ANGIOTENSIN-II RECEPTOR EXPRESSION IN THE CONDUCTION SYSTEM AND ARTERIAL DUCT OF NEONATAL AND ADULT-RAT HEARTS

Paralleling the classic circulating system, recent evidence has demons trated the presence of a cardiac renin-angiotensin system, as well as the synthesis of angiotensin II in the heart. Two receptors for angiot ensin II have been identified and classified as AT(1) and AT(2). The p roportions of these receptor subtypes vary with the tissues, species a nd stage of development, From the results of other studies, it might b e generalized that the expression of angiotensin II receptors and the proportion of AT(2) receptor subtype are much higher in fetal and neon atal tissues than in the same tissues from an adult, The aim of this s tudy was to specifically evaluate the AT(1)/AT(2) ratio in the neonata l and adult conduction systems of rat hearts by means of quantitative autoradiography, In the neonatal hearts, angiotensin II binding sites were highly concentrated in the vasculature, arterial duct, and conduc tion system, whereas their concentrations were barely detectable in th e myocardium, Incubation with selective angiotensin II receptor ligand s (losartan and CGP 42112) revealed that AT(2) was the major subtype i n vasculature (86 +/- 3%) and conduction system (73 +/- 4%), In the ad ult conduction system, the total expression of angiotensin II receptor s was greatly reduced meanwhile the AT(1) receptors represented the ma jor proportion of the binding sites (80 +/- 3%). Our results demonstra ted that the pattern of angiotensin II receptor expression in the cond uction system of the rat heart is developmentally regulated. We sugges t, as others have already, that the renin-angiotensin system plays a r ole during the early stage of cardiac development.