Comparison of the effects of high ambient temperature and clonidine on autonomic functions in man

Abstract

The effects of two interventions, high ambient temperature, a sympathetic activator, and clonidine, a centrally acting sympatholytic drug, were compared on a number of autonomic functions. Eight healthy male volunteers participated in four weekly sessions. Each session was associated with one of the following treatments: placebo (physiological saline infused intravenously over 10 min) at 20 degrees C; clonidine hydrochloride (1.5 micrograms kg-1 in 10 ml infused intravenously over 10 min) at 20 degrees C; placebo at 40 degrees C; clonidine at 40 degrees C. Subjects were allocated to treatments and sessions according to a double-blind (for drug condition) balanced design. In each session, the following indices of autonomic function were recorded: systolic and diastolic blood pressure, heart rate, salivation, body temperature, plasma noradrenaline and adrenaline concentrations, baseline and carbachol-evoked sweating, physiological finger tremor. Raised ambient temperature (40 degrees C) caused increases in heart rate, body temperature, carbachol-evoked sweating and physiological finger tremor. Clonidine (at 20 degrees C) reduced systolic blood pressure, body temperature, salivation and plasma noradrenaline concentration, but did not affect any of the other measures. Clonidine (at 40 degrees C) counteracted the increase in heart rate, but not the increases in carbachol-evoked sweating and finger tremor, evoked by high ambient temperature. The high ambient temperature condition abolished the body-temperature-lowering effect of clonidine, but did not modify the effects of clonidine on systolic blood pressure, salivation and plasma noradrenaline concentration. These result indicate that while the effects of the heat stressor are consistent with an increase in sympathetic activity, and most of the effects of clonidine are consistent with a decrease in sympathetic activity, only two functions (body temperature and heart rate) were affected in opposite directions by the two interventions. Indeed physiological antagonism between the two interventions could be demonstrated on body temperature and heart rate only, and there was no evidence for an interaction between the effects of the two variables on any of the other indices of autonomic activity. The failure of clonidine to affect two sympathetically mediated functions, carbachol-evoked sweating and physiological finger tremor, under either temperature condition, indicates that central alpha 2-adrenoceptors cannot be involved in the regulation of these functions.

Modulation of the Acoustic Startle Response by the Level of Arousal: Comparison of Clonidine and Modafinil in Healthy Volunteers

“…Clonidine reduced pupil diameter (Bitsios et al, 1996;Phillips et al, 2000c, d;Hou et al, 2005, reduced systolic and diastolic blood pressure in both the supine and the standing positions (Lal et al, 1975;Berlan et al, 1989;Morley et al, 1991;Bitsios et al, 1996;Kumari et al, 1996;Abduljawad et al, 1997Abduljawad et al, , 2001Arya et al, 1997;Phillips et al, 2000d;Hou et al, 2005, reduced salivation (Bitsios et al, 1996;Abduljawad et al, 1997Abduljawad et al, , 2001Arya et al, 1997;Phillips et al, 2000d;Hou et al, 2005, and reduced core temperature (Arya et al, 1997;) (see Figure 5 and Table 2). The autonomic effects of clonidine are consistent with the stimulation of inhibitory a 2 -adrenoceptors on central neurons.…”

“…A Braun Pro4000 tympanic thermometer (Welch Allyn UK Ltd, Buckinghamshire, UK) was used to measure temperature from the ear canal. A measure of salivation was derived by placing three cotton wool dental rolls in the subject's mouth (two buccally and one sublingually) and recording the increase in their weight over a 1-min period (Peck, 1959;Arya et al, 1997;Szabadi and Tavernor, 1999). The test was repeated twice with a 5-min interval, and the mean of the two measurements was taken as an index of salivary output.…”

Comparison of pramipexole and amisulpride on alertness, autonomic and endocrine functions in healthy volunteers

“…A Braun Pro4000 tympanic thermometer (Welch Allyn UK, Buckinghamshire, UK) was used to measure temperature from the ear canal. A measure of salivation was derived by placing three cotton wool dental rolls in the mouth (two buccally and one sublingually) and recording the increase in the weight of the three swabs over a 1-min period (Peck 1959;Arya et al 1997;Szabadi and Tavernor 1999). The test was repeated twice with a 5-min interval, and the mean of the two measurements was taken as an index of salivary output.…”

Comparison of the effects of clonidine and yohimbine on spontaneous pupillary fluctuations in healthy human volunteers

“…A battery of 16 visual analogue self-rating scales (VAS) (Bond and Lader 1974;Phillips et al 2000) was administered, and heart rate and blood pressure recordings were taken in the erect position using an electroaneroid sphygmomanometer (Boso Prestige, Jungingen, Germany). Salivation was measured using the dental roll technique (Arya et al 1997). …”