Class of 2012 Abstract / Specific Aims: We sought to determine the influence of genetic variation of ADRB2 on the airway response to albuterol in patients with CF when compared to matched healthy controls at baseline and at 60 minutes following the administration of albuterol (2.5mg diluted in 3ml normal saline). Methods: Baseline pulmonary function (forced vital capacity, FVC, forced expiratory flow in 1-second, FEV1, mid-maximal expiratory flow, MMF, and forced expiratory flow at 50% of the FVC) was assessed in 17 patients with CF and 31 healthy subjects. Main Results: As expected, the healthy group had higher baseline pulmonary function when compared to the CF group (FVC=97±3 vs. 83±5; FEV1=95±3 vs. 72±6; MMF=90±4 vs. 54±8, % predicted for healthy and CF, respectively, mean±SE, p<0.05 for all. We compared Arg16Arg to Arg16Gly/Gly16Gly subjects. There was no effect of genotype on the response to albuterol in healthy subjects. However, in the CF group, we found that the Arg16Arg group (n=6) had an attenuated response to β-agonist when compared to the Gly-containing group (n=11) (FVC=0±0.9 vs. 6±3: FEV1=3±1 vs. 7±4: MMF=12±3 vs. 12±5 % change, for Arg16Arg and Gly-containing groups, respectively, p<0.05 for FVC, p=0.06 for FEV1). Conclusions: These results demonstrate a differential response to β-agonists according to genetic variation of the ADRB2 at amino acid 16. Due to the differences in FVC and FEV1 but not in MMF, these data suggest that the genetic difference in airway function is primarily in bronchodilation of the larger airways.

Airway hyperresponsiveness (AHR) is a hallmark of asthma where constriction of airway smooth muscle (ASM) causes excessive airway narrowing. Asthmatics, unlike healthy subjects, cannot prevent or reverse this narrowing by stretching their airways with a deep inspiration (DI). Since stretching of isolated ASM causes dramatic reductions in force generation and asthmatics tend to have stiffer airways, researchers hypothesize that reduced ASM stretching during breathing and DIs results in hyperreactive airways. However, counterintuitively, excised measurement on intact airways show narrowing is minimally reversed by pressure oscillations simulating breathing and DIs. We hypothesized that AHR does not result from reduced capacity to stretch the airways; furthermore, each constituent of the airway wall experiences different strain magnitude during breathing and DIs. To test this, we used an intact airway system which controls transmural pressure (Ptm) to simulate breathing while measuring luminal diameter in response to ASM agonists. An ultrasound system and automated segmentation algorithm were implemented to quantify and compare the ability of Ptm fluctuations to reverse and prevent narrowing in larger (diameter=5.72±0.52mm) relative to smaller airways (diameter=2.92±0.29mm). We found the ability of Ptm oscillations to reverse airway narrowing was proportional to strain imposed on the airway wall. Further, tidal-like breathing Ptm oscillations (5-15cmH2O) after constriction imposed 196% more strain in smaller compared to larger airways (14.6% vs. 5.58%), resulting in 76% greater reversal of narrowing (41.2% vs. 23.4%). However, Ptm oscillations applied before and during constriction resulted in the same steady-state diameter as when Ptm oscillations were applied only after constriction. To better understand these results, we optimized an ultrasound elastography technique utilizing finite element-based image registration to estimate spatial distributions of displacements, strains, and material properties throughout an airway wall during breathing and bronchoconstriction. This required we formulate and solve an inverse elasticity problem to reconstruct the distribution of nonlinear material properties. Strains and material properties were radially and longitudinally heterogeneous, and patterns and magnitudes changed significantly after induced narrowing. Taken together, these data show AHR likely does not emerge due to reduced straining of airways prior to challenge, but remodeling that stiffens airway walls might serve to sustain constriction during an asthmatic-like attack.