The molecular pharmacologic bases for the attenuated cardiovascular and metabolic responses to
catecholamines, after
burn injury, have not been elucidated. In the present study, myocardial tissues were used as a model of
beta-adrenergic receptors to study
burn injury-induced alterations in receptors and in signal transduction.
DESIGN: Prospective study, randomized to treatment and control groups.
SETTING: University-hospital research laboratory.
SUBJECTS: Male Sprague-Dawley rats (180 to 210 g).
INTERVENTIONS: Myocardial membranes were isolated at 24 hrs, 7 days and 14 days after 50% body surface area scald or
sham injury. (-)125I-iodocyanopindolol was used to assess maximal binding capacity and affinity of the
beta-adrenergic receptor. Basal and stimulated concentrations of second messenger, cyclic
adenosine monophosphate (cAMP), were also assessed. Production of cAMP during
isoproterenol stimulation tested the integrity of the
beta-adrenergic receptor-mediated signal transduction.
Forskolin, which stimulates
adenylate cyclase enzyme directly (bypassing the receptor and
G protein) to produce cAMP, tested the efficacy of the
enzyme itself. Maximal binding capacity was unaltered between the experimental and control groups, but the affinity (mean +/- SEM) was significantly decreased in burned animals at 7 days (125.4 +/- 15.5 picomoles [pmol]; p = .01) and at 14 days (216.7 +/- 50.7 pmol; p = .001) compared with controls (75.5 +/- 8.4 pmol). In different set experimental and control groups, basal concentrations of cAMP in myocardial membranes were significantly decreased in burned animals at 7 days (control 38.6 +/- 4.2 vs. 5.8 +/- 0.9 pmol/mg of
protein/min; p = .003) and at 14 days (control 47.4 +/- 3.2 vs 28.3 +/- 6.6 pmol/mg of
protein/min; p = .002). The
forskolin (direct)-stimulated synthesis of cAMP was decreased in burned animals at 24 hrs (control 339.0 +/- 40.5 vs. 214.4 +/- 16.6 pmol/mg of
protein/min; p = .01), at 7 days (control 289.0 +/- 34.4 vs. 32 +/- 13.0 pmol/mg of
protein/min; p = .01), and at 14 days (control 322.9 +/- 28.6 vs. 137.0 +/- 46.1 pmol/mg of
protein/min; p = .01). The
isoproterenol or receptor-mediated stimulation of cAMP production was also significantly (p < .001) impaired in burned animals compared with controls at 24 hrs (control 134.7 +/- 11.9 vs. 83.1 +/- 13.3 pmol/mg of
protein/min), and at 14 days (control 128.2 +/- 7.2 vs. 92.8 +/- 17.7 pmol/mg of
protein/min).
CONCLUSION: The etiology of the decreased responses in the myocardium to exogenous and endogenous
beta-adrenergic receptor agonists after
burn injury may be attributed to decreased affinity for
ligands, and also to impaired receptor-mediated signal transduction and to decreased
adenylate cyclase enzyme activity, resulting in decreased basal and stimulated second messenger (cAMP) production.