Effects of imidazoline and non-imidazoline alpha-adrenergic agents on canine platelet aggregation

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.

Imidazoline use in sinonasal surgery

The nasal mucosa is very vascular, receiving more blood flow per cubic centimeter of tissue than does muscle, brain or liver (Drettner and Aust, 1974; [1]). This vascularity can present a major problem during sinus surgery. Surgeons routinely use topical vasoconstrictors in endoscopic sinus surgery however, the optimal regimen is not clear. Imidazoline nasal sprays are often used up to 1hour before sinonasal surgery to aid in intraoperative vasoconstriction. After the induction of anaesthesia, epinephrine-based topical and submucosal preparations are subsequently administered to further enhance vasoconstriction. Imidazolines are non-selective, partial alpha adrenoceptor agonists with a higher affinity, yet lower potency, for alpha adrenoceptors when compared to epinephrine. It is hypothesized that imidazolines block the action of epinephrine on the alpha adrenoceptors of the nasal mucosa resulting in less vasoconstriction, and a poorer intra-operative field, when compared to the use of epinephrine alone. This paper hypothesizes that preoperative imidazoline administration may adversely affect optimal intra-operative vasoconstriction.

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.

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

BACKGROUND/AIMS

Imidazoline α2-adrenergic agents exert complex effects on mammalian platelet aggregation. Although non-adrenergic, imidazoline (I) receptors have been revealed in human platelets, there is limited information about imidazoline's action on platelet aggregation. This study aimed to investigate aggregatory and anti-aggregatory effects of various imidazoline or non-imidazoline α-adrenergic agents on rabbit platelets.

METHODS

Aggregatory responses of agents on rabbit platelets were examined by turbidimetric method. Radioligand binding assay to platelet I1 and I2 receptors was performed using [(3)H]-clonidine and [(3)H]-idazoxan, respectively.

RESULTS

Aggregation was not induced by α-adrenoceptor agonists alone. Adrenaline and noradrenaline produced dose-dependent potentiation of ADP- or collagen-induced aggregation. Imidazoline adrenoceptor agonists clonidine and p-aminoclonidine also potentiated ADP-induced platelet aggregation. The α2-adrenoceptor antagonists and/or certain imidazoline adrenergic agents inhibited adrenaline-potentiated aggregation in a dose-dependent manner, whereas α1-adrenoceptor antagonists and non-imidazoline α-adrenergic agents were either ineffective or less effective in inhibiting adrenaline-potentiated aggregation. Rabbit platelets did not have I1 receptors, but had I2 receptors, indicating that adrenaline-potentiated platelet aggregation was inhibited by idazoxan, but not by imidazoline compounds clonidine and oxymetazoline.

CONCLUSIONS/IMPLICATIONS

These results demonstrated that α2-adrenoceptor-blocking agents and/or imidazoline α-adrenergic agents effectively inhibit adrenaline-potentiated platelet aggregation. It is proposed that imidazoline structure in part plays a role in the inhibition of adrenaline-potentiated aggregation.

Apparent heterogeneous responsiveness of human platelet rich plasma to catecholamines

The platelet-rich plasma (PRP) from 35 healthy Korean volunteers were challenged by four catecholamines and clonidine, which were known as alpha-adrenergic agonists. Wide individual variations were observed with the respect to the pattern and the degree of aggregation in response to each agent. They fall into five distinct groups; Group A (2.9%) was responsive to all five agonists; Group B (28.6%) aggregated in response to (-)-epinephrine, (-)- norepinephrine and epinine; Group C (37.1%) aggregated in response to (-)-epinephrine and (-)-norepinephrine;Group D (11.4%) aggregated only by (-)-epinephrine; Group E (20%) showed impaired responsiveness to all the alpha-agonists tested. All of the non-responding PRP were capable of induction of aggregation in response to (-)-epinephrine, (-)-norepinephrine and epinine in the presence of near-threshold concentration of collagen. Variations were also observed between groups with dopamine and clonidine. Dopamine and clonidine failed to induce secondary aggregation, even in the presence of low concentration of collagen, in most of the PRP preparations belonging to Groups D and E and only slight improvements were observed in the preparations belonging to groups Band C. The observation on heterogeneous responsiveness to catecholamines with PRP of Korean volunteers is quite different from the previous report with PRP of presumably mostly Caucasians.

Guanabenz, an antihypertensive centrally acting alpha2-agonist, suppresses morning elevations in aggregation of human platelets

To determine whether the antihypertensive agent guanabenz affects the circadian rhythm in the hemorheologic properties of the platelet, we evaluated the aggregability of platelets collected from 11 healthy subjects in the morning and the evening after the oral administration of this agent, daily for 2 weeks. We analyzed platelet aggregation by the turbidimetric method. In an in vitro study, guanabenz, 10 nM-100 microM, did not affect platelet aggregation, whereas epinephrine induced platelet aggregation at an EC50 of 1.5 microM. The healthy volunteers demonstrated a diurnal variation in platelet aggregability that was high in the morning and low in the evening (66 +/- 10% and 56 +/- 11% respectively, of the percent platelet aggregation induced by epinephrine). The same variation was seen with the platelet aggregation induced by adenosine diphosphate (ADP) (62 +/- 8% [morning] vs. 51 +/- 7% [evening]). After the administration of guanabenz, platelet aggregability was significantly reduced in the morning compared with that before drug administration, when platelet aggregation was induced by epinephrine (49 +/- 9%, p < 0.05) or ADP (48 +/- 7%, p < 0.05), although the plasma levels of catecholamine were unchanged. A suppressive effect of guanabenz on platelet aggregability was observed in the evening, as the platelets were stimulated by epinephrine (38 +/- 9%, p < 0.05), but not by ADP (49 +/- 5%). Findings suggest that guanabenz mainly suppressed the morning enhancement in platelet aggregability, which contributes to the formation of intravascular thrombi. Thus, in addition to its antihypertensive actions, guanabenz may help to reduce the risk of vascular accidents, which frequently occur in the morning.

Epinephrine induced platelet aggregation in rat platelet-rich plasma

Epinephrine at even high concentrations neither caused shape change nor aggregation of rat platelets. However, epinephrine induced aggregation in the presence of low (near-threshold) concentrations of collagen at which concentration only platelet shape change was induced without aggregation. The platelet aggregation was also induced by some other catecholamines and clonidine but not by beta-agonist isoproterenol. The aggregatory potencies of R-(-)-isomers, (-)-epinephrine and (-)-norepinephrine were higher than the corresponding desoxy derivatives, epinine and dopamine. In addition, the epinephrine-induced rat platelet aggregation was inhibited by alpha2-antagonists, yohimbine and phentolamine, but not by alpha1-antagonist prazosin or beta-antagonist propranolol. These results suggested that the epinephrine-induced rat platelet aggregation is occurring through alpha2-adrenergic receptors as is the case with human platelets.

Atipamezole, an imidazoline-type alpha(2)-adrenoceptor inhibitor, binds to human platelets and inhibits their adrenaline-induced aggregation more effectively than yohimbine

To investigate the usefulness of atipamezole [MPV-1248, 4-(2-ethyl-2, 3-dihydro-1H-inden-2-yl)-1H-imidazole], a novel alpha(2)-adrenoceptor-specific antagonist, as a tool in platelet studies, the ability of this antagonist: (1) to bind to platelet alpha(2)-adrenoceptors, and (2) to inhibit adrenaline-induced platelet aggregation was compared to that of yohimbine, another commonly used alpha(2)-adrenoceptor antagonist. It was found that atipamezole binds to platelet alpha(2)-adrenoceptors more effectively than yohimbine: [3H]atipamezole has more than three times higher alpha(2)-adrenoceptor binding affinity in intact gel-filtered human platelets (equilibrium dissociation constant (K(d)) 0.7+/-0.21 vs. 2.9+/-0.77 nM, p<0.05), but only one-third of the binding capacity of [3H]yohimbine (B(max) 27.0+/-3.8 vs. 100+/-19 pM/10(5) cells, p<0.01). Functionally, in comparison with yohimbine, an almost threefold lower concentration of atipamezole inhibited adrenaline (5 microM)-induced platelet aggregation. A concentration of atipamezole, which inhibited this aggregation by 50% (IC(50)), was 0.37+/-0.07 microM, whereas IC(50) for yohimbine was 0.98+/-0.12 microM, p<0.0001. Thus, atipamezole represents a functionally undisputed alpha(2)-adrenoceptor antagonist, more effective than yohimbine. Its distinct binding profile as a radioligand also suggests the presence of imidazol(in)e binding sites in platelets.