I RECETTORI DELL'ANGIOTENSINA II NEL TESSUTO DI CONDUZIONE CARDIACO

Angiotensin II exerts a positive chronotropic effect on mammalian heart. This effect can be mediated either by activation of the sympathetic nervous system or binding to specific receptors located in cardiac conduction tissue. Recently, two angiotensin II receptor subtypes have been identified in rat tissues. In the present study, we have investigated tissue distribution of angiotensin II receptor subtypes in conduction tissue obtained from rat hearts, employing an in situ autoradiographic technique. Receptor subtypes were identified by use of the peptide antagonists losartan (AT₁) and PD123177 (AT₂). Angiotensin II binding was homogeneously distributed in right and left ventricle, and interventricular septum, with each subtype accounting for approximately 50% of the specific binding. Receptor density in the conduction tissue, identified by acetylcholinesterase histochemistry, was quantitated by counting the silver grains on tissue sections which, following incubation, were dipped in photographic emulsion and developed. In both senoatrial and atrioventricular node AT₁ and AT₂ receptors were homogeneously distributed. Receptor density in senoatrial node was comparable to myocardium, whereas it was significantly higher (p<0.02) in atrioventricular node. In conclusion, both AT₁ and AT₂ receptors are homogeneously distributed in cardiac conduction tissue. Binding density is higher in the atrioventricular node than in the other structures examined in our study, suggesting a possible role of angiotensin II in the atrioventricular conduction.