Role of vessel wall in acute resetting of aortic baroreceptors.

Previously we found that baroreceptor (BR) pressure-frequency (PF) curves rapidly (5 min) reset to changes in mean arterial pressure (MAP) exclusive of sympathetic efferents or circulating hormones. Since BRs are mechanically coupled to vessel wall structures, resetting may be due to changes in the wall. This was examined using an in vitro rat aortic arch-aortic nerve preparation and a photoelectric device to measure arch diameter. Pressure-diameter (PD) curves were constructed at selected MAPs following the resetting protocol used previously [Am. J. Physiol. 244 (Heart Circ. Physiol. 13): H672-H680, 1983]. Overall there was no consistent relationship between the PD curves and MAP, which was in sharp contrast to BR PF resetting. To determine if smooth muscle tone could affect aortic or BR function, the arch was constricted or relaxed with norepinephrine (NE) or sodium nitroprusside (NP), respectively. BR PF and aortic PD curves were constructed before and after drug treatment. NE reduced and NP increased discharge at given pressures. Diameter-frequency (DF) plots were not altered by either drug. PD and PF curves at constant MAP were stable after the initial responses to NP relaxation were completed. Subsequent MAP elevations in NP reset BRs to higher pressures, whereas PD curves shifted to lower pressures. Hence, DF plots shifted to higher diameters. These results show that neither smooth muscle activity nor mechanical changes in the whole wall can account for BR rapid resetting. The process probably occurs in the receptors.