Smooth muscle tone and rapid resetting of rat aortic baroreceptors

Changes in conditioning mean arterial pressure (cMAP) selectively alter the set point of arterial baroreceptors and baroreflexes without affecting gain. Changes in smooth muscle tone at constant cMAPs shift the pressure-discharge curves of aortic baroreceptors in a similar manner. Using an in vitro preparation of the rat aortic arch, we tested whether near maximal changes in smooth muscle tone affect rapid resetting in single regularly discharging aortic baroreceptors. Discharge, pressure, and aortic diameter were simultaneously measured. By using vasoactive drugs (phenylephrine, angiotensin II, Bay K 8644, and nitroprusside), rapid resetting to cMAP changes was tested during different smooth muscle tone conditions (control, constricted, and dilated). Baroreceptor discharge-response curves were periodically assessed with slow ramps of increasing pressure at each of three different cMAP levels (10-15 minutes each). Rapid resetting relations were constructed for pressure threshold (Pth) and diameter threshold (Dth) plotted against cMAP and conditioning diameter (cD), respectively. Vasoconstriction decreased Pth in all baroreceptors (n=13, p<0.05). Baroreceptor resetting ability as indicated by the slopes of the resetting relations (pressure- or diameter-resetting ratios, ΔPth/ΔcMAP or ΔDth/ΔcD, respectively) was unaffected by large increases in smooth muscle tone (p>0.12). Vasoconstriction, however, offset the pressure-resetting relation, shifting the linear relation in a parallel manner to higher Pth values. In contrast, Dth values during vasoconstriction were not offset but instead fell along a single diameter-resetting relation coincident with the control relation for each baroreceptor. This last result suggests that acute alteration of vessel mechanics by vasoconstriction does not alter the basic rapid resetting process. These new results reinforce the notion that changes in the prevailing degree of baroreceptor distortion are the direct stimulus for a rapid resetting and that modulation of distortion threshold and baroreceptor performance are similar whether conditioning is produced passively by changes in cMAP or actively by altering local smooth muscle tone.