Which of the following is NOT true concerning clonidine?
A. It is a central acting alpha-2 agonist working in the pontomedullary region of the CNS
B. The net-effect of its administration is a decrease in sympathetic nervous system activity
C. It can be used in the diagnosis of pheochromocytoma
D. Binding of clonidine to presynaptic alpha-2 agonist receptors decreases presynaptic sodium levels, and inhibits the release of norepinephrine (NE)
The answer is D. Binding of the receptors will decrease presynaptic CALCIUM levels and decrease NE release. All of the others are true. Clonidine's effect on reducing circulating epinephrine by a central mechanism was used in the past as an investigatory test for pheochromocytoma which is a catecholamine-synthesizing tumor, usually of the adrenal medulla. In a clonidine suppression test plasma catecholamines levels are measured before and 3 hours after a 0.3 µg/kg oral test dose has been given to a patient. A positive test occurs if there is no decrease in plasma levels. I.E., if a patient has a pheo, the levels of catecholamines (NE) will remain elevated.