Electrically Induced Relaxation of the Noradrenaline Contracted Isolated Urethra from Rabbit and Man

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Abstract
Isolated urethral preparations from rabbit and man responded to transmural electrical stimulation with contraction when the basal tension was low and with relaxation when the preparations were contracted by noradrenaline [norepinephrine], clonidine, 5-hydroxytryptamine and lysine vasopressin. Both contractant and relaxant responses were abolished by tetrodotoxin, suggesting that they were caused by the action of transmitters released from nerves. The electrically induced contractions in the rabbit urethra had a threshold frequency of 5-6 Hz and maximum was reached at 40 Hz. The responses were markedly reduced by .alpha.-receptor blockers suggesting that the released contraction-mediating transmitter was mainly NE. The relaxant response was immediate and rapidly reversible. It was obtained at a threshold frequency of 1-2 Hz, and maximum was reached at 10-15 Hz. It was not inhibited by propranolol, indomethacin, atropine or peptides such as substance P, VIP [vasoactive intestinal polypeptide], vasopressin or somatostatin. Prostaglandin E2, isoprenaline, ATP and VIP relaxed the NE contracted rabbit urethra with a time course different from that of the electrically induced relaxation. Nifedipine and Ca-free solution decreased the NE induced contraction but did not influence the relaxant response to electrical stimulation. Apparently, the relaxant response of the isolated NE-contracted urethra to electrical stimulation is caused by an unknown transmitter released from nerves.