Tachykinin NK1 receptor in the guinea-pig isolated proximal urethra: characterization by receptor selective agonists and antagonists.

1. The tachykinin receptor mediating contraction of the guinea-pig isolated proximal urethra has been characterized by use of receptor selective agonists and antagonists. All experiments were performed in the presence of peptidase inhibitors (bestatin, captopril and thiorphan, 1 microM each) in order to reduce peptide degradation. 2. The natural tachykinins, substance P and neurokinin A produced a concentration-dependent contraction of rings of the proximal urethra which approached the same maximum (about 50% of the response to 80 mM KCl). Substance P (EC50 155 nM) was slightly (3.6 times) more potent than neurokinin A (EC50 560 nM). 3. The tachykinin NK1 receptor selective agonist, [Sar9]substance P sulphone (EC50 62 nM), was slightly more potent than substance P and produced the same maximal response of natural tachykinins. The NK2 receptor selective agonist, [beta Ala8] neurokinin A(4-10), was active only at microM concentrations and its maximal effect did not exceed 20% of that to substance P or neurokinin A. The NK3 receptor selective agonist, senktide, was ineffective up to 30 microM. 4. The response to [Sar9]substance P sulphone was antagonized in a competitive manner by either (+/-)-CP 96,345 (pA2 7.75, slope - 1.10) or GR 82,334 (pA2 7.31, slope - 1.26), which are selective NK1 receptor antagonists, while it was unaffected (up to 10 microM) by MEN 10,376, a selective NK2 receptor antagonist. 5. The response to 10 microM [beta Ala8]neurokinin A (4-10) was abolished by either 0.2 microM (+/-)-CP 96,345 or 1 microM GR 82,334, suggesting the involvement of NK1 receptors.6. Electrical field stimulation (5 and 10 Hz, 0.25 ms, 100 V, trains of 5 s duration) produced tetrodotoxin-sensitive phasic contractions of the urethra which were abolished by atropine plus phentolamine (3 microM each). Capsaicin (1 microM) produced a small transient contraction of the urethra which was abolished by ( )-CP 96,345 (0.1 microM). ( )-CP 96,345 did not modify the response to electrical field stimulation.7. We conclude that tachykinin NK, receptors are the main if not the only mediators of the contractile response of guinea-pig proximal urethra to peptides of this family and that this preparation is useful for assessing the affinities of various ligands for the NK, receptor. Endogenous tachykinins released from peripheral endings of capsaicin-sensitive primary afferents produce urethral contraction by activating NK, receptors.