Unique effect of tetrapentylammonium ions on sympathetic transmission in vat vas deferens

1. Effects of tetrapentylammonium ions (TPA⁺) on contractile responses induced by electric field stimulation and by exogenous agonist such as noradrenaline, ATP and high K⁺ were examined in the isolated epididymal half of the rat vas deferens. 2. The electrically evoked monophasic contractions (0.5 Hz, 1 ms pulse duration, 40 V) were potentiated by TPA⁺ at low concentrations (1-3 μM) and inhibited by TPA⁺ at a concentration greater than 10 μM. 3. TPA⁺ (0.3-3 μM) potentiated the contractile responses induced by noradrenaline and ATP, whilst, TPA⁺ (10 μM) concentration-dependently reduced the agonist-induced contractions. TEA⁺ enhanced both electrically evoked and agonist-induced contractions. 4. TPA⁺ (0.3 μM) potentiated 30 mM K⁺-induced phasic contraction which was inhibited by pretreatment with α,β-methylene ATP (3 μM) and prazosin (3 μM). 5. TPA⁺ (10-300 μM) reduced the contractions induced by 100 mM extracellular K⁺ while TEA⁺ and TEA⁺ had no effect. 6. The present results show that TPA⁺ at low concentrations may act at prejunctional nerve membranes to enhance release of contractile transmitters and act as a putative K⁺ channel blocker at postjunctional membranes to increase muscle contractility in rat vas deferens; whilst at high concentrations TPA⁺ mainly acts on smooth muscle probably as a non-selective relaxant against the agonist-induced contraction probably through inhibition of Ca²⁺ influx; this inhibitory effect appears unique for TPA⁺ since TEA⁺ and TBA⁺ did not induce muscle relaxation.