EPAC signalling pathways are involved in low P_O_2 chemoreception in carotid body chemoreceptor cells

摘要

Chemoreceptor cells of the carotid bodies (CB) are activated by hypoxia and acidosis, responding with an increase in their rate of neurotransmitter release, which in turn increases the electrical activity in the carotid sinus nerve and evokes a homeostatic hyperventilation. Studies in isolated chemoreceptor cells have shown that moderate hypoxias (P_O_2 = 46 mmHg) produces smaller depolarisations and comparable Ca~(2+) transients but a much higher catecholamine (CA) release response in intact CBs than intense acidic/hypercapnic stimuli (20% CO_2, pH 6.6). Similarly, intense hypoxia (P_O_2 = 20 mmHg) produces smaller depolarizations and Ca~(2+) transients in isolated chemoreceptor cells but a higher CA release response in intact CBs than a pure depolarizing stimulus (30-35 him external K~+). Studying the mechanisms responsible for these differences we have found the following. (1) Acidic hypercapnia inhibited I_Ca (~60%; whole cell) and CA release (~45%; intact CB) elicited by ionomycin and high K~+. (2) Adenylate cydase inhibition (SQ-22536; 80 mum) inhibited the hypoxic release response (>50%) and did not affect acidic/hypercapnic release, evidencing that the high gain of hypoxia to elicit neurotransmitter release is cAMP dependent. (3) The last effect was independent of PKA activation, as three kinase inhibitors (H-89, KT 5720 and Rp-cAMP; > 10 x IC_50) did not alter the hypoxic release response. (4) The Epac (exchange protein activated by cAMP) activator (8-pCPT-2'-O-Me-cAMP, 100mum) reversed the effects of the cydase inhibitor. (5) The Epac inhibitor brefeldin A (100 mum.) inhibited (54%) hypoxic induced release. Our findings show for the first time that an Epac-mediated pathway mediates O_2 sensing/transduction in chemoreceptor cells.