Airway hyperresponsiveness to adenosine induced by lipopolysaccharide in Brown Norway rats

Abstract

We have explored the effects of bacterial endotoxin (lipopolysaccharide; LPS) on the response of the airways of Brown Norway (BN) rats to adenosine. Comparisons have been drawn with the effects on responses to methacholine and 5-hydroxytryptamine. In vehicle-challenged animals, adenosine, given i.v. was only a weak bronchoconstrictor. In contrast, 1 h following intratracheal administration of LPS, 0.3 mg kg-1, bronchoconstrictor responses to adenosine were markedly and selectively enhanced. At this time point, there were no significant changes in leukocyte numbers, eosinophil peroxidase and myeloperoxidase activities or protein concentrations in bronchoalveolar lavage (BAL) fluid. Twenty-four hours after challenge, the sensitivity of the airways to both adenosine and methacholine was reduced relative to the earlier time point and there were substantial increases in each marker of inflammation in BAL fluid. The bronchoconstrictor response to adenosine was blocked selectively by methysergide, disodium cromoglycate and the broad-spectrum adenosine receptor antagonist, 8-SPT, but not by DPCPX or ZM 243185, selective antagonists for the A1 and A2A receptors, respectively. Thus, the response to adenosine augmented following LPS is mast cell mediated and involves a receptor which can be blocked by 8-SPT but not by selective A1 or A2A receptor antagonists. It thus bears similarity to the augmented response to adenosine induced by allergen challenge in actively sensitized BN rats. Exposure to LPS could be a factor along with allergen in determining the increased sensitivity of the airways of asthmatics to adenosine.

Figure 1

Increases in pulmonary resistance induced by intravenous administration of adenosine and methacholine starting 1 h post intratracheal instillation of lipopolysaccharide (LPS; 0.1, 0.3 or 1 mg kg⁻¹), or vehicle (saline, 0.2 ml), in Brown Norway rats. Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^*P<0.05 indicates a significant difference between the LPS-treated and equivalent vehicle-treated group.

Figure 2

Increases in pulmonary resistance induced by intravenous administration of adenosine and methacholine starting 1, 3 or 24 h post intratracheal instillation of lipopolysaccharide (LPS; 0.3 mg kg⁻¹), or vehicle (saline, 0.2 ml), in Brown Norway rats. Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^*P<0.05 indicates a significant difference between the equivalent LPS-and vehicle-treated groups.

Figure 3

Comparison of the bronchoconstrictor and cardiovascular (mean arterial blood pressure, MABP; heart rate, HR) effects of adenosine in Brown Norway rats challenged intratracheally 1 h previously with vehicle (saline, 0.2 ml) or LPS (0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^**P<0.01, ^***P<0.001 that the value is significantly different from the equivalent value in vehicle-challenged animals.

Figure 4

Effect of intratracheal instillation of lipopolysaccharide (LPS, 0.3 mg kg⁻¹) or vehicle (saline, 0.2 ml) on the number of inflammatory cells, the eosinophil peroxidase (EPO) and myeloperoxidase (MPO) activities and the protein and TNFα concentrations in BAL fluid of Brown Norway rats. Animals were killed either 1 or 24 h following LPS administration; vehicle-treated animals were given saline and killed 1 h after treatment. Histograms represent the mean values (±s.e.means) from 10 animals per group. ^*P<0.05, ^**P<0.01, ^***P<0.001 indicates a significant difference from the equivalent value in the vehicle-treated group.

Figure 5

Effect of methysergide (30 μg kg⁻¹ given i.v. 5 min prior to the start of the injection sequence) on bronchoconstrictor responses to adenosine, methacholine and 5-HT in Brown Norway rats treated intratracheally 1 h previously with lipopolysaccharide (LPS, 0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^*P<0.05, ^**P<0.01, ^***P<0.001 indicates significant difference between animals given LPS plus the vehicle for methysergide (saline) and animals given the vehicle for LPS plus the vehicle for methysergide. #P<0.05, ##P<0.01, ###P<0.001 indicates significant difference between LPS-challenged animals given methysergide and the corresponding saline-treated control value.

Figure 6

Effect of disodium cromoglycate (DSCG, 20 mg kg⁻¹ given i.v. 5 min prior to the start of the injection sequence) on bronchoconstrictor responses to 5-HT and adenosine in Brown Norway rats treated with lipopolysaccharide (LPS, 0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^***P<0.001 indicates significant difference between vehicle- and corresponding DSCG-treated animals.

Figure 7

Effect of 8-SPT (40 mg kg⁻¹ given i.v. 5 min prior to adenosine) on responses to adenosine, 5-HT and NECA with respect to pulmonary resistance, mean arterial blood pressure (MABP) and heart rate (HR) in Brown Norway rats 1 h post intratracheal instillation of lipopolysaccharide (LPS, 0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^**P<0.01, ^***P<0.001 indicates significant difference between vehicle- and corresponding 8-SPT-treated animals.

Figure 8

Effect of DPCPX (100 μg kg⁻¹ given i.v. 5 min prior to adenosine) on responses to adenosine, 5-HT and CPA with respect to pulmonary resistance, mean arterial blood pressure (MABP) and heart rate (HR) in Brown Norway rats 1 h post intratracheal instillation of lipopolysaccharide (LPS, 0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^*P<0.05, ^**P<0.01, ^***P<0.001 indicates significant difference between vehicle- and corresponding DPCPX-treated animals.

Figure 9

Effect of ZM 241385 (30 μg kg⁻¹ given i.v. 5 min prior to adenosine) on responses to adenosine, 5-HT and CGS 21680 with respect to pulmonary resistance, mean arterial blood pressure (MABP) and heart rate (HR) in Brown Norway rats 1 h post intratracheal instillation of lipopolysaccharide (LPS, 0.3 mg kg⁻¹). Results are expressed as means±s.e.means of the number (n) of animals shown in parentheses. ^*P<0.05, ^**P<0.01, ^***P<0.001 indicates significant difference between vehicle- and corresponding ZM 24138-treated animals.