Apamin and nonadrenergic inhibition of guinea pig trachealis

Role of carbon monoxide in electrically induced non-adrenergic, non-cholinergic relaxations in the guinea-pig isolated whole trachea

BACKGROUND AND PURPOSE

Nitric oxide (NO) and vasoactive intestinal peptide (VIP) are considered transmitters of non-adrenergic, non-cholinergic (NANC) relaxations in guinea-pig trachea, whereas the role of carbon monoxide (CO) is unknown. This study was designed to assess the participation of CO, and to investigate the localization of haem oxygenase-2 (HO-2), the CO-producing enzyme, in tracheal neurons.

EXPERIMENTAL APPROACH

NANC responses to electrical field stimulation (EFS) at 3 and 10 Hz were evaluated in epithelium-free whole tracheal segments as intraluminal pressure changes. Drugs used were: L-nitroarginine methyl ester (L-NAME, 100 microM) to inhibit NO synthase (NOS), alpha-chymotrypsin (2 U ml(-1)) to inactivate VIP, zinc protoporphyrin-IX (ZnPP-IX, 10 microM) to inhibit HO-2, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM), a soluble guanylyl cyclase inhibitor. For immunohistochemistry, tissues were exposed to antibodies to PGP 9.5, a general neuronal marker, HO-2 and NOS, and processed with an indirect immunofluorescence method.

KEY RESULTS

alpha-Chymotrypsin did not affect NANC relaxations. ODQ inhibited NANC responses by about 60%, a value similar to that obtained by combining L-NAME and ZnPP-IX. The combination of ODQ, L-NAME and ZnPP-IX reduced the responses by 90%. Subpopulations of HO-2 positive neurons containing NOS were detected in tracheal sections.

CONCLUSIONS AND IMPLICATIONS

In the guinea-pig trachea, NANC inhibitory responses at 3 and 10 Hz use NO and CO as main transmitters. Their participation is revealed following inhibition of NOS, HO-2 and soluble guanylyl cyclase. The involvement of CO as a relaxing transmitter paves the way for novel therapeutic approaches in the treatment of airway obstruction.

Effect of apamin on the responses to VIP, ATP and NANC neurone stimulation in the rat and cat gastric fundus

1. The influence of apamin on non-adrenergic non-cholinergic (NANC) relaxation and on the effect of the putative transmitters VIP and ATP was examined in the rat and cat gastric fundus. 2. In longitudinal muscle strips of the rat gastric fundus, ATP induced a biphasic effect, relaxation followed by contraction. The relaxant effect of ATP was blocked by apamin, whereas the relaxations induced by VIP and NANC neurone stimulation were not influenced by apamin. 3. In circular and longitudinal muscle strips of the cat gastric fundus, ATP only induced relaxation at high concentrations. The ATP-induced relaxation was increased in the presence of apamin, whereas the VIP-induced and NANC relaxations were not influenced. 4. It is concluded that the relaxant effect of ATP might be related to activation of apamin-sensitive Ca(2+)-dependent K(+)-channels in the rat gastric fundus. In the cat gastric fundus, apamin did not antagonize the relaxant effect of ATP so that it cannot be used to investigate an ATP involvement in inhibitory NANC neurotransmission in this tissue. No evidence for a presynaptic inhibitory action of apamin on VIP-ergic NANC neurones was obtained.

A novel neurogenic vasodilator mechanism in bovine mesenteric artery

The presence of a neurogenic vasodilator mechanism was investigated in isolated bovine mesenteric arteries (BMAs) that were precontracted with phenylephrine. Electrical field stimulation induced tetrodotoxin-sensitive relaxations in guanethidine-pretreated BMAs. The relaxation occurred after a delay of about 5-8 seconds and amounted to 25-35% in different sets of experiments. The relaxation was not affected by classical receptor antagonists such as atropine (1 microM), cimetidine (3.9 microM), clemastine (2.8 microM), naloxone (1.2 microM), 8-phenyltheophylline (1 microM), propranolol (3.4 microM), ritanserin (5 microM), or droperidol (13 microM). The nicotinic acetylcholine-receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium iodide (10 microM) was without effect on the relaxation, and removal of the endothelium of the arteries also had no effect. The bee venom component apamin (1 microM), which has been shown to block the nonadrenergic, noncholinergic relaxation in intestinal and vascular smooth muscle from other species, was also found to be without effect on the relaxation induced by electrical field stimulation in BMAs. Pretreatment of the arteries with capsaicin (1 microM) had no effect per se and did not affect the relaxation induced by a subsequent stimulation. Capsaicin has been suggested to release neurotransmitter and eventually deplete neurons containing substance P and calcitonin gene-related peptide. Furthermore, exogenously applied calcitonin gene-related peptide (1-100 nM), substance P (10 nM-1 microM), and vasoactive intestinal peptide (0.3-30 nM) gave relaxations amounting to less than 10%. It is postulated that electrical field stimulation induces a neurogenic relaxation of a nonadrenergic, noncholinergic nature in BMAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alaproclate, potassium channel blockers, and lidocaine on the release of 3H-acetylcholine from the guinea-pig ileum myenteric plexus

The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, preincubated with 3H-choline or 3H-noradrenaline, was mounted in an organ bath and superfused with Tyrode's solution. Alaproclate (2-(4-chlorophenyl)-1,1-dimethyl 2-aminopropanoate) (0.01-0.5 mmol/l) reduced (IC50 = 0.1 mmol/l) and at about 0.5 mmol/l completely blocked the electrically evoked 3H-acetylcholine secretion. The depressing effect of alaproclate (0.2 mmol/l) was not counteracted by atropine (0.01, 1 or 10 mumol/l), hexamethonium (0.1 mmol/l), phentolamine (1 mumol/l) yohimbine (1 mumol/l), haloperidol (1 mumol/l), 8-phenyltheophylline (10 mumol/l), cyproheptadine (1 mumol/l), metitepine (1 mumol/l), bicuculline (10 mumol/l), picrotoxinin (0.1 mmol/l), forskolin (25 mumol/l), 3-isobutyl-1-methylxanthine (5 mmol/l), nifedipine (1 mumol/l), verapamil (1 mumol/l), dilitiazem (1 mumol/l), high calcium (6 mmol/l), high potassium (10 or 15 mmol/l), tetraethylammonium (2 mmol/l), 4-aminopyridine (0.5 mmol/l), apamin (0.5 mumol/l), barium (0.5 mmol/l) or quinine (0.1 mmol/l). Among the potassium channel blockers tested only quinine (at 0.5 or 1 mmol/l), in the same manner as lidocaine, reduced the evoked secretion of 3H-acetylcholine. The results are in agreement with the hypothesis that the effect of alaproclate on the evoked 3H-acetylcholine secretion is not mediated by a neurotransmitter receptor, or a potassium channel sensitive to tetraethylammonium, 4-aminopyridine, apamin, or barium or quinine, but is due to a local anaesthetic effect. In contrast to the evoked secretion, the spontaneous release of 3H-acetylcholine was enhanced by high concentrations of alaproclate (0.4-1 mmol/l). The mechanism underlying the effect of alaproclate on the spontaneous release remains to be established. Alaproclate (0.25 or 0.5 mmol/l) also enhanced the spontaneous release and reduced the electrically evoked 3H-noradrenaline secretion.