NEURALLY EVOKED-RESPONSES OF HUMAN ISOLATED RESISTANCE ARTERIES ARE MEDIATED BY BOTH ALPHA-1-ADRENOCEPTORS AND ALPHA-2-ADRENOCEPTORS

1 Human subcutaneous resistance arteries (internal diameter 113-626-mu-m) 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-mu-M) was highly variable but unrelated to vessel calibre. Contractile responses to both continuous and intermittent stimulation were abolished by 1-mu-M tetrodotoxin. 2 Prazosin (100 nM and 1-mu-M, al-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-mu-M 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-mu-M) 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-mu-M. 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.