A HCO₃⁻-Dependent Mechanism Involving Soluble Adenylyl Cyclase for the Activation of Ca²⁺ Currents in Locus Coeruleus Neurons

Hypercapnic acidosis activates Ca ²⁺ channels and increases intracellular Ca ²⁺ levels in neurons of the locus coeruleus, a known chemosensitive region involved in respiratory control. We have also shown that large conductance Ca ²⁺ -activated K ⁺ channels, in conjunction with this pathway, limits the hypercapnic-induced increase in firing rate in locus coeruleus neurons. Here, we present evidence that the Ca ²⁺ current is activated by a HCO ₃ ⁻ -sensitive pathway. The increase in HCO ₃ ⁻ associated with hypercapnia activates HCO ₃ ⁻ -sensitive adenylyl cyclase (soluble adenylyl cyclase). This results in an increase in cyclic adenosine monophosphate levels and activation of Ca ²⁺ channels via cyclic adenosine monophosphate-activated protein kinase A. We also show the presence of soluble adenylyl cyclase in the cytoplasm of locus coeruleus neurons, and that the cyclic adenosine monophosphate analogue db-cyclic adenosine monophosphate increases Ca ²⁺ _i . Disrupting this pathway by decreasing HCO ₃ ⁻ levels during acidification or inhibiting either soluble adenylyl cyclase or protein kinase A, but not transmembrane adenylyl cyclase, can increase the magnitude of the firing rate response to hypercapnia in locus coeruleus neurons from older neonates to the same extent as inhibition of K ⁺ channels. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.