B-PO02-020 PDE4D MEDIATES DESENSITIZATION OF Β-ADRENERGIC RECEPTORS IN THE SINOATRIAL NODE IN HYPERTENSIVE HEART DISEASE

Abstract

Chronotropic incompetence is common in hypertensive heart disease due to impaired β-adrenergic receptor (β-AR) responsiveness of the sinoatrial node (SAN). To determine the mechanism for impaired β-AR signaling in the SAN of angiotensin II (AngII) induced hypertensive heart disease. C57BL/6 mice were infused with saline or AngII (2.5mg/kg/day) for 21 days to induce hypertensive heart disease. Heart rate (HR) and SAN function in response to the β-AR agonist isoproterenol (ISO) were studied by in vivo electrocardiography, in isolated atrial preparations by optical mapping, in isolated SAN myocytes by patch-clamping, and by molecular techniques. AngII infused mice had impaired HR increases in response to ISO (100mg/kg) in vivo (171.1±7.1 vs. 134±9.8 bpm; n=5-6 mice; P<0.05) and in isolated atrial preparations (10μM ISO; 160.4±9.7 vs. 96.6±7.8, n=5-6 hearts; P<0.05). ISO (10μM) induced increases in SAN conduction velocity were also smaller in AngII infused hearts (1.6±0.1 vs. 1.1±0.1 cm/s, n=5-6 hearts; P<0.05). The effects of ISO (10μM) in isolated SAN myocytes after AngII infusion were impaired as indicated by smaller increases in spontaneous action potential (AP) firing (54.5±4.6 vs. 28.3±2.7 AP/min; P<0.001), diastolic depolarization (DD) slope (33.2±3.8 vs. 19.0±3.5 mV/s; P<0.05), peak “funny” current (-3.7±0.2 vs. -1.8±0.1 pA/pF; P<0.05), and peak L-type Ca2+ current (-6.9±1.3 vs. -3.5±0.7 pA/pF; P<0.05; n=7-15 cells). SAN myocytes from AngII infused mice had diminished cAMP generation in response to ISO (126.5±4.7 vs. 103.7±5.3 nM/1000 cells; P<0.05). No differences were seen in the protein expression of β1- or β2-ARs (both n=6; P>0.05); however, AngII infused mice had increased SAN PDE4 activity (1.4±0.3 vs. 3.9±0.9 pmol/mg/min, n=5; P<0.05). Total PDE4 expression was unaltered in the SAN of AngII infused mice but localization of PDE4D surrounding both β1- and β2-ARs was increased (n=6; P<0.05). Acute application of the PDE4 inhibitor rolipram (10μM) normalized ISO (10nM) induced increases in AP firing and DD slope in SAN myocytes (n=8-10) from AngII infused mice. AngII infusion results in blunted HR responses to β-AR stimulation due to upregulation of PDE4D specifically around β-ARs in the SAN.