Effects of imidazoline and non-imidazoline α-adrenergic agents on rabbit platelet aggregation

Green and rapid and instrumental one-pot method for the synthesis of imidazolines having potential anti-SARS-CoV-2 main protease activity

The Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2) is responsible for ongoing epidemics in humans and some other mammals and has been declared a public health emergency of international concern. In this project, several small non-peptide molecules were synthesized to inhibit the major proteinase (M^pro) of SARS-CoV-2 using rational strategies of drug design and medicinal chemistry. M^pro is a key enzyme of coronaviruses and plays an essential role in mediating viral replication and transcription in human lung epithelial and stem cells, making it an attractive drug target for SARS-CoV. The antiviral potential of imidazoline derivatives as inhibitors of (SARS-CoV-2) M^pro was evaluated using in-silico techniques such as molecular docking simulation, molecular dynamics (MD), and ADMET prediction. The docking scores of these imidazoline derivatives were compared to that of the N3 crystal inhibitor and showed that most of these compounds, particularly compound E07, interacted satisfactorily in the active site of the coronavirus and strongly interacted with the residues (Met 165, Gln 166, Met 165, His 41, and Gln 189). Furthermore, the results were confirmed by MD simulations after exposure to long-term MD simulations and ADMET predictions.

A literature perspective on the pharmacological applications of yohimbine

Introduction: Phytochemicals have garnered much attention because they are useful in managing several human diseases. Yohimbine is one such phytochemical with significant pharmacological potential and could be exploited for research by medicinal chemists. It is an indole alkaloid obtained from various natural/synthetic sources.Aims and Results: The research on yohimbine started early, and its use as a stimulant and aphrodisiac by humans has been reported for a long time. The pharmacological activity of yohimbine is mediated by the combined action of the central and peripheral nervous systems. It selectively blocks the pre and postsynaptic α₂-adrenergic receptors and has a moderate affinity for α1 and α2 subtypes. Yohimbine also binds to other behaviourally relevant monoaminergic receptors in the following order: α-2 NE > 5HT-1A>, 5HT-1B > 1-D > D3 > D2 receptors.Conclusion: The current review highlights some significant findings that contribute to developing yohimbine-based drugs. It also highlights the therapeutic potential of yohimbine against selected human diseases. However, further research is recommended on the pharmacokinetics, molecular mechanisms, and drug safety requirements using well-designed randomized clinical trials to produce yohimbine as a pharmaceutical agent for human use.Key MessagesYohimbine is a natural indole alkaloid with significant pharmacological potential.Humans have used it as a stimulant and aphrodisiac from a relatively early time.It blocks the pre- and postsynaptic α2-adrenergic receptors that could be exploited for managing erectile dysfunction, myocardial dysfunction, inflammatory disorders, and cancer.

Effects of imidazoline and nonimidazoline α-adrenoceptor agonists and antagonists, including xylazine, medetomidine, dexmedetomidine, yohimbine, and atipamezole, on aggregation of feline platelets

OBJECTIVE

To examine the effects of imidazoline and nonimidazoline α-adrenergic agents on aggregation of feline platelets.

SAMPLE

Blood samples from 12 healthy adult cats.

PROCEDURES

In 7 experiments, the effects of 23 imidazoline and nonimidazoline α-adrenoceptor agonists or antagonists on aggregation and antiaggregation of feline platelets were determined via a turbidimetric method. Collagen and ADP were used to initiate aggregation.

RESULTS

Platelet aggregation was not induced by α-adrenoceptor agonists alone. Adrenaline and noradrenaline induced a dose-dependent potentiation of ADP- or collagen-induced aggregation. Oxymetazoline and xylometazoline also induced a small potentiation of ADP-stimulated aggregation, but other α-adrenoceptor agonists did not induce potentiation. The α₂-adrenoceptor antagonists and certain imidazoline α-adrenergic agents including phentolamine, yohimbine, atipamezole, clonidine, medetomidine, and dexmedetomidine inhibited adrenaline-potentiated aggregation induced by ADP or collagen in a dose-dependent manner. The imidazoline compound antazoline inhibited adrenaline-potentiated aggregation in a dose-dependent manner. Conversely, α₁-adrenoceptor antagonists and nonimidazoline α-adrenergic agents including xylazine and prazosin were ineffective or less effective for inhibiting adrenaline-potentiated aggregation. Moxonidine also was ineffective for inhibiting adrenaline-potentiated aggregation induced by collagen. Medetomidine and xylazine did not reverse the inhibitory effect of atipamezole and yohimbine on adrenaline-potentiated aggregation.

CONCLUSIONS AND CLINICAL RELEVANCE

Adrenaline-potentiated aggregation of feline platelets may be mediated by α₂-adrenoceptors, whereas imidazoline agents may inhibit in vitro platelet aggregation via imidazoline receptors. Imidazoline α-adrenergic agents may have clinical use for conditions in which there is platelet reactivity to adrenaline. Xylazine, medetomidine, and dexmedetomidine may be used clinically in cats with minimal concerns for adverse effects on platelet function.

Sympathetic stimulation facilitates thrombopoiesis by promoting megakaryocyte adhesion, migration, and proplatelet formation

NE and EPI promote megakaryocyte adhesion, migration, and proplatelet formation via α2-adrenoceptor-ERK1/2 signaling. Sympathetic stimulation enhances platelet production, which may facilitate recovery of thrombocytopenia or aggravate atherosclerosis.

Bidirectional effects of dexmedetomidine on human platelet functions in vitro

Platelets express the imidazoline (I)-receptor, I1 and I2, as well as the α2-adrenoceptor. Although dexmedetomidine, a selective α2-adrenoceptor agonist with some affinity for the I-receptor is expected to affect platelet function, the effects of dexmedetomidine on platelet functions remain unclear. In the present study, we investigated the effects of dexmedetomidine on human platelet functions in vitro. The effects of dexmedetomidine on platelet aggregation were examined using aggregometers. The formation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in platelets was measured by an enzyme immunoassay. In addition, P-selectin expression in platelets was estimated by flow cytometry. We showed that dexmedetomidine enhances platelet aggregation. But in the presence of yohimbine, an α2-antagonist, dexmedetomidine suppressed platelet aggregation. Efaroxan, an I1-antagonist, and methylene blue, a soluble guanylate cyclase inhibitor, abolished the suppressive effect of dexmedetomidine, whereas idazoxan, an I2-antagonist, showed no effect. Dexmedetomidine suppressed cAMP formation and enhanced P-selectin expression in platelets, and these effects were inhibited by yohimbine. Dexmedetomidine increased cGMP formation in platelets in the presence of yohimbine, and this increase was suppressed by efaroxan. These results demonstrated that dexmedetomidine has both enhancing and suppressive effects on human platelet functions through its action on the α2-adrenoceptor and on the I1-imidazoline receptor, respectively.

Identification and characterization of platelet α2-adrenoceptors and imidazoline receptors in rats, rabbits, cats, dogs, cattle, and horses

This study aimed to pharmacologically identify and characterize α2-adrenoceptors and imidazoline (I) receptors (I1- and I2-subtype) on canine, feline, bovine, equine, murine, and leporine platelet membranes. Saturation binding studies with both (3)H-yohimbine and (3)H-clonidine showed that α2-adrenoceptors were expressed on canine, leporine, feline, and murine platelets but not on bovine and equine platelets. In competition studies, the rank order of affinity of 6 compounds for canine platelet α2-adrenoceptors was similar to that of potency at α2A-subtype reported in human platelets. Saturation binding studies in the presence of norepinephrine showed that canine, feline, bovine, and equine platelets had I1-receptors defined by (3)H-clonidine binding, but neither murine nor leporine platelets had I1-receptors; whereas, platelets of all species had I2-receptors defined by (3)H-idazoxan binding. In competition studies, more potent compounds displayed biphasic competition curves with (3)H-clonidine. The rank orders of affinity of I1 compounds for high-affinity components of I1-receptors of canine, feline, bovine, and equine platelets and I2-receptors of all species platelets were similar to those of compounds for high-affinity components reported in human I1- and I2-receptors, respectively. Guanine nucleotides inhibited the high-affinity component of naphazoline binding to canine I1-receptors, but not to I2-receptors. Furthermore, guanine nucleotides dose-dependently inhibited (3)H-clonidine binding to I1-receptors; whereas, they did not interfere with (3)H-idazoxan binding to I2-receptors, supporting the notion that Il-receptors may belong to a G protein-coupled receptor superfamily in canine platelets. Interspecific variations of platelet α2-adrenoceptor and imidazoline receptor expressions may explain different platelet responses to catecholamines and imidazoline α-adrenergic agents.