Abstract 15647: Myocardial Gradients in Caveolar Number Modulate Cardiomyocyte Contractile Response to Specific β2AR Stimulation
The ratio of β2 adrenergic receptors (β2AR) to β1adrenergic receptors has been reported to be higher in the apical myocardium. We investigated apical/basal differences in cardiomyocytes due to our interest in apical ballooning. Apical cardiomyocytes (ApCMs) isolated from the rat increased their contractility significantly more in response to β2AR stimulation than basal cardiomyocytes (BCMs). In normal circumstances this may give a balanced myocardial contraction. In healthy CMs the β2AR-cAMP response is normally stringently localized in sub-cellular domains such as t-tubules and caveolae. We assessed the effect of β2AR stimulation on cellular contractility using the IonOptixTM system. We utilized a FRET technique employing the EPAC2-camps sensor to investigate the relative role of the β2AR-cAMP response. A combination of scanning ion conductance microscopy (SICM) and FRET was used to investigate sub-cellular cAMP responses. Finally, transmission electron microscopy was utilized to quantify cellular caveolae number. The peak amplitude of cAMP transients upon β2AR stimulation was not different between the apex and base (ApCMs 4.00%±0.7 vs. BCMs 4.02±0.6 p=NS, unpaired t-test: %rawFRET). However, the β2AR-cAMP signal was more sustained in apical cardiomyocytes (ApCM 74.7 ± 11.6 n=10 vs. BCM 94.1 ± 1.8 n=11 p≤0.05: %removal 200s post max response). A diffuse β2AR-cAMP signal is observed in ApCMs but not BCMs using SICM FRET. Treatment of the cardiomyocytes with cyclodextrin which disrupts caveolae equilibrated the contractile response of the apex and base to β2AR stimulation. An electron microscopy study revealed that apical cardiomyocytes possess far fewer caveolae than their basal counterparts (ApCM 0.47 ± 0.03 vs BCM 4.28 ± 0.12 caveolae per µm, p≤0.001 n=3 animals 100 images per animal). These data suggest that the greater localization of β2AR in caveolae in BCMs leads to rapid cAMP hydrolysis and consequently less contractile response. The regional gradient in β2AR responses will result in increased positively inotropic support at the apical myocardium which has less stimulation via neural input. Physiologically, this may provide an evenly balanced contraction however in hyper adrenergic scenarios it may have pathological consequences.
- © 2012 by American Heart Association, Inc.