ABSTRACT
Systolic blood pressure, diastolic blood pressure, norepinephrine, cortisol, growth hormone, and prolactin plasma levels were investigated in 46 normal subjects, 28 high-intestinal tone (IT) and 18 low-IT, before and after the administration of a single intramuscular dose of clonidine. Postsynaptic and presynaptic pharmacological mechanisms are postulated in order to explain these two different patterns of responses to the challenge of clonidine. In this chapter, the author distinguishes two types of subjects according to the changes induced by clonidine on their distal colon motility: those shows a high-IT who respond to the drug with an increase of tone and those showing a low-IT and frequently high phasic activity. In the latter, clonidine suppresses the phasic activity. Two main reasons argue against the possibility that clonidine-induced changes were peripherally induced: the raised norepinephrine plasma levels registered in low-IT subjects would favor peripheral pre- and postsynaptic adrenoceptors subsensitivity and in peripheral tissues, a2-adrenoceptors are predominantly presynaptically located on norepinephrine terminals.
