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Angiotensin AT₁ receptor-mediated excitation of rat carotid body chemoreceptor afferent activity

A. M. Allen

1. A high density of angiotensin II receptors was observed in the rat carotid body by in vitro autoradiography employing ¹²⁵I-[Sar¹,Ile⁸]-angiotensin II as radioligand. Displacement studies demonstrated that the receptors were of the AT₁ subtype.

2. The binding pattern indicated that the AT₁ receptors occurred over clumps of glomus cells, the principal chemoreceptor cell of the carotid body. Selective lesions of the sympathetic or afferent innervation of the carotid body had little effect on the density of receptor binding, demonstrating that the majority of AT₁ receptors were intrinsic to the glomus cells.

3. To determine the direct effect of angiotensin II on chemoreceptor function, without the confounding effects of the vasoconstrictor action of angiotensin II, carotid sinus nerve activity was recorded from the isolated carotid body in vitro. The carotid body was superfused with Tyrode solution saturated with carbogen (95 % O₂, 5 % CO₂), maintained at 36 °C, and multi-unit nerve activity recorded with a suction electrode.

4. Angiotensin II elicited a dose-dependent excitation of carotid sinus nerve activity (maximum increase of 36 ± 11 % with 10 nM angiotensin II) with a threshold concentration of 1 nM. The response was blocked by the addition of an AT₁ receptor antagonist, losartan (1 µM), but not by the addition of an AT₂ receptor antagonist, PD123319 (1 µM).

5. In approximately 50 % of experiments the excitation was preceded by an inhibition of activity (maximum decrease of 24 ± 8 % with 10 nM angiotensin II). This inhibitory response was markedly attenuated by losartan but not affected by PD123319.

6. These observations demonstrate that angiotensin II, acting through AT₁ receptors located on glomus cells in the carotid body, can directly alter carotid chemoreceptor afferent activity. This provides a means whereby humoral information about fluid and electrolyte homeostasis might influence control of cardiorespiratory function.

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