Differentiation of neurogenic and myocardial angiotensin II receptors in isolated rabbit atria

Abstract

The effect of angiotensin on the action of tyramine was studied in isolated rabbit left atria paced by point and field stimulation to more clearly define the interaction of angiotensin with the sympathetic nervous system. Administration of angiotensin resulted in similar increases in contractility in both point and field stimulated atria. In point stimulated preparations only the muscle is stimulated to contract, whereas in field stimulated preparations both nerve and muscle are stimulated. 1 Sar 8 Ala angiotensin II completely blocked the direct inotropic effect of angiotensin in a molar dose ratio of 3:1 in both point and field stimulated preparations. However, angiotensin (0.05-10 ng/ml) potentiated the inotropic effect of tyramine in field stimulated atria only. This facilitatory effect was not inhibited by 1 Sar 8 Ala angiotensin II at a molar dose ratio of 3:1; indeed, a ratio of 500:1 was necessary for complete blockade of this angiotensin induced potentiation. This antagonist in doses of 0.1-1000 ng/ml was without contractile effect in any preparation, regardless of whether tyramine was present. The data suggest the presence of a presynaptic angiotensin receptor that, in the presence of sympathetic nerve stimulation, modulates the release of norepinephrine and of a second angiotensin receptor in cardiac tissue that directly influences myocardial contractility.