Peptidergic modulation of mechanotransduction in rat arterial baroreceptors

Regularly discharging baroreceptors in a rat in vitro aortic arch preparation were exposed to increasing concentrations of one of four vasoactive peptides: angiotensin II, arginine vasopressin, atrial natriuretic factor, or substance P. Slow ramps of pressure evoked discharge responses in single-fiber baroreceptors. Instantaneous discharge frequency was measured simultaneously with aortic diameter and pressure. During constriction induced by angiotensin II or arginine vasopressin, baroreceptor diameter threshold (Dth) decreased and pressure threshold (Pth) tended to increase; these effects were reduced or eliminated by nitroprusside. Atrial natriuretic factor and substance P by themselves were without effect on vessel diameter or on baroreceptor discharge. In preparations preconstricted with a moderate concentration of phenylephrine (10^-8 M), atrial natriuretic factor reduced the phenylephrine-induced constriction and increased Dth and decreased Pth. Substance P, even at high concentrations, was less effective than atrial natriuretic factor in reducing phenylephrine constriction and in altering baroreceptor discharge. Baroreceptor gain was unaffected by any of these peptides. Thus, changes in smooth muscle tone altered mechanotransduction by shifts in 1) the vessel pressure-diameter relation and 2) baroreceptor threshold requirements (Pth and Dth). Changes in the baroreceptor mechanical threshold (Dth) reduced the effects on Pth expected from changes in vessel wall mechanics. Pth reflects the net effects of vessel wall and Dth changes. Pth generally increased during constrictions and decreased during dilations. The changes in Dth and their selectivity (no changes in gain) during vasoactive peptide action closely resemble rapid resetting of baroreceptors. We propose that vascular smooth muscle lies in a parallel arrangement with aortic baroreceptors and that a common compensatory mechanism regulates Dth during sustained changes in vessel diameter. Activation of smooth muscle and reductions in transmural pressure would reduce loading of baroreceptors, and the proposed compensatory mechanism would tend to keep discarge constant by decreasing Dth. Our experiments, however, cannot distinguish between hypotheses for local micromechanical changes in coupling or for changes modulating excitability within the baroreceptor neuron itself as the basis for Dth adjustments.