic function tests. By contrast, clear inhibition was demonstrated with atropine indicating an important role of vagal nerves on variability. However, in men, inhibition was not complete with atropine; thus, also other mechanisms play a role but these are, at present, largely unknown.

1 The effect of metoprolol (200 mg once daily) and guanfacine (2 mg once daily) on 12 h ambulatory blood pressure and heart rate was compared with placebo in a double-blind cross-over randomized protocol. Twenty-six patients with moderate essential hypertension were studied. 2 Metoprolol significantly decreased both systolic and diastolic blood pressure; every point of the blood pressure curve over the 12 h was significantly lower with metoprolol than with placebo and the histograms of systolic and diastolic blood pressure were significantly shifted to the left. 3 Guanfacine significantly reduced systolic and diastolic blood pressure over the 12 h recording period. The antihypertensive effect lasted at least 24 h. 4 There was a significant decrease in heart rate with metoprolol but no significant effect on heart rate could be shown with guanfacine. 5 Neither metoprolol nor guanfacine caused a significant change in the amplitude of blood pressure variations regardless of whether variability was expressed as standard deviation of the mean, as variation coefficient or as range between the highest and the lowest blood pressure.

I . The aim of the present study was to examine the influence of the carotid baroreceptors on the peripheral vascular responses during static exercise and its interference with the 'central command' and the ergoreceptors.2. Ten healthy subjects performed an isometric handgrip with 30% of maximal voluntary contraction for 1.5 min. Five seconds prior to the end of the exercise the arterial circulation to the exercising forearm muscles was occluded for 1.5 min. The carotid baroreceptors were stimulated by neck suction at -40 mmHg for 0.5 min in each period, and also during control and recovery phase. Contralateral forearm and calf blood flow were measured simultaneously with ECG-triggered venous occlusion pleth ysmography.3. The present findings suggest that there are 'central command' and ergoreceptors which modulate carotid baroreflex function to the afferent output to the heart and the peripheral circulation.

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