Endothelium-mediated and direct actions of acetylcholine on rabbit aortic baroreceptors

Abstract

Effects of exogenous acetylcholine on single-unit aortic baroreceptors were studied using a rabbit in vitro aortic arch/aortic nerve preparation. The arch was perfused with Krebs-Henseleit solution at constant pressure while simultaneously recording arch diameter and baroreceptor discharge frequency. Administration of acetylcholine over a wide range of concentrations (10-9 to 10-4 mol/L), evoked multiple dose-dependent changes in baroreceptor activity. 'Low' concentrations (<10-6 mol/L) consistently dilated arch and increased baroreceptor discharge (n=11). These responses were prevented by prerelaxing smooth muscle with sodium nitroprusside (10-7 mol/L, n=6) and were augmented by precontracting smooth muscle with norepinephrine (0.5 x 10-7 mol/L, n=11). Thus, the baroreceptor responses were dependent quantitatively on the level of preexisting vasoactive tone. The responses also were antagonized by atropine (10-7 mol/L, n=6), which inhibits release of endothelium-derived relaxing factor (EDRF), and by N-monomethyl-L-arginine (10-5 mol/L, n=6), a specific blocker of nitric oxide formation (primary relaxing factor responsible for acetylcholine vasorelaxation). Hexamethonium was ineffective (n=6). These results indicate that 'low-dose' responses to acetylcholine were mediated solely by release of EDRF. In contrast, 'high' concentrations of acetylcholine (>10-6 mol/L) constricted arch and reduced baroreceptor discharge in most units (66%). These responses were blocked by atropine, but not by hexamethonium, and were attributed to direct contraction of the smooth muscle. In the remaining baroreceptors (33%), discharge increased irrespective of changes in diameter or preexisting vasoactive tone, but not in the presence of hexamethonium. Therefore, some baroreceptors were stimulated chemically by acetylcholine (or an intermediary substance), probably through activation of nicotinic receptors on the sensory endings. The effects of acetylcholine on the baroreceptor pressure-frequency curve also were examined by injecting slow pressure ramps before and during acetylcholine treatment. Low concentrations that relaxed smooth muscle shifted the curve to lower pressures, high concentrations that contracted smooth muscle shifted the curve to higher pressures. Last, when acetylcholine was given with the diameter rather than the pressure held constant, the baroreceptor reciprocal responses to smooth muscle relaxation and contraction were directionally reversed but remained consistent with the changes in wall tension. In summary, acetylcholine altered baroreceptor discharge through three mechanisms: (1) endothelium-mediated vasorelaxation, which either increased or decreased discharge depending on the directional change in wall tension, (2) direct smooth muscle contraction, which had the opposite effect as relaxation, but only at high concentrations, and (3) chemical activation, which occurred in a minority of baroreceptors and also only at high concentrations.