Non–Voltage-Gated Ca 2+ Influx Through Mechanosensitive Ion Channels in Aortic Baroreceptor Neurons

Abstract

Abstract The mechanisms underlying mechanotransduction in baroreceptor neurons (BRNs) are undefined. In this study, we specifically identified aortic baroreceptor neurons in primary neuronal cell cultures from nodose ganglia of rats. Aortic baroreceptor neurons were identified by labeling their soma with the fluorescent dye 1,1′-dioleyl-3,3,3′,3′-tetramethylindocarbocyanine (DiI) applied to the aortic arch. Using Ca 2+ imaging with fura 2, we examined these BRNs for evidence of Ca 2+ influx and determined its mechanosensitivity and voltage dependence. Mechanical stimuli were produced by ejecting buffer from a micropipette onto the cell surface with a pneumatic picopump, producing a shift in the center of mass of the cell that was related to intensity of stimulation. Ninety-three percent of DiI-labeled neurons responded to mechanical stimulation with an increase in [Ca 2+ ] i . The magnitude of the increases in [Ca 2+ ] i was directly related to the intensity of the stimulus and required the presence of external Ca 2+ . The trivalent cations Gd 3+ or La 3+ in equimolar concentrations (20 μmol/L) eliminated the K + -induced rises in [Ca 2+ ] i , demonstrating that both trivalent cations are equally effective at blocking voltage-gated Ca 2+ channels in these baroreceptor neurons. In contrast, the mechanically induced increases in [Ca 2+ ] i were blocked by Gd 3+ (20 μmol/L) only and not by La 3+ (20 μmol/L). Stretch-activated channels (SACs) have been shown in other preparations to be blocked by Gd 3+ specifically. Our data demonstrate that (1) BRNs, specifically identified as projecting to the aortic arch, have ion channels that are sensitive to mechanical stimuli; (2) mechanically induced Ca 2+ influx in these cells is mediated by a Gd 3+ -sensitive ion channel and not by voltage-gated Ca 2+ channels; (3) the magnitude of the Ca 2+ influx is dependent on the intensity of the stimulus and the degree and duration of deformation; and (4) repeated stimuli of the same intensity result in comparable increases in [Ca 2+ ] i . We conclude that mechanical stimulation increases Ca 2+ influx into aortic BRNs independent of voltage-gated Ca 2+ channels. The results suggest that Gd 3+ -sensitive SACs are the mechanoelectrical transducers in baroreceptors.