Nonadrenergic Noncholinergic Relaxation Responses Research Articles

Enhanced Electrical Field Stimulated Nitrergic and Purinergic Vasoreactivity in Distal vs Proximal Internal Pudendal Arteries

BackgroundThe internal pudendal arteries (IPAs) supply blood to the penis and are highly susceptible to vascular remodeling in rodent models of diabetes, hypertension, aging, and chronic kidney disease, thus contributing to erectile dysfunction. Interestingly, vascular remodeling primarily occurs in the distal and not in the proximal IPA, suggesting distinct local physiologic signaling differences within the IPA. AimTo examine the role of purinergic signaling and neurotransmitter release by electrical field stimulation (EFS) in the regulation of proximal and distal IPA vascular tone. MethodsProximal and distal IPAs were mounted in wire myographs and vascular responses to phenylephrine, acetylcholine, and 2-(N,N-diethylamino)-diazenolate-2-oxide, diethyl-ammonium salt (DEA NONOate) were measured. EFS-mediated contraction and non-adrenergic non-cholinergic (NANC) relaxation were evaluated in the absence and presence of a nitric oxide synthase antagonist. Purinergic agonist and NANC relaxation responses were assessed in the presence and absence of P2X1 and P2Y1 antagonists. Protein expression of P2X1 and P2Y1 receptors was measured by western blot. Main Outcome MeasuresProximal and distal IPA contraction and relaxation were measured during increasing agonist administration and EFS in the presence and absence of antagonists. ResultsProximal and distal IPA concentration response curves to phenylephrine, acetylcholine, and DEA NONOate did no differ. Interestingly, distal IPA exhibited greater EFS-mediated contraction and NANC relaxation compared with proximal IPA. Nitric oxide synthase inhibition completely inhibited distal IPA NANC relaxation but did not affect proximal IPA relaxation. P2X1 or P2Y1 receptor antagonism during NANC relaxation increased distal IPA relaxation but decreased proximal IPA relaxation. Combined P2X1 and P2Y1 receptor antagonism had no effect on proximal IPA relaxation but significantly increased distal IPA NANC relaxation. Clinical TranslationUnderstanding neurovascular regulation of IPA vascular tone through nitrergic and purinergic mechanisms could yield new therapeutic targets to improve IPA blood flow and treat vasculogenic erectile dysfunction. Strengths and LimitationsThis study is the first to illustrate the differences in mechanisms responsible for regulating vascular tone in the proximal and distal IPAs. All presented findings are currently limited to ex vivo vascular function. ConclusionThe regulation of vascular tone differs regionally in the IPA. The distal IPA is controlled through neurotransmitter-mediated NO-dependent mechanisms and increased sensitivity to purinergic P2X1 and P2Y1 receptor inhibition.Odom MR, Pak ES, Brown DA, Hannan JL. Enhanced Electrical Field Stimulated Nitrergic and Purinergic Vasoreactivity in Distal vs Proximal Internal Pudendal Arteries. J Sex Med 2017;14:1285–1296.

Effects of chronic l-DOPA administration on neurogenic and endothelium-dependent relaxation responses in rabbit corpus cavernosum

Dopamine is a crucial central neurotransmitter that plays a fundamental role in the autonomic and somatic components of penile reflexes in animals and humans. Similar to the erectile responses of dopamine, systemic administration of l-DOPA induces yawning and penile erection in some species. The possible effects of l-DOPA on nitric oxide (NO)-dependent and -independent non-adrenergic non-cholinergic (NANC) relaxation responses mediated by electrical field stimulation (EFS) and endothelium-dependent relaxation were investigated in this study. Thirty-two adult albino male rabbits, in two- and four-week-treatment groups, were divided into three subgroups: control group (saline-injected) (n=4), 3mg/kg/day (low dose) l-DOPA-injected groups (n=6) and 12mg/kg/day (high dose) l-DOPA-injected groups (n=6). After the intraperitoneal injection treatments, the corpus cavernosum tissues were placed in organ bath chambers. The EFS-mediated responses, and the concentration-response curve to carbachol, sodium nitroprusside (SNP), sildenafil were assessed. The two-week treatment with high-dose l-DOPA decreased the NO-dependent NANC relaxation responses, while there was no change in the low-dose two- and four-week treatment groups. The NO-independent NANC relaxation responses in the two-week groups decreased, and the responses in the four-week groups were unchanged when compared to the controls. The relaxation responses to carbachol showed no differences among all groups except for the high-dose four-week l-DOPA group. The relaxation responses of SNP and sildenafil were increased in all of the treatment groups when compared to the controls. The observed increases in SNP- and sildenafil-induced responses, along with the decreased EFS-mediated responses, suggest increased sensitivity in the NO-signalling pathway following l-DOPA administration.

Calcium channel subtypes for cholinergic and nonadrenergic noncholinergic neurotransmission in isolated guinea pig trachea

The Ca(2+) channel subtypes in the neurotransmission of isolated guinea pig trachea were elucidated by monitoring the effects of specific Ca(2+) channel blockers on cholinergic contractions and nonadrenergic noncholinergic (NANC) relaxation elicited by electrical field stimulation (EFS). In isolated guinea pig trachea, cholinergic contractile responses to low- and high-frequency EFS were inhibited by the selective N-type calcium channel blocker, ω-conotoxin MVIIA. ω-Agatoxin IVA (a selective P-type blocker), ω-conotoxin MVIIC (a nonselective N-, Q-, and P-type blocker), and nifedipine (a selective L-type blocker) were ineffective, whereas Ni(2+) (a T- and R-type blocker) facilitated cholinergic contractions and produced a late contracture when its concentration exceeded 30μM. The more the concentration of Ni(2+) increased, the greater the number of incidences and the late contracture areas which occurred. Late contracture did not seem to be due to the effects of acetylcholine, tachykinins, or other polypeptides, but disappeared in the absence of indomethacin. The NANC relaxant responses elicited by the low- and high-frequency EFS were inhibited by ω-conotoxin MVIIA or Ni(2+), but unaffected by ω-Agatoxin IVA, ω-conotoxin MVIIC, and nifedipine. In the absence of indomethacin, Ni(2+) did not alter the ω-conotoxin MVIIA (100nM)-resistant component of cholinergic contraction, but significantly further inhibited that of NANC relaxation. These results suggest that in isolated guinea pig trachea, cholinergic contraction is regulated by N-type calcium channels which may mask T- and R-type calcium channels and may be co-modulated by both, while NANC relaxation is mainly and independently controlled by N-, T-, and R-type calcium channels.

Effect of anandamide on nonadrenergic noncholinergic-mediated relaxation of rat corpus cavernosum

The purpose of this study was to investigate the effect of the endogenous cannabinoid anandamide on the nonadrenergic noncholinergic (NANC) relaxant responses to electrical field stimulation in isolated rat corpus cavernosum. The corporal strips were mounted under tension in a standard oxygenated organ bath with guanethidine sulfate (5 μM) and atropine (1 μM) (to produce adrenergic and cholinergic blockade). The strips were precontracted with phenylephrine hydrochloride (7.5 μM) and electrical field stimulation was applied at different frequencies to obtain NANC-mediated relaxation. The expression of CB 1, CB 2 and vanilloid receptor proteins within the rat corpus cavernosum was evaluated using western blot analysis. The results showed that the relaxant responses to electrical stimulation were significantly enhanced in the presence of anandamide at 1 and 3 μM. The potentiating effect of anandamide (1 μM) on relaxation responses was significantly attenuated by either the selective cannabinoid CB 1 receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1 H-pyrazole-3-carboxamide (AM251; 1 μM) or the vanilloid receptor antagonist capsazepine (3 μM), but not by the selective cannabinoid CB 2 receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl) ethyl]-1 H-indol-3-yl (4-methoxyphenyl)methanone (AM630; 1 μM). Neither of these antagonists had influence on relaxation responses. Indomethacin (20 μM) had no effect on NANC-mediated relaxation in the presence or absence of anandamide (1 μM). Preincubation with N w-Nitro- l-Arginine Methyl Ester ( l-NAME; 1 μM) significantly inhibited the relaxation responses in the presence or absence of 1 μM anandamide. Although at 30 nM, l-NAME did not cause a significant inhibition of relaxant responses individually, it significantly inhibited the potentiating effect of anandamide (1 μM) on relaxation responses. Anandamide (1 μM) had no influence on concentration-dependent relaxant responses to sodium nitroprusside (10 nM–1 mM), a nitric oxide (NO) donor. The western blotting of corporal tissues demonstrated the existence of both vanilloid and CB 1 receptors in corporal strips. In conclusion, our results showed that anandamide has a potentiating effect on NANC-mediated relaxation of rat corpus cavernosum through both CB 1 and vanilloid receptors and the NO-mediated component of the NANC relaxant responses to electrical stimulation is involved in this enhancement.

Effects of platelet‐activating factor on nitrergic transmission in rabbit corpus cavernosum

The information currently available suggests that nonadrenergic noncholinergic (NANC) transmitters, particularly nitric oxide, are involved in the relaxation of penile erectile tissues. Platelet-activating factor (PAF) is a chemical mediator and is involved in many physiological and pathophysiological events. It is well known that several of the vascular actions of PAF are mediated by the generation of nitric oxide. We designed this study to test the hypothesis that PAF has an effect on NANC responses in rabbit corpus cavernosum strips. Rabbit corpus cavernosum strips were precontracted with phenylephrine (10(-5) mol/L). Isometric tension changes produced by carbachol (10(-9)-10(-5) mol/L), sodium nitroprusside (10(-8)-10(-5) mol/L) and electrical field stimulation (for 10 s at sequential frequencies of 2, 4, 8, 16, and 32 Hz as square-wave pulses of 50 mV) were recorded with a pressure transducer. These relaxations were compared to those obtained in the presence of PAF. PAF had no effect on endothelium-dependent, endothelium-independent or electrical field stimulation-induced NANC relaxation responses in isolated rabbit corpus cavernosum strips. There was no statistically significant difference between the pD(2) and E(max) values for carbachol or sodium nitroprusside in the presence of PAF. Our results suggest that PAF does not modify the endothelium-dependent, endothelium-independent or electrical field stimulation-induced NANC relaxation responses in isolated rabbit corpus cavernosum strips.

Purines and pyrimidines are not involved in NANC relaxant responses in the rabbit vaginal wall.

1. Non-adrenergic non-cholinergic (NANC) relaxant responses were elicited by electrical field stimulation (EFS) in rabbit vaginal wall strips after treatment with guanethidine and scopolamine and raising smooth muscle tone with phenylephrine. Under these conditions treatment with NOS inhibitors revealed a non-nitrergic NANC relaxant response. The possible role of purines and pyrimidines in these non-nitrergic NANC responses was investigated. 2. Exogenous application of ATP, ADP, adenosine, UTP, or UDP (all at 0.03-10 mM) induced concentration-dependent relaxant responses. 3. Responses to exogenous application of ATP were reduced by the general P2 antagonist cibacron blue (500 micro M), but not by suramin (100 micro M) and were unaffected by L-NAME (500 micro M), omega-conotoxin GVIA (omega-CTX, 500 nM) or tetrodotoxin (TTX, 1 micro M). 4. Responses to exogenous application of adenosine were reduced by the A(2A) antagonist ZM-241385 (30 micro M). 5. ATP- and ADP-induced responses were unaffected by the G-protein inhibitor pertussis toxin (100 ng ml(-1)), whilst ADP- but not ATP-induced responses were reduced by GDPbetaS (100 micro M), which stabilizes G-proteins in their inactive state. 6. EFS-induced non-nitrergic NANC relaxant responses were unaffected by suramin, cibacron blue, ZM-241385, pertussis toxin or GDPbetaS, but were completely inhibited by TTX. 7. Exogenous application of ATP (10 mM) and adenosine (10 mM) increased intracellular cyclic adenosine-3', 5'-monophosphate (cAMP). However, non-nitrergic NANC responses were not associated with increased cAMP. Neither non-nitrergic NANC responses nor responses to ATP or adenosine were associated with increased intracellular cyclic guanosine-3', 5'-monophosphate (cGMP) concentrations. 8. These results suggest that adenosine A(2A) receptors and P2 receptors are present in the rabbit vaginal wall, but that they are not involved in non-nitrergic NANC relaxant responses.

Characterization of the non-nitrergic NANC relaxation responses in the rabbit vaginal wall.

Electrical field stimulation (EFS)-induced non-adrenergic non-cholinergic (NANC) relaxation responses in the rabbit vaginal wall were investigated. These NANC responses were partially inhibited with the nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME; 500 microM), N(G)-nitro-L-arginine (300 microM) or N-iminoethyl-L-ornithine (500 microM) or the selective soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 microM). Application of L-NAME and ODQ concomitantly did not increase the degree of inhibition. L-NAME or ODQ were observed to be more effective at low frequencies. The resistant part of the responses was more pronounced at higher frequencies and was completely inhibited by tetrodotoxin (1 microM). Exogenous application of the peptides vasoactive intestinal peptide (VIP), pituitary adenylate cyclase activating peptide (PACAP-27 and PACAP-38), peptide histidine methionine (PHM), peptide histidine valine (PHV), helospectin-I or -II induced a relaxation response. Calcitonin gene-related peptide or substance P did not cause any relaxation. The peptidase alpha-chymotrypsin (type II; 2 units ml(-1)) did not affect non-nitrergic NANC responses, although it did inhibit relaxation responses elicited by exogenous VIP, PACAP-27, PACAP-38, PHM, PHV, helospectin-I or -II. K(+) channel inhibitors apamin (1 microM) or charybdotoxin (100 nM) when used alone or in conjunction did not affect non-nitrergic NANC responses. The non-nitrergic NANC responses were not associated with any increase in intracellular cyclic adenosine-3', 5'-monophosphate (cyclic AMP) or cyclic guanosine-3', 5'-monophosphate (cyclic GMP) concentrations. The peptide-induced relaxations were all associated with increases in cyclic AMP concentrations. These results suggest that a neuronal factor elicits non-nitrergic NANC responses in the rabbit vaginal wall. The identity of this factor remains to be established.

Nitrergic neurotransmission mediates the non-adrenergic non-cholinergic responses in the clitoral corpus cavernosum of the rabbit.

The corpus cavernosum is the erectile tissue in the penis and clitoris. Although nitrergic neurotransmission has been characterized in detail in the penile corpus cavernosum, functional studies on the inhibitory non-adrenergic non-cholinergic (NANC) transmission in the clitoral corpus cavernosum have been lacking. Here we demonstrate that electrical field stimulation (EFS) induces NANC relaxation responses in the clitoral corpus cavernosum of the rabbit. These responses were inhibited by NG-nitro-L-arginine methylester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) or tetrodotoxin. The inhibitory effect of L-NAME was partially reversed by L-arginine but not by D-arginine. EFS-induced relaxations were enhanced by an inhibitor of type V cyclic GMP phosphodiesterase, zaprinast. These results suggest that nitrergic neurotransmission is responsible for the NANC relaxation responses in the clitoral corpus cavernosum of the rabbit.

Evidence for a role for nitric oxide in relation of the frog oesophageal body to electrical field stimulation.

1. Electrical field stimulation (EFS) (1-10 Hz, 30 V, 2 ms) of frog oesophageal body strips resulted in frequency-dependent non-adrenergic, non-cholinergic (NANC) relaxations. 2. Tetrodotoxin (TTX) (10(-6)-10(-5) M) had no effect on EFS evoked relaxations with a 2 ms pulse width. At a pulse width of 0.5 ms only the responses to the highest frequency (10 Hz) were significantly inhibited by TTX at 10(-5) M. Relaxation at 2 ms pulse width were unaffected by omega-conotoxin (10(-6) M), nifedipine (10(-6) M) or cobalt (5 x 10(-4) M). 3. NG-nitro-L-arginine (L-NOARG) (10(-6)-10(-4) M), a nitric oxide synthase (NOS) inhibitor, caused a concentration-dependent inhibition of the EFS-induced NANC relaxant responses. The inhibitory effect of L-NOARG was both prevented and reversed by L-arginine but not D-arginine (5 x 10(-3) M). 4. The phosphodiester type V inhibitor (PDE V), SK&F 96231 (10(-7)-10(-4) M), caused a concentration-dependent potentiation of both the percentage relaxation and the duration of the relaxant responses to EFS. 5. ODQ (10(-7)-10(-5) M), a guanylate cyclase inhibitor, produced a concentration-dependent inhibition of EFS-evoked NANC relaxations. 6. Oxyhaemoglobin (10(-6) M), which binds nitric oxide (NO), inhibited NANC relaxations to EFS. 7. The NO donor sodium nitroprusside (SNP) (10(-8)-10(-4) M) produced a concentration-dependent inhibition of evoked tone. L-NOARG (10(-4) M) had no effect on the SNP evoked relaxations. Preincubation with oxyhaemoglobin (10(-6) M) caused a reduction in the SNP (10(-6)-10(-5) M) induced relaxations. 8. These results suggest NO is the relaxant transmitter of the frog oesophageal body and the source of NO may be non-neuronal.

Phosphodiesterase inhibitors and endothelin as modulators of respiratory neurotransmission.

1. Airway smooth muscle receives cholinergic, adrenergic and non-adrenergic, non-cholinergic (NANC) neural input. In guinea-pig airways cholinergic and NANC nerves provide contractile innervation, while adrenergic and NANC nerves provide relaxant pathways. In contrast, the major relaxant innervation in human airways is NANC in nature. 2. The present review describes the effects of selective phosphodiesterase (PDE) inhibitors on NANC relaxant and contractile responses in guinea-pig trachea as well as on NANC relaxations in human bronchus. 3. The effects of endothelin-1 on cholinergic contractile responses obtained in a variety of species are also assessed.

Nitric oxide, an enteric nonadrenergic‐noncholinergic relaxant transmitter: evidence using phosphodiesterase V and nitric oxide synthase inhibition

1. The effects of NG-nitro-L-arginine (L-NOARG), a nitric oxide synthase inhibitor, and SK&F 96231, a phosphodiesterase type V inhibitor, on electrical field stimulated (EFS) nonadrenergic noncholinergic (NANC) relaxations of rat fundal strips, guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, and guinea-pig taenia caeci were investigated. 2. Reproducible repeated control random EFS frequency-response curves were obtained for all three tissues. 3. Depending on the frequency of stimulation, L-NOARG (10(-4)-5 x 10(-3) M) caused either a complete or partial inhibition of the NANC-induced relaxations of the rat fundal strips and the guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not of the guinea-pig taenia caeci. The inhibitory action of L-NOARG was partially or totally reversed, depending on the tissue, by L-arginine (5 x 10(-3) M). 4. SK&F 96231 (10(-6)-10(-4) M) caused a concentration- and frequency-dependent potentiation of both the size and duration of the EFS-induced NANC relaxant response of rat fundal strips and guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not of the guinea-pig taenia caeci. 5. Zaprinast, another phosphodiesterase type V inhibitor (10(-6)-10(-4) M) caused a concentration- and frequency-dependent potentiation of the NANC relaxant responses to EFS of rat fundal strips. 6. SK&F 96231 and zaprinast alone (10(-6)-10(-4) M) caused a concentration-dependent relaxation of the agonist-induced tone of all three tissues with the maximum degree of relaxation found to be in the order stomach < ileum < caecum. This is the reverse order for ability of SK&F 96231 to potentiate relaxant responses to EFS. 7. These results suggest NO is involved in the NANC nerve-mediated relaxation of rat fundal strips and guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not the guinea-pig taenia caeci.

Sensory nerve- and neuropeptide-mediated relaxation responses in airways of Sprague-Dawley rats

We examined the role of sensory nerves in mediating nonadrenergic inhibitory responses in airway segments isolated from male Sprague-Dawley rats. In the presence of adrenergic blockade, capsaicin (Cap; 1 microM) elicited marked relaxation responses in isolated bronchi precontracted with bethanechol (Beth). Cap-induced inhibitory responses were unaffected by tetrodotoxin (TTX), were attenuated by incubation of the airway with indomethacin (Indo), phosphoramidon, or RP-67580, but were abolished by previous exposure of the airway to Cap and by denuding the epithelium. Substance P (SP; 1 microM), neurokinins A and B (1 microM), and calcitonin gene-related peptide (0.1 microM) relaxed Beth-contracted airway segments to a similar extent. The SP-induced responses were unaffected by adrenergic blockade or by pretreatment with either TTX, phosphoramidon, or Cap, but were attenuated by RP-67580 and abolished by Indo and by denuding the epithelium. In anesthetized mechanically ventilated rats, Cap (50 and 100 micrograms/kg i.v.) elicited a dose-dependent reversal of the increase in lung resistance induced by an infusion of Beth. The Cap-induced bronchodilation was unaffected by pretreatment with propranolol alone or in combination with hexamethonium. SP (44 nmol/kg iv) also evoked bronchodilatory responses in intact animals, which were unaffected by propranolol and hexamethonium but were abolished by treatment of the animals with Indo. Electrical-field stimulation (EFS) evoked nonadrenergic noncholinergic relaxation responses in contracted airway segments. These EFS-induced inhibitory responses were markedly attenuated by treatment of the airway segment with TTX, Cap, or RP-67580. We conclude that neuropeptides released from Cap-sensitive sensory nerves have potent inhibitory effects in rat airways that are mediated, in part, by activation of neurokonin NK1 receptors on epithelium and subsequent release of an inhibitory prostaglandin(s).

Modulation of neurotransmission in guinea-pig airways by galanin and the effect of a new antagonist galantide

Galanin is localised to sensory nerve fibres and cholinergic nerves in airways. Galantide has been shown to be a novel high affinity antagonist to galanin, since it inhibits galanin-mediated inhibition of glucos-induced insulin secretion and the neuronal action of galanin in the brain. We investigated the effects of galanin on cholinergic and non-adrenergic, non-cholinergic (NANC) responses to electrical field stimulation in guinea-pig airways, and examined whether galantide antagonised the effect of galanin on neurotransmission. Galanin (10 −6M) had no effect on cholinergic bronchoconstrictor responses and inhibitory NANC relaxation responses in trachea, but significantly inhibited excitatory NANC bronchoconstrictor responses in bronchi which is due to the release of tachykinins. Galantide (10 −8–10 −6M) had no effect on the galanin-induced inhibition of the excitatory NANC responses. Galanin may be important in the modulation of excitatory NANC responses but not cholinergic and inhibitory NANC responses in guinea-pig airways. This modulatory effect may be via a different type of galanin receptor than is present in other organs.

Non‐adrenergic, non‐cholinergic contractions in the electrically field stimulated guinea‐pig trachea

1. The effects of drugs and altering stimulus parameters on neurogenic responses to electrical field stimulation (EFS) have been investigated in distal and proximal portions of the guinea-pig trachea. 2. In the presence of indomethacin (3 microM) and propranolol (1 microM) two contractile phases were evident in both the proximal and distal trachea. The first rapid phase was abolished by atropine (0.1 microM), whereas the prolonged, second phase was abolished by capsaicin (10 microM) pretreatment. Tetrodotoxin (3 microM) abolished the initial response and greatly inhibited the second phase. In proximal trachea this second phase was evident only in 9 of 22 preparations. The addition of the peptidase inhibitor thiorphan (10 microM) however, caused a second phase to be seen in all the proximal tissues examined. 3. The two phases of the contractions to EFS were differentially sensitive to the pulse duration applied. The initial, cholinergic contractions were evident at lower pulse durations than were the prolonged capsaicin-sensitive contractions, with the first phase being approximately 10 fold more sensitive than the second phase. 4. The magnitude of the capsaicin-sensitive contraction to EFS was significantly greater in the distal trachea than in the proximal trachea. This difference prevailed in the presence of thiorphan, an inhibitor of neutral endopeptidase. In contrast, concentration-response curves to capsaicin were similar in segments of proximal and distal trachea. 5. The non-adrenergic non-cholinergic (NANC) relaxant responses were studied in tissues in which excitatory neurogenic responses were pharmacologically abolished by capsaicin and atropine treatment. The NANC relaxant responses in the proximal trachea were evident at lower pulse frequencies and were of greater magnitude compared with NANC relaxant responses in the distal trachea. 6. These results indicate that, by pharmacologically manipulating the trachea and by selecting optimum stimulation parameters, a NANC contractile response to EFS can be seen throughout the length of the guinea-pig trachea. This NANC response is most likely to be due to the release of tachykinins from capsaicin-sensitive sensory fibres. It is suggested that NANC relaxant responses mask NANC contractile responses especially in the proximal trachea where NANC relaxant responses predominate.

Enzymatic Modulation of Vasoactive Intestinal Peptide and Nonadrenergic Noncholinergic Inhibitory Responses in Guinea Pig Tracheae

The airways of the guinea pig are innervated by four types of autonomic nerves: cholinergic excitatory, adrenergic inhibitory, nonadrenergic noncholinergic (NANC) excitatory, and NANC inhibitory. Tachykinins (neurokinins A and B and substance P) are believed to mediate NANC excitatory responses, and vasoactive intestinal peptide (VIP) has been proposed as the chemical mediator of the NANC inhibitory system. Enzymatic degradation represents an important means by which the biologic actions of neurotransmitters are terminated. In the present study, relaxation responses of guinea pig tracheae to NANC nerve stimulation and to exogenous VIP administration were compared in the absence and presence of various peptidase inhibitors. NANC inhibitory responses elicited by electrical field stimulation were unaffected by aprotinin or soybean trypsin inhibitor but were depressed by thiorphan or leupeptin. Concentration-response curves to exogenous VIP were shifted to the left by soybean trypsin inhibitor but were not affected by aprotinin, leupeptin, or thiorphan. After tachykinin depletion with capsaicin, thiorphan also induced a leftward shift in the VIP concentration-response curve. Under the same conditions, thiorphan failed to influence NANC inhibitory responses. These results indicate that the NANC inhibitory neurotransmitter is not metabolized by enzymes susceptible to inhibition by aprotinin, leupeptin, soybean trypsin inhibitor, or thiorphan and, accordingly, distinguish NANC nervous responses from those induced by VIP. The results also suggest that the NANC excitatory system can interact functionally with the NANC inhibitory system, as evidenced by the blunting of NANC relaxation responses following inhibition of tachykinin metabolism and elimination of this effect by capsaicin.

The effects of antagonists of vasoactive intestinal peptide on nonadrenergic noncholinergic inhibitory responses in feline airways

Relaxations of the feline intrapulmonary bronchus (IPB) induced by VIP or nonadrenergic noncholinergic (NANC) inhibitory nervous stimulation were unaffected by the VIP receptor antagonist [Ac-Tyr 1,D-Phe 2]-GRF (1–29) (30 μM). A second VIP antagonist, [pCl-D-Phe 6,Leu 17]-VIP (30 μM), also had no effect on NANC relaxation responses or IPB sensitivity to VIP. However, responses to three of the four highest VIP concentrations were inhibited by this antagonist. These results indicate that [Ac-Tyr 1,D-Phe 2]-GRF (1–29) and [pCl-D-Phe 6,Leu 17]-VIP are not effective competitive antagonists of VIP receptors in feline airways and, hence, have but limited applicability in determining the role of VIP in mediating airway NANC inhibitory responses in this tissue.

The effect of epithelium removal on non-adrenergic, non-cholinergic inhibitory responses in the isolated central airways of the cat and guinea pig

Epithelium removal from the feline or the indomethacin-treated guinea pig trachea had no effect on tissue sensitivity or responsiveness to the contractile actions of pharmacological agonists or electrical field stimulation. In the feline hilar bronchus, epithelium removal had no effect on tissue sensitivity or responsiveness to acetylcholine or electrical field stimulation but increased bronchial sensitivity to serotonin without affecting responsiveness. Non-adrenergic non-cholinergic (NANC) relaxation responses elicited by electrical field stimulation in airway preparations from either species were unaffected by epithelium removal. These results suggest that the epithelium does not modulate contractile responses in the feline trachea but may modulate the actions of specific contractile agonists in the feline hilar bronchus. Further, NANC relaxation responses appear to occur independently of the airway epithelium.