Effects of clonidine on central sympathetic tone

The antihypertensive mechanism of clonidine in man. Evidence against a generalized reduction of sympathetic activity.

“…To our knowledge, the effect of clonidine on MSA has not been studied previously but in recordings from cervical sympathetic trunk, cardiac, splanchnic, and renal nerves, clonidine always caused a reduction of sympathetic activity. 6 '•• 20 ' 21 The reason for the difference is not clear, but several factors may contribute. Our experiments were conducted on conscious patients who received doses of 1.4 to 3.3 Mg/kg body weight, whereas previous studies were carried out on animals subjected to general anesthesia and surgery, and given higher doses (10 to 30 Mg/kg body weight).…”

“…Although this obviously may be one explanation, the fact that different nerves were studied could also be important. In humans, sympathetic outflow to skin is entirely different from that to muscles, 21 and in animals, the degree of baroreflex control is known to differ between different vascular beds. 23 Quantitative differences in the effect ofclonidine on different sympathetic nerves have also been reported 3 ' " and therefore, the effect ofclonidine on sympathetic outflow to CLONIDINE CONC.…”

Central noradrenergic neurones and the cardiovascular actions of clonidine in the rabbit

“…However, it is unlikely that clonidine directly influences the baroreflex arc (Klupp et al, 1970) as the drug does not influence the diving reflex of duck (Kobinger & Oda, 1969) or the response to Valsalva's manoeuvre in man (Muir, Burton & Lawrie, 1969). If clonidine had a selective action activating inhibitory modulating neurones in the medulla, then following destruction of both medullary and bulbospinal tracts by 6-OHD the actions of clonidine would be abolished.…”

“…Previous studies have indicated that clonidine exerts a hypotensive action when injected into the cisterna magna or lateral cerebral ventricle of several animal species (Kobinger & Walland, 1967a); Schmitt, Schmitt, Boissier, Guidicelli & Fichelle, 1968; Sherman, Grega, Woods & Buckley, 1968), after vertebral artery infusion (Sattler & van Zweiten, 1967) and in cross circulation experiments (Sherman et al, 1968). The hypotensive effect appears to be achieved by a reduction in central sympathetic tone, and a reduction in peripheral sympathetic discharges to the heart and resistance vessels (Kobinger & Walland, 1967a;Schmitt et al, 1968;Klupp, Knappen, Otsuka, Streller & Teichman, 1970). The site of central action is at the level of the medulla (Schmitt & Schmitt, 1969;Klupp et al, 1970;Shaw, Hunyor & Korner, 1971b).…”

“…The hypotensive effect appears to be achieved by a reduction in central sympathetic tone, and a reduction in peripheral sympathetic discharges to the heart and resistance vessels (Kobinger & Walland, 1967a;Schmitt et al, 1968;Klupp, Knappen, Otsuka, Streller & Teichman, 1970). The site of central action is at the level of the medulla (Schmitt & Schmitt, 1969;Klupp et al, 1970;Shaw, Hunyor & Korner, 1971b).…”