Journal of Applied Physiology
Vol. 83, No. 3, pp. 912-917, September 1997
PULMONARY CIRCULATION AND LUNG FLUID BALANCE

Ragweed sensitization alters pulmonary vascular responses to bronchoprovocation in beagle dogs

Andreas Theodorou¹, Natalie Weger¹, Kathleen Kunke¹, Kyoo Rhee¹, David Bice², Bruce Muggenberg², and Richard Lemen¹

¹ Respiratory Sciences Center and Steele Memorial Children's Research Center, University of Arizona, Tucson, Arizona 85724-5073; and ² Inhalation Toxicology Research Institute, Albuquerque, New Mexico 87115

ABSTRACT

INTRODUCTION

METHODS

RESULTS

DISCUSSION

FOOTNOTES

REFERENCES

ABSTRACT

Theodorou, Andreas, Natalie Weger, Kathleen Kunke, Kyoo Rhee, David Bice, Bruce Muggenberg, and Richard Lemen. Ragweed sensitization alters pulmonary vascular responses to bronchoprovocation in beagle dogs. J. Appl. Physiol. 83(3): 912-917, 1997.In ragweed (RW)-sensitized beagle dogs, we tested the hypothesis that reactivity of the pulmonary vasculature was enhanced with aerosolized histamine (Hist) and RW. Seven dogs were neonatally sensitized with repeated intraperitoneal RW injections, and 12 dogs were controls (Con). The dogs were anesthetized with intravenous chloralose, mechanically ventilated, and instrumented with femoral arterial and pulmonary artery catheters. Specific lung compliance (CL_sp), specific lung conductance (G_sp), systemic vascular resistance index, and pulmonary vascular resistance index (PVRI) were measured before and after bronchoprovocation with Hist and RW. After Hist inhalation (5 breaths of 30 mg/ml), both Con and RW dogs had significant (P < 0.05) decreases in CL_sp (51 ± 4 and 53 ± 5%, respectively) and G_sp (65 ± 5 and 69 ± 3%, respectively), but only RW-sensitized dogs had a significant increase in PVRI (38 ± 10%). After RW inhalation (60 breaths of 0.8 mg/ml), only RW-sensitized dogs had significant increases (62 ± 20%) in PVRI and decreases in G_sp (77 ± 4%) and CL_sp (65 ± 7%). We conclude that, compared with Con, RW-sensitized beagle dogs have increased pulmonary vasoconstrictive responses with Hist or RW inhalation.

pulmonary hypertension; pulmonary circulation; asthma; airway reactivity; LabView

INTRODUCTION

DOGS NEONATALLY SENSITIZED to ragweed (RW) have been demonstrated previously by us (23) and others (5, 9) to develop specific and nonspecific airway hyperresponsivness. The mechanism by which RW sensitization enhances airway responsiveness is unknown. However, release of bronchoconstricting mediators (such as platelet-activating factor, substance P, leukotrienes, and others) and upregulation of ganglionic muscarinic receptors are proposed by current theories (5, 11). We reasoned that pulmonary vascular responses, especially of vessels in close proximity to airways, also may be influenced by these mechanisms and contribute to the pathophysiology of human asthma. Thus we evaluated the hemodynamic effects of histamine (Hist) and RW bronchoprovocation in dogs that were neonatally sensitized to RW. Our studies suggest that the pulmonary vascular smooth muscle, like airway smooth muscle, is hyperresponsive to Hist or RW.

METHODS

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RESULTS

Baseline ages, weights, and pulmonary function in Con and RW dogs were not significantly different, as illustrated in Table 1. We had a predominance of female dogs in the RW-sensitized group.

Table  1.   Baseline physical and pulmonary function data Controls RW-Sensitized P Value n 12 7 Age, days 301 ± 35  228 ± 31  0.18   Range 140-432 154-343 Weight, kg 9.1 ± 0.5  9.3 ± 0.5  0.79   Range  7.0-11.4  7.7-10.5 RL, cmH2O · ml1 · s 8.34 ± 1.2  8.71 ± 0.6  0.81   Range  3.9-13.8  6.7-11.1 Cdyn, ml/cmH2O 40.3 ± 4.6  37.4 ± 4.3  0.67   Range 18.0-68.8 26.0-53.4 Gender (M:F) 7:5 1:6 Values are means ± SE or ranges; n, no. of dogs. RW, ragweed; RL, lung resistance; Cdyn, dynamic compliance; M, male; F, female.

Pulmonary functions for both groups are illustrated in Fig. 1, A and B. After five breaths of saline (Sal1), neither Con nor RW-sensitized dogs had a significant change in G_sp (P = 0.08 and 0.27, respectively) or CL_sp (P = 0.14 and 0.63, respectively). On the other hand, after inhaling aerosolized Hist, both Con and RW-sensitized dogs had significant (P < 0.001) decreases in G_sp (65 and 69%, respectively) and CL_sp (51 and 53%, respectively). After 60 breaths of saline (Sal2), G_sp decreased significantly in Con (19%, P = 0.02) and in RW (10.5%, P = 0.004) compared with BL3, but these changes in G_sp were not significant (P > 0.05) between groups. Comparing BL3 with Sal2 data, CL_sp also decreased significantly (13%, P = 0.035) in Con dogs but not in RW-sensitized dogs (5%, P = 0.21). However, these differences were again not significantly different between groups (P = 0.9), and both groups recovered pulmonary function (BL4) to pre-Sal2 levels.

Lung G_sp and CL_sp decreased significantly (20%, P = 0.006 and 13%, P = 0.034, respectively) in Con dogs after RW bronchoprovocation, but these decreases were not different from values after 60 breaths of saline (Sal 2) bronchoprovocation (P = 0.20). On the other hand, in RW-sensitized dogs, both G_sp and CL_sp decreased markedly (77%, P = 0.00001, and 66%, P = 0.0003, respectively), and these decreases were three- to fourfold larger (P = 0.004) than from saline alone (Sal2 compared with BL3).

Data on arterial blood gases after Hist and RW bronchoprovocation are illustrated in Table 2. After Hist brochoprovocation, arterial PO₂ (Pa_O2) decreased significantly (P < 0.05) within groups compared with BL2 and between groups. On the other hand, Pa_O2 remained >130 Torr for both groups because we gave supplemental O₂. The Pa_O2 in Con did not change after RW bronchoprovocation, but RW-sensitized dogs had a significant decrease (P < 0.05) in Pa_O2, although the group mean remained >100 Torr.

Table  2.   Arterial blood-gas data with histamine and ragweed bronchoprovocation Parameter by Group BL2 Histamine BL4 Ragweed PaO2, Torr   Controls 230 ± 9  177 ± 14 218 ± 10  214 ± 8    RW 191 ± 14  131 ± 13*, 191 ± 14  106 ± 16*, PaCO2, Torr   Controls 37 ± 1  42 ± 1 38 ± 1  38 ± 1    RW 35 ± 2  41 ± 3 38 ± 2  47 ± 4*, Arterial pH   Controls 7.29 ± 0.01  7.25 ± 0.01 7.27 ± 0.01  7.27 ± 0.01    RW 7.32 ± 0.02* 7.27 ± 0.02 7.29 ± 0.02  7.21 ± 0.02 Values are means ± SE. PaO2, partial pressure of arterial O2; PaCO2, partial pressure of arterial CO2; RW, ragweed-sensitized dogs; BL2 and BL4, second and fourth baselines, respectively. * Significant difference between control and RW; Significant difference (P < 0.05) from BL.

The pH and Pa_CO2 had statistically significant (P < 0.05) changes in both Con and RW-sensitized dogs after Hist bronchoprovocation, although there was no significant difference between Con and RW-sensitized groups. RW bronchoprovocation in RW-sensitized dogs led to a decrease in pH and increase in Pa_CO2 (P < 0.05) in the RW-sensitized dogs, with no change in Con dogs.

As illustrated in Fig. 2A, CI is not significantly different at BL1, Sal1, or BL2 between or within Con and RW groups. On the other hand, CI increased significantly after Hist bronchoprovocation in Con dogs compared with pre-Hist levels (BL2) and RW-sensitized dogs. The CI in RW-sensitized dogs did not significantly increase after Hist compared with BL2. After Hist bronchoprovocation, CI in the RW group returned to previous BL levels within the group but remained significantly lower than Con at BL3 (P = 0.007), Sal2 (P = 0.006), and BL4 (P = 0.003). After RW bronchoprovocation, CI did not change significantly in Con or RW-sensitized dogs compared with their preceding baseline (BL4). However, CI in Con dogs remained significantly larger (P = 0.02) than in RW-sensitized dogs as found in their preceding BL values. HR, Fig. 2B, increased significantly (P = 0.0008) in Con dogs and approached a significant increase (P = 0.08) in RW-sensitized dogs after Hist bronchoprovocation compared with BL2 but was not different between groups.

Ppa (Fig. 3A) was not different (P > 0.05) between or within groups for BL1, Sal1, or BL2 data. After Hist bronchoprovocation, Ppa increased significantly compared with BL2 data in the Con group (47%, P = 0.0002) and the RW group (41%, P = 0.006), but Ppa was not different between groups (P 0.4). After recovery, Ppa returned to baseline levels (BL3) and was not significantly different for BL3, Sal2, or BL4 between groups. In contrast, after RW bronchoprovocation, Ppa increased markedly in RW-sensitized dogs (49%, P = 0.001) compared with BL4 values and was significantly larger than for Con dogs (P = 0.03), and Ppa in Con dogs did not change compared with BL4 levels.

As illustrated in Fig. 3B, PVRI in Con dogs is unchanged during Hist and RW bronchoprovocations. In the RW-sensitized dogs, PVRI increased markedly after Hist bronchoprovocation (38%, P = 0.01) compared with BL2 data and was significantly increased compared with Con dogs (P = 0.04). The PVRI also increased markedly (62%, P = 0.03) in RW-sensitized dogs after RW bronchoprovocation compared with BL4 data and was significantly increased compared with Con dogs (P = 0.02).

MAP (Fig. 4A) in Con or RW groups was not different between groups for BL1, Sal1, and BL2. The MAP was lower in RW-sensitized dogs than in Con dogs but was not significantly lower (P = 0.08) at baseline (BL1); however, it decreased further after five breaths of saline (Sal1) in RW-sensitized dogs and reached significance (P = 0.04) but recovered spontaneously (BL2). Hist bronchoprovocation significantly decreased MAP compared with BL2 data in Con (13%, P = 0.005) and RW groups (32%, P = 0.02) and within groups, and these decreases were significantly different (P = 0.049) between groups. After spontaneous recovery in each group, BL3 data were not different within or between groups compared with BL1 and BL2 data. After RW bronchoprovocation, MAP decreased (21%, P = 0.051) in RW-sensitized dogs compared with BL4 data and was significantly different (P = 0.02) between groups.

As illustrated in Fig. 4B, SVRI decreased markedly after Hist bronchoprovocation in both groups and was not significantly different between groups. This was associated with a decrease in MAP (Fig. 4A) in both groups but an increase in CI (Fig. 2A) in only Con dogs. On the other hand, the RW bronchoprovocation resulted in a large decrease in SVRI in RW-sensitized dogs (44%, P = 0.006) compared with BL4 data but did not change in Con dogs and approached significant differences (P = 0.06) between groups.

DISCUSSION

Our study indicates that RW-sensitized beagle dogs have increased airway and pulmonary vascular responses to Hist and RW bronchoprovocation. Although both Con and RW groups had similar changes in pulmonary function (i.e., G_sp and CL_sp) after Hist bronchoprovocation, only RW-sensitized dogs had pulmonary vasoconstrictive responses to RW inhalation. Thus pulmonary vasoconstrictive responses to RW bronchoprovocation appear to be independent of the changes in pulmonary functions in RW-sensitized dogs.

Pulmonary hypertension occurs in asthma patients (7, 8, 10, 12, 21, 25). In one report (25), two children with congenital heart defects and asthma were found to have unexplained pulmonary hypertension on cardiac catheterization. Ppa decreased after aggressive asthma treatment. Thus alterations in pulmonary vascular responses may be an important component of asthmatic attacks.

We (23) and others (5, 9) have demonstrated that repeated RW injections in beagle puppies beginning on the first day of life sensitize them to RW, as illustrated by positive skin tests, anti-RW immunoglobulin (Ig) G and IgE, and airway hyperresponsiveness to RW bronchoprovocation. In our present studies, we used an intact-chest preparation to study the cardiopulmonary interactions induced in vivo by Hist or RW bronchoprovocation in RW-sensitized dogs. Our previous data (23) suggest that the airways of both Con and RW-sensitized beagle dogs respond to Hist (5 breaths of 30 mg/ml) with bronchoconstriction. Because we observed decreases in G_sp and CL_sp in both control and RW-sensitized dogs after Hist bronchoprovocation, we believe that the changes in PVRI and SVRI indicate changes in pulmonary vascular smooth muscle responses in RW-sensitized dogs.

We are not aware of previous studies of the effects of aerosolized Hist or RW on pulmonary vascular responses. In studies by others (1, 3, 16, 24, 26) intravenous Hist increases pulmonary vascular resistance by small- and large-vein constriction in isolated blood-perfused dog lungs. This response is thought to be mediated via H₁ receptors on the vascular smooth muscle. Hist may also have pulmonary artery vasodilator actions due to stimulation of H₂ receptors if pulmonary vascular tone is elevated (2). In addition, there are H₁ receptors located on the vascular endothelium, which, when stimulated, may lead to the release of other mediators, such as nitric oxide. After aerosolized Hist, RW-sensitized dogs increased PVRI, presumably due to stimulation of H₁ receptors. Thus we speculate that Hist or other mediators released from the lungs, as well as absorbed across the alveolar surface, could have contributed to the effects of aerosolized Hist on the pulmonary circulation in our studies.

We also noted that only RW-sensitized dogs increased PVRI after RW inhalation, suggesting that RW-sensitized dogs had pulmonary vascular smooth muscle hyperresponsiveness compared with Con dogs. It is difficult to compare our in vivo data with other studies (26, 27) where blood flow is held constant. Increased CI in Con dogs could result from decreased SVRI and the release of catacholamines, with subsequent increase in HR. We think this may have hidden a small increase in PVRI, because CO and PVR are inversely related when vascular tone is constant (30). Despite large changes in SVRI and HR in both groups, RW-sensitized dogs had no change in CI. Thus the increase in PVRI reflects an intrinsic increase in pulmonary vascular tone in RW-sensitized dogs that is clearly not present in Con.

We anticipated changes in pulmonary functions and arterial blood gases to Hist. Therefore, we took precautions to minimize their effects on PVRI. We gave supplemental O₂ throughout the bronchoprovocation studies and continuously monitored O₂ saturation and end-tidal CO₂. Although we observed significant changes in blood gases with Hist bronchoprovocation in both groups and with RW in the RW-sensitized dogs, we feel the differences were not clinically important. Therefore, we do not think changes in Pa_O2 or Pa_CO2 contributed to the pulmonary vasoconstrictor response we observed. Furthermore, Hist bronchoprovocation led to similar changes in CL_sp and G_sp in both Con and RW-sensitized dogs. Despite this similarity, the Con dogs did not develop increased PVRI. Again, this finding supports our conclusion that RW sensitization altered the normal responsiveness of the pulmonary circulation to Hist and RW inhalation.

It has been well established that allergic bronchoprovocation results in the release of numerous mediators, including products of arachidonic acid metabolism (prostaglandins, thromboxanes, and leukotrienes) and Hist (4, 6, 17, 18). These substances are vasoactive, with predominantly pulmonary vasoconstrictor and systemic vasodilator properties. In our RW-sensitized dogs, the RW bronchoprovocation resulted in airway and pulmonary vascular changes that were similar to those observed after Hist. Therefore, our data support the conclusion that Hist release after RW bronchoprovocation may have contributed, at least in part, to our findings. Endothelin-1 (15) also may be a significant mediator in human asthma. Because our RW-sensitized dogs exhibit several similarities to human asthma (i.e., anti-RW IgE and skin-test responses), we speculate that changes in endothelin-1 may also play a role in our model.

In summary, our results indicate that RW sensitization is associated with increased responsiveness of the pulmonary vasculature to RW bronchoprovocation in beagle dogs. Increases in pulmonary vascular resistance were observed after nonallergic (Hist) and allergic (RW) bronchoprovocation in RW-sensitized dogs and were independent of acute changes in pulmonary functions or arterial blood gases. We speculate that hyperresponsivness of the pulmonary vasculature may be an important mechanism in severe asthmatic attacks in humans.

FOOTNOTES

Address for reprint requests: A. Theodorou, Pediatrics/3302, 1501 N. Campbell Ave., PO Box 245073, Tucson, AZ 85724-5073.

Received 17 June 1996; accepted in final form 29 April 1997.

REFERENCES