Bicarbonate-regulated soluble adenylyl cyclase (sAC) mRNA expression and activity in peripheral chemoreceptors

Peripheral arterial chemoreceptors in the carotid body (CB) are modulated by pH/CO₂. Soluble adenylyl cyclase (sAC) is directly stimulated by bicarbonate ions (HCO₃ ^-). Because CO₂/HCO ₃ ^- mediates depolarization in chemoreceptors, we hypothesized that sAC mRNA would be expressed in the CB, and its expression and function would be regulated by CO₂/HCO₃ ^-. Sprague-Dawley rats at postnatal days 16-17 were used to compare sAC mRNA gene expression between CB and non-chemosensitive tissues: superior cervical (SCG), petrosal (PG) and nodose ganglia (NG) by quantitative real time-PCR. Rat sAC gene expression was standardized to the expression of GAPDH (housekeeping gene) and the data were analyzed with the Pfaffl method. Gene and protein expression, and sAC regulation in the testis was used as a positive control. To determine the regulation of sAC mRNA expression and activity, all tissues were exposed to increasing concentrations of bicarbonate (0, 24, 44 mM, titrated with CO ₂ and maintained a constant pH of 7.40). RESULTS: sAC mRNA expression was between 2-11% of CB expression in the SCG, PG and NG. Furthermore, only in the CB did HCO₃ ^- upregulate sAC gene expression and increase cAMP levels. Conclusion: sAC mRNA and protein expression is present in peripheral arterial chemoreceptors and non-chemoreceptors. In the CB, CO ₂/HCO₃ ^- not only activated sAC but also regulated its expression, suggesting that sAC may be involved in the regulation of cAMP levels in response to hyper/hypocapnia.