Differences in tyrosine hydroxylase expression after short-term hypoxia, hypercapnia or hypercapnic hypoxia in rat carotid body

In the carotid body (CB), it has been reported that the expressions of tyrosine hydroxylase (TH) mRNA and TH protein are enhanced by exposure to hypoxia. However, it is not known whether CO2 affects the expression of TH in the CB. We examined the expression of TH mRNA and the immunoreactivity for TH in the CB of rats exposed to hypoxia (10% O2), hypercapnia (10% CO2) and hypercapnic hypoxia (10% O2 and 10% CO2) for 2–24 h. The expression of TH mRNA in the CB was markedly enhanced in rats exposed to hypoxia for 4 h (6.6-fold), 6 h (6.0-fold) and 8 h (7.8-fold), and in rats exposed to hypercapnic hypoxia for 12 h (4.8-fold). The most intense TH immunoreactivity was observed in the CB from rats exposed to hypoxia for 12 and 24 h and to hypercapnic hypoxia for 24 h. The expressions of TH mRNA and the immunoreactivity for TH were not altered in the CB of rats exposed to hypercapnia. It is suggested that CO2 does not affect TH expression in the CB, and that it inhibits hypoxia-enhanced TH expression.