Sympathoexcitation by Bradykinin Involves Ca²⁺-Independent Protein Kinase C

Abstract

<jats:p>Bradykinin has long been known to excite sympathetic neurons via B<jats:sub>2</jats:sub>receptors, and this action is believed to be mediated by an inhibition of M-currents via phospholipase C and inositol trisphosphate-dependent increases in intracellular Ca<jats:sup>2+</jats:sup>. In primary cultures of rat superior cervical ganglion neurons, bradykinin caused an accumulation of inositol trisphosphate, an inhibition of M-currents, and a stimulation of action potential-mediated transmitter release. Blockade of inositol trisphosphate-dependent signaling cascades failed to affect the bradykinin-induced release of noradrenaline, but prevented the peptide-induced inhibition of M-currents. In contrast, inhibition or downregulation of protein kinase C reduced the stimulation of transmitter release, but not the inhibition of M-currents, by bradykinin. In cultures of superior cervical ganglia, classical (α, βI, βII), novel (δ, ε), and atypical (ζ) protein kinase C isozymes were detected by immunoblotting. Bradykinin induced a translocation of Ca<jats:sup>2+</jats:sup>-independent protein kinase C isoforms (δ and ε) from the cytosol to the membrane of the neurons, but left the cellular distribution of other isoforms unchanged. This activation of Ca<jats:sup>2+</jats:sup>-independent protein kinase C enzymes was prevented by a phospholipase C inhibitor. The bradykinin-dependent stimulation of noradrenaline release was reduced by inhibitors of classical and novel protein kinase C isozymes, but not by an inhibitor selective for Ca<jats:sup>2+</jats:sup>-dependent isoforms. These results demonstrate that bradykinin B<jats:sub>2</jats:sub>receptors are linked to phospholipase C to simultaneously activate two signaling pathways: one mediates an inositol trisphosphate- and Ca<jats:sup>2+</jats:sup>-dependent inhibition of M-currents, the other one leads to an excitation of sympathetic neurons independently of changes in M-currents through an activation of Ca<jats:sup>2+</jats:sup>-insensitive protein kinase C.</jats:p>