Martha L. McNee

Secretogogue responses of leukotriene C4, D4: Comparison of potency in canine trachea invivo

By the use of close arterial injection of leukotrienes into the circulation supplying the upper cervical canine trachea, it has been possible to assess the secretogogue effects of leukotriene C4, and D4 on mucus secretion. Both LTC4 and LTD4 increased mucus secretion over baseline levels by a statistically significant level (p = less than 0.05). LTD4 was more potent than C4 with relative potencies of 2500, 320, 630, and 500 based on hillock formation (a measure of secretion) at 1, 2, 3, and 4 minutes after injection. The overall difference in potency in this animal model of mucus production was LTD4 greater than C4 by 1000-fold.

Adenosine‐induced secretion in the canine trachea: modification by methylxanthines and adenosine derivatives

1 Adenosine alone at 0.1 and 1.0 mg per tracheal segment stimulated mucus secretion by 52% and 88%, respectively, compared to baseline (P < 0.0001). 2 The site of the potent secretagogue effect of adenosine in canine trachea was consistent with A2 activation. 3 A2 site activation and enhanced secretion were also induced by N‐ethylcarboxamide adenosine and dipyridamole. 4 N6‐R‐phenylisopropyl adenosine (PIA) and 2′,5′‐dideoxyadenosine (ddAdo) inhibited the adenosine‐induced secretion (35% and 42%, respectively). However, when PIA or ddAdo were administered in conjunction with the potent phosphodiesterase inhibitor, methylisobutylxanthine (MIX), the effects of PIA were potentiated and the effects of ddAdo were reversed, yielding stimulation (A2) and antagonism (A1) of secretion, respectively. 5 8‐Phenyltheophylline by aerosol was a very potent antagonist of the secretagogue effect of adenosine (70% inhibition; P < 0.00001).

The Activity of a New, Novel Inhibitor of Leukotriene Synthesis in Rhesus Monkey Ascaris Reactors

The IgE-mediated hypersensitivity to Ascaris antigen in reactor rhesus primates was used to assess the pharmacologic profile of U-60,257, a pyrrole analog of prostacyclin. Whether the compound was given by aerosol or intravenously, it effectively inhibited the bronchopulmonary effects of antigen challenge. Dose responses by the aerosol route showed a dose-dependent inhibition of resistance (RL) and compliance (Cdyn) changes (100% +/- 0 to 10.2% +/- 14.5 for RL and 79.9% +/- 20 to 0% for Cdyn after 15 breaths of 1.0-0.01% solutions delivered into the lungs; n = 16). When administered at 1.0% by aerosol the duration of the inhibitory effect was 6 h (n = 3). Dose-dependent inhibition in both RL (93.3% +/- 5.6 to 69.4% +/- 34.8) and Cdyn (51.4 +/- 40.4 to 47.0% +/- 28.5) was also seen when the compound was given intravenously (5.0-0.01 mg/kg; n = 16). U-60,257 is a selective inhibitor of leukotriene synthesis in in vitro systems. Therefore, the finding of synergism in the inhibition of response of primates to Ascaris between this compound and the H1 histamine blocker diphenhydramine suggests that the leukotrienes may play an etiologic role in this response.

Leukotriene C4 and Dimethylphenylpiperazinium-Induced Responses in Canine Airway Tracheal Muscle Contraction and Fluid Secretion

Leukotrienes have been implicated as putative mediators in several air way diseases. In previous canine studies it was shown that leukotriene C4 (LTC4) enhanced fluid secretion over baseline values and this enhancement could be blocked by hexamethonium. This indicates that leukotrienes have as one of their actions, stimulation of ganglionic motor neurons. In the present study, we determined that LTC4 acts at a similar site as the specific nicotinic receptor agonist dimethylphenylpiperazinium (DMPP). Both LTC4 and DMPP when given alone enhanced mucus secretion and induced tracheal muscle contraction over control baseline (p less than 0.05). When added to DMPP, LTC4 enhanced the DMPP effect of muscle contraction at 5 and 8 micrograms by a synergistic amount, while the secretion was only additive. The slopes of the dose-response curves for DMPP + LTC4 did not differ by a statistically significant amount. LTC4 and DMPP act on a similar, if not the same, ganglionic receptor.

Inhibitors of metal catalyzed lipid peroxidation reactions inhibit mucus secretion and 15 HETE levels in canine trachea.

Inhibition of canine mucus secretion in vivo induced by arachidonic acid administration was correlated with a reduction of 15 HETE levels in canine mucus. Antioxidants and inhibitors of lipid peroxidation were effective inhibitors of both mucus secretion and 15 HETE production. This same series of inhibitors also dose dependently inhibited Fe2+ dependent oxidation of arachidonic acid in vitro as assessed by an inhibition of thiobarbituric acid reactive material and conjugated diene formation. These data argue for an involvement of reactive oxygen species and lipid peroxidation in the generation and elaboration of mucus secretion.

Secretagogue action of adenosine in the in vivo canine tracheal mucus model

Adenosine injected via the cranial thyroid artery route into canine tracheas was a very potent and quick acting secretagogue in a unique model of mucus production. It is not known if adenylate cyclase activation was involved, but adenosine caused a statistically significant, dose related enhancement over baseline secretion rates in seventeen mongrel dogs (0.01 mg 18.4 +/- 5.04% increase N.S.; 0.1 mg 62.5 +/- 9.00% P less than 0.001; and 1.0 mg 91.2 +/- 14.1% P less than 0.001). The response was complete in less than two minutes and muscle relaxation usually accompanied the response. Adenosine activation of submucosal glands, mast cells or airway tissue may mean that adenosine is an agonist in pathophysiological abnormalities of the airways.

Regulation of Canine Mucus Secretion by a Novel Leukotriene Synthesis Inhibitor (U-60,257)

A novel leukotriene synthesis inhibitor, piriprost (U-60,257) was characterized in three quantitated assays of secretion in the canine in vivo trachea. The normal baseline secretion rate was inhibited in a dose-related fashion when the inhibitor was given via the cranial thyroid artery (decrease compared to control dogs: 27 +/- 3.11% at 0.1 mg, n = 3, p less than 0.05; 56 +/- 4.85% at 0.5 mg, n = 3, p less than 0.01; 61.5 +/- 6.8% at 1.0 mg, n = 14, p less than 0.001). Hypoxia gas mixtures (8% O2 and 92% N2 4 min) delivered into the lung caused a mean increase in secretion of 160 +/- 11.2% (p less than 0.05). When the leukotriene inhibitor (1.0 mg) was given into the close arterial segment before hypoxia, this increased secretion response was blocked (160 +/- 11.2 to 40 +/- 16.7%; p less than 0.05). Finally, arachidonic acid caused a potent and long-lasting enhancement in secretion (179 +/- 9.0% of control; p less than 0.01) that was partially blocked by 5 mg/kg indomethacin 30 min before (90 +/- 12.7% of arachidonic acid; p less than 0.01), and when 1.0 mg U-60,257 preceded the arachidonic acid, an additional inhibition of 50% of the indomethacin inhibition was seen (p less than 0.01; n = 14). These findings are consistent with the hypothesis that canine secretion is modulated by arachidonic acid metabolites that act as agonists for secretion both by the cyclooxygenase and lipoxygenase pathways.