Neurally evoked responses of human isolated resistance arteries are mediated by both alpha 1- and alpha 2-adrenoceptors.
1. Human subcutaneous resistance arteries (internal diameter 113-626 microns) were mounted in an isometric myograph. Electrical field stimulation was applied either continuously in the form of a frequency-response curve or intermittently at 16 Hz. The magnitude of the maximum contraction induced by continuous stimulation expressed as a percentage of the response to a supramaximal concentration of noradrenaline (10 microM) was highly variable but unrelated to vessel calibre. Contractile responses to both continuous and intermittent stimulation were abolished by 1 microM tetrodotoxin. 2. Prazosin (100 nM and 1 microM, alpha 1-adrenoceptor antagonist) inhibited responses to continuous stimulation over a range of frequencies (2-8 Hz). The response to continuous stimulation at 8 Hz was inhibited by 78 +/- 6% by 1 microM prazosin. Rauwolscine (100 nM, alpha 2-adrenoceptor antagonist) had a smaller effect on contractions induced by continuous stimulation. Rauwolscine inhibited the response at 8 Hz by 36 +/- 6%. Rauwolscine at a higher concentration (1 microM) caused further inhibition of the response to continuous stimulation. Prazosin and rauwolscine in combination almost completely inhibited the response to continuous stimulation at concentrations of 1 microM. 3. Prazosin and rauwolscine inhibited responses to intermittent stimulation in a concentration-dependent manner. The IC50 for this action of prazosin was 3.7 +/- 1.6 nM and the maximum inhibition induced by 100 nM prazosin was 78 +/- 6%. The IC50 of rauwolscine was 12.0 +/- 1.3 nM and 100 nM rauwolscine caused a 86 +/- 7% reduction in the response to intermittent stimulation.Prazosin and rauwolscine in combination (each at 100 nM) caused marked inhibition of the response to intermittent stimulation leaving only 7.0 +/- 2.6% of the response.4. These data suggest that neurally released noradrenaline evokes contractions of human resistance arteries by activation of both alpha 1,- and alpha 2-adrenoceptors postjunctionally.
Parkinson
N A
NA
Department of Clinical Pharmacology, St. Mary's Hospital Medical School, London.
Thom
S M
SM
Hughes
A D
AD
Sever
P S
PS
Mulvany
M J
MJ
Nielsen
H
H
D001158:Arteries / Q000502:physiology*; D004558:Electric Stimulation; D006801:Humans; D066298:In Vitro Techniques; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D014661:Vasoconstriction / Q000187:drug effects / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D011224:Prazosin