1. Electrical field stimulation (EFS) of muscle strips in vitro elicited a tetrodotoxin (TTX)-sensitive biphasic contractile response consisting of a phasic component followed by a tonic one. 2. The amplitude of both components of the response was impaired by N omega-nitro-L-arginine and potentiated by sodium nitroprusside. Cystamine caused a reduction in amplitude of both phasic and tonic components of the response to EFS. Neither N omega-nitro-L-arginine, sodium nitroprusside, nor cystamine induced changes in the resting muscle tone, or in the contractile response to exogenous agonists ATP and noradrenaline (NA). 3. The nitric oxide scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, induced a reduction in amplitude of both components of the response to EFS. 4. These results reveal a facilitatory prejunctional modulatory role for nitric oxide in sympathetic neurotransmission in rat vas deferens. Endogenous nitric oxide released in the extracellular space is presumed to potentiate neurotransmission by acting at prejunctional level via cGMP.

The pre- and postjunctional activities of a number of diadenosine polyphosphates were examined in the guinea-pig isolated vas deferens at the level of the membrane potential, using a modified sucrose-gap technique. P1,P3-Di(adenosine 5')triphosphate (Ap3A), P1,P4-di(adenosine 5')tetraphosphate (Ap4A) and P1,P5-di(adenosine 5')pentaphosphate (Ap5A) all caused concentration-dependent depolarization of the smooth muscle membrane. The potency order was: Ap5A > Ap4A > or = Ap3A. P1,P2-Di(adenosine 5')pyrophosphate (Ap2A) did not evoke depolarization even at the highest concentration tested (1 mM). All the dinucleotides caused a reduction in the amplitude of evoked excitatory junction potentials (e.j.ps). The potency order was: Ap5A = Ap4A > Ap3A > Ap2A. The depolarizations evoked by the dinucleotides were markedly reduced by the selective P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM), as was the amplitude of the fully facilitated e.j.p. The inhibition of the e.j.p. evoked by Ap3A and Ap2A was reduced by the P1-purinoceptor antagonist, 8-p-sulphophenyltheophylline (8-pSPT, 50 microM), but that evoked by Ap5A and Ap4A was not. Thus, Ap3A, Ap4A and Ap5A evoke depolarization of the guinea-pig vas deferens via P2X-purinoceptors, and additionally Ap2A and Ap3A exert a prejunctional effect via P1-purinoceptors. The prejunctional activity of Ap4A and Ap5A is mediated via an undefined purinoceptor, which is neither P1 nor P2X.

Using a single sucrose gap apparatus, experiments were performed to determine the involvement of nitric oxide (NO) in the generation of nonadrenergic, noncholinergic (NANC) inhibitory junction potentials in circular muscle of rat proximal colon. Inhibitors of NO synthase, N omega-nitro-L-arginine and its methyl ester, reduced the amplitude of the electrically evoked inhibitory junction potentials, without affecting membrane resting potential. Such an effect was stereospecific and it was prevented by L-arginine but not by D-arginine. Sodium nitroprusside induced a tetrodotoxin-resistant hyperpolarization, which was not affected by NO synthase inhibitors. Apamin reduced sodium nitroprusside induced hyperpolarization, as well as NANC inhibitory junction potentials, and alpha-chymotrypsin decreased the amplitude of electrical field stimulation evoked responses. Residual responses after NO synthase inhibitors or after alpha-chymotrypsin were further reduced by pretreatment with alpha-chymotrypsin or NO synthase inhibitors, respectively. These results suggest that, in rat colonic circular muscle, NO plays an important role in NANC inhibitory junction potential generation. However, another mechanism, peptidergic in nature, is also involved.

No abstract

Using the sucrose-gap technique, we attempted to assess a role for tachykinins (TKs) in mediating noncholinergic excitatory junction potential (EJP) and contraction, in the circular muscle of rat proximal colon. Excitatory responses were evoked by submaximal electrical field stimulation (EFS) in the presence of atropine (1 microM), guanethidine (1 microM), indomethacin (10 microM), and N(omega)-nitro-L-arginine methyl ester (L-NAME) (100 microM). The NK1 receptor antagonist, SR 140,333 (up to 3 microM) or the NK2 receptor antagonists, SR 48,968 and MEN 10,627 (up to 5 microM) produced a partial inhibition of the excitatory responses to EFS. The co-administration of the selective NK1 and NK2 receptor antagonists produced additive effects on the responses to EFS. Selective NK1 receptor agonist, [Sar9, Met (O2)11]-substance P, induced depolarization and contraction, antagonized by SR 140,333, but not by NK2 receptor antagonists. NK2 receptor agonist, [betaAla8]-neurokinin A (4-10), also produced electrical and mechanical excitatory effects that were antagonized by SR 48,968 or MEN 10,627, but not by the NK1 receptor antagonist. Our results provide evidence that, in circular muscle of rat colon, endogenous tachykinins are the main excitatory transmitters for nonadrenergic, noncholinergic (NANC) excitation and their action is mediated by both NK1 and NK2 receptors.