Pacing Redistributes Glycogen within the Developing Myocardium

Ectopic pacing at physiological rate improves postanoxic recovery of the developing heart

“…As we stressed previously (24), the advantage of the embryonic heart model at the investigated stage is that it represents a relatively homogeneous population of cardiomyocytes (e.g., absence of vascularization, extrinsic neural regulation, fibroblasts, endothelium, and smooth muscle), beats spontaneously, reacts rapidly to anoxia-reoxygenation, and, most importantly, displays pacing-induced remodeling within only a few hours (36,42), instead of weeks or months, as observed in adult animal models (35,52). Moreover, access to the working heart and in vivo ectopic stimulation are easy and do not disturb embryonic development.…”

“…After a window was made in the shell, the heart of the chick embryo was paced over a 12-h period in ovo under controlled conditions of temperature (37°C) and humidity (70%) in a thermostabilized incubator and developed normally to stage 25HH. A stimulator (model 5880 A, Medtronic, Minneapolis, MN) and a platinum wire (0.2 mm diameter), isolated with a plastic sheath as a cathode, were used to induce pacing, which consisted of a discontinuous, asynchronous stimulation at the surface of the ventricular apex (5 min on-10 min off) at 110% of the intrinsic rate (i.e., 160-200 beats/min) with supraliminar intensities (2 times diastolic threshold), according to our recent study (24). Such a protocol allowed for 4 h of effective pacing, with visual testing of ventricular capture.…”

“…At the end of each experiment, hearts were carefully dissected into atria, ventricle, and conotruncus, which were stored at Ϫ20°C for subsequent biochemical determination, as described previously (24). Protein content was measured according to Lowry et al (22), with bovine serum albumin used as standard.…”

“…Moreover, access to the working heart and in vivo ectopic stimulation are easy and do not disturb embryonic development. Although there is no functional specialized conduction system in the embryonic heart before stage 33-36HH (13), the endogenous cardiac impulse propagates from the sinus venosus (51) and results in sequential activation of the atrium and ventricle (24,42), and the slow-conducting properties of the AV junction and its long-lasting contraction fulfill the valve function (27). We previously observed that pacing the heart at the apex of the ventricle results in the opposite pattern of activation and also induces reduction in end-diastolic volume, stroke volume, and cardiac output (24), similar to adult heart models of pacing.…”

“…Indeed, intermittent pacing over a 24-to 48-h period at 110% of the spontaneous rate leads to 1) a remodeling of the heart, with thickening of the atrial wall distant from the pacing site and thinning of the ventricular wall at the apex close to the pacing site (42), 2) a decrease of myocardial glycogen concentration in the regions remote from the pacing site (24), and 3) a decrease of the production of ROS during reoxygenation after 30 min of anoxia (23).…”