Prejunctional alpha 2-adrenoceptors and norepinephrine release in the forearm of normal humans.
To investigate the prejunctional alpha 2-adrenoceptor-mediated autoinhibitory feedback for norepinephrine (NE) release, we measured the response of forearm blood flow (FAF, mercury in silastic strain gauge plethysmography) and changes of the forearm venous-arterial NE difference (delta VA-NE, deep forearm vein, brachial artery norepinephrine concentration) induced by six intrabrachial artery 5-min infusions of clonidine in 13 normal volunteers. NE spillover into the forearm circulation was estimated as the product of delta VA-NE and FAF after three clonidine infusions. Clonidine caused dose-dependent decreases in FAF with a maximum reduction of FAF after the highest dose of clonidine of 32 +/- 8%, (N = 7, p less than 0.01) indicating postjunctional alpha-adrenergic stimulation. delta VA-NE increased slightly yet calculated norepinephrine spillover remained statistically unchanged after clonidine. The results suggest that auto-inhibitory feedback control of neuronal transmitter release via prejunctional alpha 2-adrenoceptors may not to an important degree modulate norepinephrine release in the forearm vasculature of healthy subjects.
Kiowski
W
W
Hulthén
U L
UL
Ritz
R
R
Bühler
F R
FR
D000328:Adult; D001132:Arm / Q000098:blood supply; D001794:Blood Pressure / Q000187:drug effects; D003000:Clonidine / Q000494:pharmacology; D005246:Feedback; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D009638:Norepinephrine / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012039:Regional Blood Flow / Q000187:drug effects
D011942:Receptors, Adrenergic, alpha; D003000:Clonidine; D009638:Norepinephrine