Effects of changes in extracellular ionic concentrations on aortic baroreceptors with nonmyelinated afferent fibers

The effects of ionic changes upon one of the two classes of aortic baroreceptors, those having unmyelinated axons or C-fibers, have not been examined heretofore. Differences from results in aortic baroreceptors with myelinated axons might be expected because of differences in accessible surface area-volume relationships. Recordings were obtained using an in vitro aortic arch-aortic nerve preparation from 21 aortic C-fibers in 15 normotensive Wistar Kyoto rats (WKR) 13-17 weeks old. During a slow pressure ramp, the endings responded with an irregular discharge. However, if the discharge was averaged over 10-20 seconds, the average discharge correlated well with the pressure level in the arch. The maximal discharge increased with faster ramp rates but the threshold was not shifted over the range of ramp rates employed. The relationship between pressure and discharge was tested during alterations in the extracellular concentrations of Na, K, and Ca ([Na](o), [K](o), and [Ca](o)). A 12% decrease in [Na](o) clearly decreased the firing in the aortic C-fibers during a ramp stimulus. During a pressure step, the initial discharge was not decreased but the steady state discharge was clearly depressed. When the aortic arch was continuously at constant pressure, a decrease in Na of only 6% depressed the activity. Doubling [K](o) increased firing for a few minutes and was followed by a reduction of discharge. Previous reports have shown no effect on myelinated baroreceptor discharge by a 12% decrease in [Na](o), a change which significantly depressed firing in all C-fiber baroreceptors in this study. Thus, C-fiber baroreceptors are more sensitive to changes in extracellular ionic composition than are baroreceptors with myelinated axons.