Ligands of Alpha‐1 Adrenergic Receptors: Common Signaling Pathway, but Different Sensitivity to Nitric Oxide in Sepsis

Abstract

We have shown that arteries present differential responses to α1 adrenergic receptors (α1ARs) agonists in sepsis (Bernardelli et al., Can J Physiol Pharmacol, 94, 1227–36, 2016). Indeed, in animals subjected to the cecal ligation and perforation (CLP) model of sepsis, activation of α1ARs by norepinephrine (NE) results in a small degree of hyporesponsiveness, compared with the responses generated by phenylephrine (PE). It is well known that increased amounts of nitric oxide (NO) are produced in sepsis. NO plays a crucial role in septic vasoplegia, so we hypothesized that α1ARs agonists could differentially modulate the activity of NO synthases (NOS). Here, endothelium‐intact aortic rings from male Wistar rats subjected to CLP were studied in organ baths at 6 h after the septic insult. Control experiments were conducted with arteries from naïve animals. The effects of increasing concentrations (1 nM–300 mM) of PE and NE were evaluated in preparations previously incubated with L‐NAME (a non‐selective inhibitor of NOS; 100 μM), 1400W (a selective inhibitor of the inducible isoform of NOS; 10 μM), or S‐Methyl‐L‐thiocitrulline (a selective inhibitor of the neuronal isoform of NOS; 10 μM). We also investigated the influence of sodium nitroprusside (SNP; 0.1 μM, during 30 min) in the responses of control arteries to PE and NE. Aortic sections were also evaluated in experiments using the fluorescent BODIPY® FL prazosin (QAPB) (250 nM) and DAF‐FM (10 μM). Incubation with L‐NAME increased the contractile effects of PE and NE in both the control and CLP group. On the other hand, both 1400W and S‐methyl‐L‐thiocitrulline had no influence in the effects of PE and NE in control preparations, and increased the responses to PE (but not to NE) from 1.31 ± 0.06 g in control to 2.24 ± 0.11 and 2.21 ± 0.14 g, respectively, in the CLP group (p < 0.05). The fluorescence of QAPB in aortic sections did not differ between the groups, regardless of the previous incubation with PE or NE, suggesting that there were no significant differences in the density of α1ARs between the groups. Besides, neither PE nor NE were able to increase NO production, as detected by the NO sensitive fluorescence probe DAF‐FM. Interestingly, incubation with SNP reduced the maximal contractile effect of NE and PE by 35 and 65%, respectively (p < 0.05). These results suggest that neither an impaired interaction of PE and NE with α1ARs nor a differential modulation of NO production account for the distinct degree of vascular hyporeactivity observed. Nevertheless, our findings reveal remarkable differences between the intracellular signaling activated by PE and NE, regarding their sensitivity to NO. The better comprehension of the behavior of α1ARs can allow the development of ligands with enhanced efficacy for the management of vasoplegia and hypotension in sepsis. Support or Funding Information Financial Support: CNPq (Brazil), with a Ph.D. fellowship to Bernardelli, A. K. This study was approved by the Animal Ethics Committee of UFSC (6327180718).