RT-PCR and Pharmacological Analysis of L-and T-Type Calcium Channels in Rat Carotid Body

Abstract

Mechanisms involved in carotid body (CB) chemoreceptor cells O²-sensing and responses are not fully understood. So far, it is known that hypoxia depolarizes chemoreceptor cells via O²-sensitive K⁺-channel inhibition; calcium influx via voltage-gated channels and neurotransmitter secretion follow. Presence of high voltage activated (HVA) calcium channels in rat CB chemoreceptor cells is well documented, but the presence of low voltage activated (LVH) or T-type calcium channels has not been reported to date. The fact that O²-sensitive PC12 cells express T-type channels and that they are inducible by chronic hypoxia (CH) lead us to hypothesize they could be present and play a role in the genesis of the hypoxic response in rat CB chemoreceptor cells. We have analyzed the expression of the three isoforms of T-type calcium channels (α1G, α1H and α1I) and the isoforms α1C and α1D of L-type calcium channels in rat CB by RT-PCR. We found that rat CB expresses α1G and α1C subunits. After chronic hypoxic treatment of adult rats (10° O², 8 days), expression of α1G seems to be down-regulated whereas α1C expression is up-regulated. Functionally, it was found that the release of catecholamine induced by hypoxia and high external K^{+} from CB chemoreceptor cells was fully sensitive to L-type channel inhibition (nisoldipine, 2 μM), while specific inhibition of T-channels (mibefradil, 2 μM) inhibited exclusively hypoxia-induced release (50°). As a whole, present findings demonstrate the presence of T-type as well as L-type calcium channels in rat CB and suggest a selective participation of the T-type channels in the hypoxic activation of chemoreceptor cells.