VIP antagonists enhance excitatory cholinergic neurotransmission in the human airway

Effect of vasoactive intestinal peptide (VIP)-related peptides on cholinergic neurogenic and direct mucus secretion in ferret trachea in vitro

1 We investigated whether vasoactive intestinal peptide (VIP) and its related peptides, pituitary adenylate cyclase activating peptide (PACAP) and secretin, regulate cholinergic neural mucus secretion in ferret trachea in vitro, using 35SO4 as a mucus marker. We also studied the interaction between VIP and secretin on cholinergic mucus output. 2 VIP (1 and 10 microM) increased secretion, whereas neither PACAP1 - 27, PACAP1 - 38 nor secretin (up to 10 microM) increased mucus output. In contrast, VIP, PACAP1 - 27 and PACAP1 - 38 concentration-dependently inhibited cholinergic neural secretion, with an order of potency of VIP>PACAP 1 - 38>PACAP1 - 27. Neither PACAP1 - 27 nor PACAP1 - 38 altered the secretion induced by acetylcholine (ACh). 3 Secretin increased cholinergic neural secretion with a maximal increase of 190% at 1 microM. This potentiation was blocked by VIP or atropine. Similarly, secretin (1 microM) potentiated VIP (1 microM)-induced mucus output by 160%. Secretin did not alter exogenous ACh-induced secretion. VIP vs secretin competition curves suggested these two peptides were competing reversibly for the same receptor. 4 We conclude that, in ferret trachea in vitro, VIP and PACAPs inhibit cholinergic neural secretion via pre-junctional modulation of cholinergic neurotransmission. VIP and secretin compete for the same receptor, possibly a VIP1 receptor, at which secretin may be a receptor antagonist.

Neuroregulation by vasoactive intestinal peptide (VIP) of mucus secretion in ferret trachea: activation of BK(Ca) channels and inhibition of neurotransmitter release

1. The aims of this study were to determine: (1) whether vasoactive intestinal peptide (VIP) regulates cholinergic and 'sensory-efferent' (tachykininergic) 35SO4 labelled mucus output in ferret trachea in vitro, using a VIP antibody, (2) the class of potassium (K+) channel involved in VIP-regulation of cholinergic neural secretion using glibenclamide (an ATP-sensitive K+ (K(ATP)) channel inhibitor), iberiotoxin (a large conductance calcium activated K+ (BK(ca)) channel blocker), and apamin (a small conductance K(ca) (SK(ca)) channel blocker), and (3) the effect of VIP on cholinergic neurotransmission using [3H]-choline overflow as a marker for acetylcholine (ACh) release. 2. Exogenous VIP (1 and 10 microM) alone increased 35SO4 output by up to 53% above baseline, but suppressed (by up to 80% at 1 microM) cholinergic and tachykininergic neural secretion without altering secretion induced by ACh or substance P (1 microM each). Endogenous VIP accounted for the minor increase in non-adrenergic, non-cholinergic (NANC), non-tachykininergic neural secretion, which was compatible with the secretory response of exogenous VIP. 3. Iberiotoxin (3 microM), but not apamin (1 microM) or glibenclamide (0.1 microM), reversed the inhibition by VIP (10 nM) of cholinergic neural secretion. 4. Both endogenous VIP (by use of the VIP antibody; 1:500 dilution) and exogenous VIP (0.1 microM), the latter by 34%, inhibited ACh release from cholinergic nerve terminals and this suppression was completely reversed by iberiotoxin (0.1 microM). 5. We conclude that, in ferret trachea in vitro, endogenous VIP has dual activity whereby its small direct stimulatory action on mucus secretion is secondary to its marked regulation of cholinergic and tachykininergic neurogenic mucus secretion. Regulation is via inhibition of neurotransmitter release, consequent upon opening of BK(Ca) channels. In the context of neurogenic mucus secretion, we propose that VIP joins NO as a neurotransmitter of i-NANC nerves in ferret trachea.

Involvement of K+ channels in the relaxant effect of vasoactive intestinal peptide and atrial natriuretic peptide in isolated guinea-pig trachea

The possible contribution of K+ channel activation to airway smooth muscle relaxation induced by vasoactive intestinal peptide (VIP) and atrial natriuretic peptide (ANP) was investigated in isolated guinea-pig trachea. Concentration-relaxation (CR) curves were assessed in preparations precontracted by 30 mM K+, 124 mM K+ or histamine either alone or in the presence of a K+ channel blocker: iberiotoxin (IbTX), glipizide, tetraethylammonium (TEA) or Ba2+. VIP completely relaxed contractions induced by histamine but had a lower effectiveness against those induced by 30 mM K+ and 124 mM K+. IbTX and TEA shifted the CR curve for VIP 5 and 14 times to the right, respectively. Glipizide and Ba2+ did not significantly antagonize the action of VIP. ANP relaxed contractions induced by histamine and 30 mM K+ but failed to relax those elicited by 124 mM K+. IbTX and TEA shifted the CR curve for ANP 8 and 46 times to the right, respectively. Glipizide and Ba2+ suppressed the maximal effect produced by ANP, and glipizide also shifted the CR curve to the left. The K+ channel opener levcromakalim relaxed tracheal contractions induced by histamine and 30 mM K+ but not those induced by 124 mM K+. Glipizide caused a 5-fold rightward shift of the CR curve for levcromakalim whereas IbTX shifted the curve to the left and increased the maximal relaxant effect. The Ca2+ channel blocker isradipine completely relaxed contractions induced by 30 mM K+ and 124 mM K+ but only partially relaxed those contracted by histamine. All four K+ channel blockers increased the maximal relaxant effect and shifted the CR curve for isradipine to the left. The results suggest that airway smooth muscle relaxation produced by VIP and ANP involves activation of large-conductance Ca(2+)-activated K+ channels (BKCa) and further that ANP may possibly activate other types of K+ channels additional to BKCa.

Inhibition by endothelin-1 of cholinergic nerve-mediated acetylcholine release and contraction in sheep isolated trachea

1. The relative roles of ETA and ETB receptor activation on cholinergic nerve-mediated contraction and acetylcholine (ACh) release were examined in sheep isolated tracheal smooth muscle. 2. Electrical field stimulation (EFS; 90 V, 0.5 ms duration, 1 Hz, 10 s train) applied to sheep isolated tracheal smooth muscle strips induced monophasic contractile responses that were abolished by either 1 microM tetrodotoxin or 0.1 microM atropine, but were insensitive to 10 microM hexamethonium and 100 microM L-NAME. Thus, EFS-induced contractions resulted from the spasmogenic actions of ACh released from parasympathetic, postganglionic nerves. 3. As expected, sheep isolated tracheal smooth muscle preparations did not contract in response to the ETB receptor-selective agonist, sarafotoxin S6c (0.1-100 nM). However, sarafotoxin S6c caused a concentration-dependent and transient inhibition of EFS-induced contractions. The inhibitory effect induced by a maximally effective concentration of sarafotoxin S6c (10 nM; 72.1 +/- 5.7%, n = 6) was abolished in the presence of the ETB receptor-selective antagonist BQ-788 (1 microM). Contractile responses to exogenously administered ACh (10 nM-0.3 mM) were not inhibited by sarafotoxin S6c (1 or 10 nM; n = 7). 4. In contrast to sarafotoxin S6c, endothelin-1 induced marked contractions in sheep isolated tracheal smooth muscle. These contractions were inhibited by BQ-123, consistent with an ETA receptor-mediated response. In the presence of BQ-123 (3 microM), endothelin-1 produced a concentration-dependent inhibition of EFS-induced contractions (30 nM endothelin-1, 68.9 +/- 10.2% inhibition, n = 5). These responses were inhibited by 1 microM BQ-788, indicative of an ETB receptor-mediated process. Endothelin-1 was about 3 fold less potent than sarafotoxin S6c. 5. EFS (90 V, 0.5 ms duration, 1 Hz, 15 min train) induced the release of endogenous ACh (1.94 +/- 0.28 pmol mg-1 tissue, n = 12), as assayed by h.p.l.c. with electrochemical detection. EFS-induced release of ACh was inhibited to a similar extent by 100 nM endothelin-1 (47 +/- 4%, n = 9) and 10 nM sarafotoxin S6c (46 +/- 9%, n = 3). These effects of endothelin-1 on ACh release were inhibited by 1 microM BQ-788 alone (n = 4), by BQ-788 in the presence of 3 microM BQ-123 (n = 4), but not by 3 microM BQ-123 alone (n = 5). 6. In summary, sheep isolated tracheal smooth muscle contains two anatomically and functionally distinct endothelin receptor populations. ETA receptors located on airway smooth muscle mediate contraction, whereas ETB receptors appear to exist on cholinergic nerves that innervate tracheal smooth muscle cells and mediate inhibition of ACh release. The inhibitory effect of ETB receptor stimulation on cholinergic neurotransmission is in stark contrast to the enhancing effects hitherto described in the airways.