Contractile effects of noradrenaline and 5‐hydroxytryptamine in human hand veins: a pharmacological receptor characterization

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Abstract
The postjunctional receptors mediating contractile responses to noradrenaline (NA) and 5‐hydroxytryptamine (5‐HT) were characterized in ring segments of human hand veins by using subtype selective agonists and antagonists. The mechanical characteristics of the preparations were also examined by length‐tension measurements. The length‐active wall tension curve was bell‐shaped and reached a maximum at a length corresponding to a passive distending pressure of approximately 14 mmHg. (–)‐Phenylephrine consistently contracted the veins and was 24 times less potent than (±)‐NA, whereas clonidine produced a contraction in only two out of 11 vessel segments. Neither prazosin nor rauwolscine competitively inhibited the contractile response to NA, and large inter‐individual differences were found in the degree of inhibition produced by the antagonists. However, application of both prazosin and rauwolscine almost abolished the NA‐induced contraction. Ketanserin and methergoline inhibited the contractile response to 5‐HT; the former in an apparently competitive manner with a pA2value of 8.94, whereas the latter substantially reduced the maximum 5‐HT response. It is suggested that NA elicits contraction in human hand veins by acting at a mixed population ofα1and α2‐adrenoreceptors. The contractile response to 5‐HT, on the other hand, appears to be mediated predominantly by 5‐HT2receptors.

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