Abstract 161 Poster Session II, Sunday, 5/2 (poster 259)

Relatively little is known about the humoral factors that regulate heart rate at embryonic stages. To begin to define exogenous factors that influence cardiac physiology early in development, cultures of whole murine embryos and isolated embryonic hearts were studied from post conceptual (PC) ages 8.5 to 13. Based on what is known about humoral signals that influence cardiac function, the effects of adenosinergic, muscarinic and catecholaminergic activation were examined. At the inception of spontaneous cardiac contractility (PC 8.5), A1 adenosine receptor agonists (N6-cyclopentyl adenosine (CPA); doses 0.1 nM to 10 uM) potently slowed heart rates and induced asystole (IC50, 50 nM). In contrast, no changes in heart rate were observed after treatment with high concentrations of the beta-adrenergic agonist isoproterenol (100 uM) or the muscarinic receptor agonist bethanecol (100 uM). At PC 10, isoproterenol treatment resulted in modest increase in heart rate (120 % increase above baseline). At PC 12.5, isoproterenol effects were more pronounced (150% increase). However, no response to muscarinic agonist treatment was observed. When interactions of adenosine and adrenergic agonists were examined, CPA was able to override any isoproterenol-mediated increases in heart rate, whereas isoproterenol was much less effective in overcoming the effects of CPA. To complement functional studies, receptor G-protein coupling was examined by incubating tissue sections with [35S] GTP-gamma-S in the presence of A1AR agonists and antagonists. Autoradiographs suggested the presence of A1AR-G protein coupling. Overall, our observations indicate that the embryonic heart is responsive to humoral factors at very early stages of development. Furthermore, our observations identify the adenosinergic system as the most prominent regulator of embryonic cardiac function.

Funded by R01-HL58442