BS-454195-1 ADRENERGIC MODULATION OF CONDUCTION DURING CARDIAC STRESS IS MEDIATED BY CALCIUM CHANNEL AUTOREGULATION OF INTERCALATED DISC NANODOMAINS

Abstract

During stress, adrenergic signaling enhances cardiac conduction, although the mechanisms are not fully understood. Reducing extracellular calcium in the heart widens an intercalated disc (ID) nanodomain adjacent to gap junctions known as the perinexus. Under acute cardiac stress, the perinexus adaptively remodels; preventing this response by experimentally changing calcium levels slows conduction and increases arrhythmia risk. This raises the question of whether myocytes autoregulate calcium-mediated ID adhesion to stabilize conduction during cardiac stress. The cardiac voltage gated calcium channel (Cav1.2) is a primary source of calcium influx and a key target during adrenergic signaling. Expression of Cav1.2 within the ID has not been described. To test the hypothesis that Cav1.2 is expressed in the ID and autoregulates myocyte adhesion and conduction in the adrenergic response to cardiac stress. Ex vivo guinea pig hearts were Langendorff-perfused and ventricular tissue harvested for microscopy. Cav1.2 cellular localization was quantified by spinning disk confocal microscopy (n=5), conduction velocity (CV) was measured by optical mapping (n=9), and ID ultrastructure was quantified from transmission electron micrographs (n=17). Cav1.2 function was modulated by perfusion with the β-adrenergic agonist isoproterenol (Iso, 0.6μM), the Cav1.2 agonist BayK8644 (BayK, 0.5μM), or the Cav1.2 antagonist verapamil (Ver, 2μM). Super resolution confocal microscopy demonstrated that 2.5% of total Cav1.2 signal co-occurred with connexin43 and N-cadherin. At baseline, CV slowed at shorter CLs (150 vs 300ms = 4% slower), consistent with CV restitution. Treatment with Iso increased both Wp (20.2 vs 17.0 nm at baseline) and CV (300ms: 113% of baseline). Iso reversed CV restitution such that CV increased by 4%. Activation of Cav1.2 by BayK widened Wp to the same degree as Iso (20.1 vs 20.2 nm). Verapamil prevented both the Iso-induced increase in Wp (17.0 nm) and CV (93.9% of baseline), and exacerbated CV restitution. A subpopulation of Cav1.2 was identified within the ID. Adrenergic stimulation widened perinexal width, increased conduction, and reversed CV restitution. Blocking Cav1.2 prevented the adrenergic-mediated change in perinexal structure and conduction. Thus, Cav1.2 function regulates ID ultrastructure and may serve as an adaptive response to enhance conduction during conditions of acute cardiac stress.