|
Description:
|
Statement of the problem : Using a murine model of asthma , chronic beta blocker
treatment was found to improve airway hyperresponsiveness (AHR ) . This finding
completed a paradigm shift in the treatment of asthma using β adrenoceptor (AR ) ligands ,
which was an analogy of the paradigm shift that occurred in congestive heart failure .
Therefore , we hypothesized that chronic beta blocker treatment altered the cellular
signaling modulating airway smooth muscle tone .
Procedure or methods : Radioligand binding assays were used to measure β AR number
and further identify the subtype of β AR regulated by chronic beta blocker treatment . To
evaluate the effect of chronic nadolol treatment on Gs signaling pathways , cAMP
accumulation in the lung homogenates was measured by direct enzyme immunoassay ; the
cAMP -PKA dependent signaling was evaluated by isoproterenol induced tracheal
relaxation ; Gs , GRK2 , GRK3 , PDE4D and β arrestin 2 , important proteins regulating the
Gs signaling and playing important roles in lung physiology were measured by
immunoblotting . To evaluate the effect of chronic nadolol treatment on Gi and Gq
signaling pathways , Gαi3 , Gαi2 , Gq , GRK5 , PLCβ1 , PKCα were also measured by
immunoblotting . To evaluate the effect of chronic nadolol treatment on arachidonic acid
metabolism , cysteinyl leukotrines were measured in bronchoalveolar lavage fluid
(BALF ) , while cPLA2 , COX2 and 5 -LO were also measured by immunoblotting . In
addition , β2 AR knockout mice were treated with the same protocol as the wildtype mice
to confirm that the effects of chronic nadolol treatment on PDE4D , Gαi3 and PLCβ1 were
β2 AR dependent .
Results : Chronic beta blocker treatment increased β2 AR density and decreased PDE4D
expression . Although chronic beta blocker treatment showed no effect on cAMP -PKA
dependent airway smooth muscle relaxation , the regulation of Gi expression and Gi
signaling in airway smooth muscle were found to be altered with chronic nadolol
treatment . Also , chronic nadolol treatment decreased PLCβ1 expression and PKCα , 5 -
LO translocation . While a reduction in AHR , mucous metaplasia , and inflammatory cell
counts in bronchoalveolar lavage fluid (BALF ) was observed in wildtype mice , chronic
nadolol treatment had no effect on PDE4D , Gi3 and PLCβ1 expression in β2 AR null
mice .
Conclusion : Chronic nadolol treatment decreased airway tone in a murine model of
asthma by decreasing cAMP breakdown and Gi , Gq signaling . These effects are possibly
dependent on its chronic blockade of β2 AR . |