Acute hypoxia modifies cAMP levels induced by inhibitors of phosphodiesterase-4 in rat carotid bodies, carotid arteries and superior cervical ganglia

Abstract

Background and purpose: Phosphodiesterase (PDE) inhibitors are useful to treat hypoxia-related diseases and are used in experiments studying the effects of oxygen on 3'-5'-cyclic adenosine monophosphate (cAMP) production. We studied the effects of acute hypoxia on cAMP accumulation induced by PDE inhibitors in oxygen-specific chemosensors, the carotid bodies (CBs) and in non-chemosensitive CB-related structures: carotid arteries (CAs) and superior cervical ganglia (SCG).

Experimental approach: Concentration-response curves for the effects of a non-specific PDE inhibitor [isobutylmethylxanthine (IBMX) ], PDE4 selective inhibitors (rolipram, Ro 20-1724) and a PDE2 selective inhibitor (erythro-9-(2-hydroxy-3-nonyl)adenine) on cAMP levels were obtained in normoxic (20% O(2)/5% CO(2)) or hypoxic (5% O(2)/5% CO(2)) conditions.

Key results: Responses to the PDE inhibitors were compatible with the presence of PDE4 in rat CBs, CAs and SCG but in the absence of PDE2 in CAs and CBs. Acute hypoxia enhanced the effects of IBMX and PDE4 inhibitors on cAMP accumulation in CAs and CBs. In SCG, acute hypoxia reduced cAMP accumulation induced by all the four PDE inhibitors tested. Differences between the effects of Ro 20-1724 and rolipram on cAMP were found in CAs and CBs during hypoxia.

Conclusions and implications: The effects of PDE4 inhibitors could be potentiated or inhibited by acute hypoxia depending on the PDE isoforms of the tissue. The similarities between the characterization of PDE4 inhibitors at the CBs and CAs, under normoxia and hypoxia, did not support a specific role for cAMP in the oxygen-sensing machinery at the CB and suggested that no direct CB-mediated, hyperventilatory, adverse effects would be expected with administration of PDE4 inhibitors.

Figure 1

Effect of different pre-incubation conditions on cAMP levels of carotid body (CB, n= 6–9), superior cervical ganglia (SCG, n= 2–5) and carotid artery (CA, n= 4–5) in response to hypoxia (5% O₂/5% CO₂) and in the presence of isobutylmethylxanthine (500 µM). Tissues were pre-incubated in (A) hyperoxia (95% O₂/5% CO₂) or (B) normoxia (20% O₂/5% CO₂). P > 0.05.

Figure 2

Effects of hypoxia on cAMP levels (expressed as pmol/mg tissue) in rat carotid bodies (CBs, n= 15), superior cervical ganglia (SCG, n= 11) and carotid arteries (CAs, n= 11–12) in the absence of PDE inhibitors. Data represent means ± SEM ***P < 0.001 Mann–Whitney non-parametric test, corresponding to the differences between normoxia and hypoxia.

Figure 3

Effects of PDE4 inhibitors on cAMP levels in rat carotid bodies (CBs). Concentration–response curves of (A) rolipram, (B) Ro 20-1724 and (C) isobutylmethylxanthine (IBMX) for effects on cAMP levels induced during normoxia (20%O₂) [0% effect − 0.75 ± 0.08 pmol cAMP/mg (n= 15) ] and hypoxia (5%O₂) (0% effect − 0.63 ± 0.05 pmol cAMP/mg (n= 15). *P < 0.05 and ***P < 0.001 two-way analysis of variance with Bonferroni's post hoc test. Data values represent means ± SEM.

Figure 4

Effects of PDE4 inhibitors on cAMP levels in rat carotid arteries (CAs) incubated in normoxia [20%O₂; 0% of effect corresponds to 0.19 ± 0.02 pmol·mg⁻¹ (n= 12) ] and hypoxia [5%O₂; 0% of effect corresponds to 0.11 ± 0.01 pmol·mg⁻¹ (n= 11) ]. Concentration–response curve of (A) rolipram, (B) Ro 20-1724 and (C) isobutylmethylxanthine (IBMX) for effects on cAMP levels. **P < 0.01 and ***P < 0.001 two-way analysis of variance with Bonferroni's post hoc test. Values represent means ± SEM.

Figure 5

Concentration–response curves for the effects of (A) rolipram, (B) Ro 20-1724, (C) isobutylmethylxanthine (IBMX) and (D) erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) on cAMP levels in rat superior cervical ganglia in normoxia (20%O₂) [0% effect − pmol 0.36 ± 0.03 pmol cAMP/mg tissue (n= 11) ] and in hypoxia (5%O₂) [0% effect − 0.35 ± 0.04 pmol·cAMP·mg⁻¹ tissue (n= 11) ]. *P < 0.05, **P < 0.01 and ***P < 0.001. Data values represent means ± SEM.