Evidence for different 5-HT1B/1D receptors mediating vasoconstriction of equine digital arteries and veins

Arrhythmic Effects Evaluated on Caenorhabditis elegans: The Case of Polypyrrole Nanoparticles

Experimental studies and clinical trials of nanoparticles for treating diseases are increasing continuously. However, the reach to the market does not correlate with these efforts due to the enormous cost, several years of development, and off-target effects like cardiotoxicity. Multicellular organisms such as the Caenorhabditis elegans (C. elegans) can bridge the gap between in vitro and vertebrate testing as they can provide extensive information on systemic toxicity and specific harmful effects through facile experimentation following 3R EU directives on animal use. Since the nematodes' pharynx shares similarities with the human heart, we assessed the general and pharyngeal effects of drugs and polypyrrole nanoparticles (Ppy NPs) using C. elegans. The evaluation of FDA-approved drugs, such as Propranolol and Racepinephrine reproduced the arrhythmic behavior reported in humans and supported the use of this small animal model. Consequently, Ppy NPs were evaluated due to their research interest in cardiac arrhythmia treatments. The NPs' biocompatibility was confirmed by assessing survival, growth and development, reproduction, and transgenerational toxicity in C. elegans. Interestingly, the NPs increased the pharyngeal pumping rate of C. elegans in two slow-pumping mutant strains, JD21 and DA464. Moreover, the NPs increased the pumping rate over time, which sustained up to a day post-excretion. By measuring pharyngeal calcium levels, we found that the impact of Ppy NPs on the pumping rate could be mediated through calcium signaling. Thus, evaluating arrhythmic effects in C. elegans offers a simple system to test drugs and nanoparticles, as elucidated through Ppy NPs.

Endothelial cells and angiogenesis in the horse in health and disease-A review

The cardiovascular system is the first functional organ in the embryo, and its blood vessels form a widespread conductive network within the organism. Blood vessels develop de novo, by the differentiation of endothelial progenitor cells (vasculogenesis) or by angiogenesis, which is the formation of new blood vessels from existing ones. This review presents an overview of the current knowledge on physiological and pathological angiogenesis in the horse including studies on equine endothelial cells. Principal study fields in equine angiogenesis research were identified: equine endothelial progenitor cells; equine endothelial cells and angiogenesis (heterogeneity, markers and assessment); endothelial regulatory molecules in equine angiogenesis; angiogenesis research in equine reproduction (ovary, uterus, placenta and conceptus, testis); angiogenesis research in pathological conditions (tumours, ocular pathologies, equine wound healing, musculoskeletal system and laminitis). The review also includes a table that summarizes in vitro studies on equine endothelial cells, either describing the isolation procedure or using previously isolated endothelial cells. A particular challenge of the review was that results published are fragmentary and sometimes even contradictory, raising more questions than they answer. In conclusion, angiogenesis is a major factor in several diseases frequently occurring in horses, but relatively few studies focus on angiogenesis in the horse. The challenge for the future is therefore to continue exploring new therapeutic angiogenesis strategies for horses to fill in the missing pieces of the puzzle.

Validation of canine uterine and testicular arteries for the functional characterisation of receptor-mediated contraction as a replacement for laboratory animal tissues in teaching

Teaching practicals for receptor physiology/pharmacology in medical and veterinary schools have involved the use of in vitro experiments using tissues from laboratory animals, which have been killed for isolated vascular strip or ring preparations. However, the use of scavenged tissues has been advocated to reduce animal use. Utilising discarded tissues from routine surgical procedures, such as canine neutering, has not previously been investigated. Canine testicular and uterine tissues (discarded tissues) were obtained from routine neutering procedures performed by the veterinary team at a local animal neutering clinic for stray dogs. Rings of uterine and testicular artery were dissected and mounted on a Mulvany-Halpern wire myograph in order to characterize the adrenergic and serotonergic receptors mediating vasoconstriction. Cumulative contractile concentration-response curves were constructed for the alpha adrenoceptor agonists epinephrine (α₁ and α₂ receptors), phenylephrine (α₁ selective) and UK14304 (α₂ selective). Pre-treatment with the α₁-selective antagonist, prazosin, was also investigated. The response to serotonin (5-HT) receptor agonists were also investigated, including 5-HT (acting at both 5-HT₁ and 5-HT₂ receptors), 5-carboxamidotryptamine (5-CT; 5-HT₁ selective) and α-methyl 5-HT (5-HT₂ selective). A contractile response was observed in both canine uterine and testicular arteries to epinephrine and phenylephrine, and prazosin caused a dose-dependent parallel rightward shift in the phenylephrine dose-response curve (pA₂ values of 7.97 and 8.39, respectively). UK14304 caused a contractile response in canine testicular arteries but very little appreciable contractile response in uterine arteries. The maximum responses produced by the uterine arteries to 5-HT was significantly lower than those of the testicular arteries. In the testicular artery, the 5-HT₂ receptor selective agonist, α-methyl 5-HT, produced a similar contractile response to 5-HT but the administration of 5-CT failed to produce a response in either the testicular or uterine artery segments. These results validate the use of discarded tissue from routine canine neutering procedures as a useful source of vascular tissue for pharmacological teaching, for characterizing alpha and 5-HT receptor contractile responses.

Endothelium-dependent relaxation mechanisms involve nitric oxide and prostanoids in the isolated bovine digital vein

Endothelial dysfunction contributes to the development of ungulate's laminitis. Although extensively studied in equines, the endothelial function is not fully examined in bovine digital veins (BDVs). BDVs were studied under isometric conditions to describe the acetylcholine (ACh) endothelium-dependent relaxation. Concentration-response curves were constructed to phenylephrine, ACh, and sodium nitroprusside (SNP). Relaxation responses were evaluated using either phenylephrine or depolarizing high-potassium Krebs solution (DKS) as precontraction agents. Endothelium denudation and incubation with L-NAME (300 μM), indomethacin (10 μM) or both were used to explore endothelial-mediated mechanisms. Endothelium denudation did not modify phenylephrine and SNP responses, however, significantly (p < 0.05) converted a relaxation (63.2 ± 5%) response to ACh into a contraction (30.3±9%). The ACh-evoked relaxation was significantly (p < 0.05) reduced in the presence of indomethacin (37.5 ± 6%) and L-NAME (6.40 ± 2%). The presence of both inhibitors abolished the ACh-evoked relaxation. Although DKS caused a higher precontraction than phenylephrine, ACh-evoked relaxation (22.4 ± 3.4%) was still observed and was reduced by the combination of inhibitors (7.0 ± 1.0%). The ACh endothelium-dependent relaxation in BDVs is essentially mediated by nitric oxide and endothelium-derived prostanoids. The BDV endothelium function is a dynamic component in the control of the bovine digital blood flow, particularly under endothelial dysfunction conditions when venoconstriction might lead to postcapillary resistance increase.

Tall fescue ergot alkaloids are vasoactive in equine vasculature

Mares grazing endophyte-infected () tall fescue () typically exhibit reproductive dysfunction rather than problems associated with peripheral vasoconstriction as a primary sign of the fescue toxicosis syndrome. Research using Doppler ultrasonography demonstrated that consumption of endophyte-infected tall fescue seed causes measurable vasoconstriction in the medial palmar artery. The objective of this study was to evaluate contractile responses of medial palmar artery and vein to increasing concentrations of various tall fescue alkaloids. Medial palmar arteries and veins were collected immediately following euthanasia from 23 horses of mixed breed, age, and gender from both forelimbs, and uterine arteries were collected from females ( = 12). Vessels were separated, cleaned of excess connective and adipose tissue, divided into 2- to 3-mm cross-sections, and suspended in a multimyograph chamber with continuously oxygenated Krebs-Henseleit buffer (95% O/5% CO; pH 7.4; 37°C). Following a 90-min equilibration and recovery from reference compound exposure, increasing concentrations of norepinephrine, 5-hydroxytryptamine, ergotamine, and ergonovine for the palmar artery and vein and uterine artery and ergovaline, ergocryptine, ergocristine, ergocornine, and lysergic acid for the palmar artery and vein were added to assess vasoactivity. Data were normalized as a percentage of contractile response induced by the reference compound addition and analyzed as a completely randomized design. Both norepinephrine and serotonin were vasoactive in all 3 types of blood vessels. Neither ergotamine nor ergonovine were vasoactive in the uterine artery. All alkaloids tested with the palmar artery and vein produced a contractile response, except that neither the palmar artery nor the palmar vein responded to lysergic acid ( > 0.05). Ergovaline was the most vasoactive ergot alkaloid in both the palmar artery and the palmar vein ( < 0.05) followed by ergonovine, whereas out of the 4 remaining ergopeptine alkaloids tested, ergocristine induced the lowest contractile response. Although horses do not outwardly appear to be affected by peripheral vasoconstriction as observed in cattle, these data indicate that tall fescue alkaloids are vasoactive and suggest that potential exists for peripheral vascular effects of tall fescue alkaloids in horses. This does not appear to be the case for the uterine artery, and future research should be directed at understanding how ergot alkaloids cause equine reproductive dysfunction.

The effect of tumour necrosis factor-α and insulin on equine digital blood vessel function in vitro

OBJECTIVE AND DESIGN

Insulin and inflammatory cytokines may be involved in equine laminitis, which might be associated with digital vascular dysfunction. This study determined the effects of TNF-α and insulin on the endothelial-dependent relaxant responses of equine digital blood vessels and on equine digital vein endothelial cell (EDVEC) cGMP production.

MATERIAL

Isolated rings of equine digital arteries (EDAs) and veins (EDVs) were obtained and EDVECs were cultured from horses euthanized at an abattoir.

METHODS

The effect of incubation with TNF-α (10 ng/ml) and/or insulin (1,000 μIU/ml) for 1.5 h or overnight under hyperoxic and hypoxic conditions on carbachol (endothelium-dependent) induced relaxation was assessed. The time course and concentration dependency of the effect of TNF-α and the effect of insulin (1,000 μIU/ml) on EDVEC cGMP production was determined.

RESULTS

Incubation of EDAs overnight with TNF-α under hypoxic conditions resulted in endothelial-dependent vascular dysfunction. EDVs produced a more variable response. TNF-α increased EDVEC cGMP formation in a time- and concentration-dependent manner. Insulin had no significant effects.

CONCLUSIONS

There is a mismatch between the results obtained from isolated vessel rings and cultured endothelial cells suggesting TNF-α may reduce the biological effect of NO by reducing its bioavailability rather than its formation, leading to endothelial cell dysregulation.

Seasonal variation in maintenance of phenylephrine-induced tone in isolated equine digital arteries under hypoxic or hyperoxic conditions in vitro

Digital vasoconstriction, ischaemia and hypoxia may predispose to acute laminitis. Laminitis incidence varies seasonally, peaking in spring and summer. Direct seasonal influences on equine digital artery (EDA) contractility have not been investigated. This study assessed seasonal variation in maintenance of phenylephrine (PHE)-induced tone in isolated EDAs under hypoxic (95% nitrogen) and hyperoxic (95% oxygen) conditions. The objective was to measure change in arterial tone over time after constriction to a plateau with PHE. Tone was measured at plateau and over time and percentage change calculated. Hyperoxic EDAs maintained PHE-induced tone over 1 h with no seasonal variation. Hypoxic EDAs relaxed in fall (median [inter-quartile range] 59% [44-77%] decrease from plateau; P=0.008), contracted in spring (65% [20-192%] increase from plateau; P=0.03) and did not significantly change tone in winter (18% [0-28%] decrease; P=0.13). Continued contraction under hypoxic conditions in spring may contribute to digital vasoconstriction.

Contractile responses of isolated equine digital arteries under hypoxic or hyperoxic conditions in vitro: role of reactive oxygen species and Rho kinase

The underlying pathophysiological triggers for equine acute laminitis are unknown, although digital vasoconstriction, ischaemia, hypoxia and reperfusion injury may be involved. The contractile responses of isolated equine digital arteries (EDAs), harvested from the hindlimbs of normal horses postmortem at an abattoir, were studied acutely (up to 3 h) under hyperoxic (95% oxygen, 5% CO2 ) and hypoxic (95% nitrogen, 5% CO2 ) conditions in organ baths. Phenylephrine (PHE; 10(-6) m), 5-hydroxytryptamine (5-HT; 10(-7) m) and high potassium (K(+) ; 118 mm) caused contraction in EDAs which was significantly (P<0.0001) enhanced under hypoxic conditions. In contrast, contraction stimulated by 9,11-dideoxy-9α,11α-epoxymethanoprostaglandin F2α (U44069; 3 × 10(-8) m) was not significantly enhanced by hypoxia (P=0.75). Hypoxia-enhanced contraction in response to K(+) was greater (P<0.03) in vessels with a functional endothelium than in vessels in which the endothelium was removed by rubbing. Fasudil (10(-6) to 10(-5) m), a Rho kinase inhibitor, and apocynin (10(-3) to 3 × 10(-3) m), an NADPH oxidase inhibitor, significantly (P ≤ 0.05) inhibited hypoxia-enhanced contraction in response to PHE and 5-HT. In conclusion, hypoxia-enhanced contraction occurred in EDAs. This appears to be partially mediated by reactive oxygen species produced by NAPDH oxidase, which activate Rho kinase to increase calcium sensitisation and enhance smooth muscle contraction.

The effect of cooling on the contractility of equine digital small lamellar arteries: modulating role of the endothelium

The equine hoof displays thermoregulatory functions, and the blood vessels lying under the hoof wall are temperature sensitive. The aim of this study was to investigate the effect of cooling on the contractile responses to α-adrenoceptor and 5-HT receptor stimulation in equine small lamellar arteries using wire myography. The role of the endothelium in the response mediated by 5-HT was also evaluated. Moderate cooling caused a reduction of the contraction induced by depolarizing Krebs solution (DKS, containing 118 mm KCl) and the maximal contraction caused by UK-14304 (α(2)-adrenoceptor agonist). The potency of methoxamine (α(1)-adrenoceptor agonist) was reduced by cooling [pD(2) (-log EC(50)) at 22°C, 5.7 (5.5-6.0) versus 30°C, 5.9 (5.7-6.1)]; however, the efficacy was maintained. The contractions evoked by sumatripan and α-methyl 5-HT (5-HT receptor agonists) were not modified by moderate cooling. In contrast, a cooling-enhanced contraction was observed in response to 5-HT [maximum response (E(max)) at 22°C, 213.2 ± 13% DKS versus 30°C, 179.9 ± 11% DKS]. Furthermore, this effect was more evident in endothelium-denuded lamellar arteries (E(max) at 22°C, 270.2 ± 26% DKS versus 30°C, 172.2 ± 20% DKS), suggesting a potential modulating role of the endothelium. The L-NAME/ibuprofen-resistant relaxation in response to carbachol was reduced by cooling in small lamellar arteries precontracted with 5-HT but not phenylephrine. Therefore, a moderate reduction of temperature modulates the reactivity of small lamellar arteries by enhancing the 5-HT-mediated contraction, but inhibits the α-adrenoceptor-mediated response. Furthermore, the endothelium of these blood vessels may play an important role in preventing excessive vasoconstriction in response to 5-HT and maintaining digital blood flow in cool environmental temperatures.

Evidence for vascular and enzymatic events in the pathophysiology of acute laminitis: which pathway is responsible for initiation of this process in horses?

To date, there is a substantial amount of data to support the hypotheses that vascular and enzymatic changes are ongoing in experimental laminitis. Furthermore, there is substantial in vitro evidence that the enzymatic changes weaken the dermo-epidermal attachments leading to mechanical failure of the hoof-bone interface of the equine digit. However, investigators of both the vascular and enzymatic theories have, to date, been unable to substantiate the effects of these pathophysiological changes in vivo on laminar tissues of horses afflicted with experimentally induced or naturally acquired laminitis. In addition, the effects of laminitis-inducing treatment have not been prevented or reversed by treatment with an MMP inhibitor or a vasoactive antagonist. It is possible that there is simultaneous activation of the vascular and enzymatic pathways and/or other inflammatory processes. Moreover, the third theory involving mechanical factors cannot be discounted simply because strong evidence for vascular and enzymatic changes exists. It is common for horses with severe musculoskeletal disease affecting weightbearing on a limb to develop laminitis in the contralateral limb. It remains to be determined what factors are responsible for initiation of laminitis in these individuals. Evidence has not been presented that precludes the possibility of coincident occurrence of vascular and enzymatic changes. In fact, many of the inflammatory mediators (e.g. interleukin-1beta) found in laminitic tissues can concurrently stimulate synthesis of vasoactive substances and activate MMPs. Because enzymatic action on proteins is largely dependent on the concentrations of proteins and enzyme, the enzymatic theory is not dependent upon increased delivery of enzymes via increased capillary flow. Likewise, because vascular changes can alter tissue function via increased capillary flow and oedema formation, the vascular theory is not dependent upon decreased capillary flow. It is true that naturally acquired laminitis is widely variable in severity and predisposing diseases. Therefore, most probably there are multiple mechanisms involved in the initiation and propagation of the pathophysiologic cascade(s) and, therefore, successful intervention will necessitate multiple treatment modalities.

Evaluation of the induction of vasoactive mediators from equine digital vein endothelial cells by endotoxin

OBJECTIVE

To determine the effect of endotoxin (lipopolysaccharide [LPS]) on vasoactive mediator production by cultured equine digital vein endothelial cells (EDVECs).

SAMPLE POPULATION

EDVECs obtained from forelimb digital veins of 7 healthy adult horses.

PROCEDURES

EDVECs were incubated with or without LPS (1 microg/mL) for 0, 2, 4, 6, 22, and 24 hours. The EDVECs were incubated for 18 hours with LPS (10 pg/mL to 1 microg/mL) with or without ibuprofen, cycloheximide, or L-nitroarginine methyl ester. Medium concentrations of prostacyclin, cyclic guanosine monophosphate, endothelin-1, and thromboxane A(2) were determined. Changes in inducible nitric oxide synthase and cyclooxygenase-2 expression were determined.

RESULTS

LPS stimulated mean 4.2- and 14.1-fold increases in EDVEC prostacyclin and cyclic guanosine monophosphate production, respectively, after 22 hours. These effects were LPS concentration-dependent (LPS concentrations that induced a response halfway between the maximum response and baseline of 1.50 and 1.22 ng/mL, respectively). The LPS-induced cyclic guanosine monophosphate production was significantly inhibited (to basal concentrations) by L-nitroarginine methyl ester, and prostacyclin production was inhibited by cycloheximide and ibuprofen. Production of thromboxane A(2) by EDVECs was not detected. Endothelin-1 accumulated in the medium, but LPS did not enhance its production. Inducible nitric oxide synthase expression in EDVECs was not detected with the available antibodies, whereas LPS stimulated cyclooxygenase-2 expression in a time- and concentration-dependent manner.

CONCLUSIONS AND CLINICAL RELEVANCE

LPS stimulated vasoactive mediator production by equine endothelial cells, which may play a role in LPS-induced digital hypoperfusion.

Cooling augments vasoconstriction mediated by 5-HT1 and alpha2-adrenoceptors in the isolated equine digital vein: involvement of Rho kinase

The vasculature of the equine digit fulfils an important role in thermoregulation. In other species, it has been found that cooling may enhance the response of cutaneous vessels to 5-hydroxytryptamine (5-HT) and alpha(2)-adrenoceptor agonists. Translocation of alpha(2)-adrenoceptors to the smooth muscle cell membrane, mediated by Rho kinase, is thought to be involved in the cooling-enhanced response in mouse tail arteries. However, little is known about the effect of cooling on 5-HT receptor function. The present investigation compared the response of 5-bromo-6-(2-imidazolin-2-ylamino) quinoxaline (UK14304:1 nM to 30 microM), methoxamine (0.1 nM to 30 microM; in the presence of yohimbine 0.1 microM), 5-carboxamidotryptamine (5-CT; 0.1 nM to 10 microM) and alpha-methyl 5-HT (0.1 nM to 10 microM) in the isolated equine digital vein at 30 degrees C and 22 degrees C. The effect of the Rho kinase inhibitor, fasudil (1 microM), and the recovery of the response after the irreversible blockade of surface receptors with phenoxybenzamine (10 microM) or 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ;10 microM), was established. Moderate cooling significantly increased the maximum response to alpha-methyl 5-HT, 5-CT and UK14304 and shifted their response curves to the left. Cooling also augmented the phenoxybenzamine- and EEDQ-resistant response to UK14304 and 5-CT, respectively. Fasudil had no effect on the contractile response at 30 degrees C, but completely abrogated the effect of cooling on the response to 5-CT and UK14304. The response to methoxamine was not significantly affected by cooling. These results suggest that Rho kinase plays an important role in the cooling-enhanced response mediated by 5-HT(1B/D) receptors and alpha(2)-adrenoceptors. The exact mechanism by which Rho/Rho kinase enhances the functional responses mediated by these receptors in these vessels has yet to be determined.

Assessment of endothelium-dependent vasodilation in equine digital resistance vessels

Haemodynamic disturbances leading to ischaemia and reperfusion injury of the digit are thought to be involved in the pathophysiology of acute equine laminitis. Identification of physiological regulators of blood flow through the equine digit is important in identifying factors, which may predispose animals to laminitis. A method was developed to assess endothelium-dependent responses of the isolated Krebs-perfused equine digit by co-administration of 5-hydroxytryptamine (5-HT) with vasodilator agents, carbachol (CCh), bradykinin (BK) and substance P (SP). Bolus co-administration of CCh (0.02-2 micromol), BK and SP (0.02-0.2 nmol), caused inhibition of the 5-HT pressor response by 50-60%. The vasodilator responses were abolished by the detergent, CHAPS, indicating endothelium dependency; whereas vasoconstrictor responses to 5-HT were potentiated. CCh-induced relaxation was significantly reduced by the nitric oxide synthase inhibitor L-NAME (79.7 +/- 3.4% inhibition), whereas a large proportion of BK and SP-induced relaxation remained (34.1 +/- 6.3% and 33.6 +/- 5.3% inhibition). L-NAME potentiated vasoconstrictor responses to 5-HT. In conclusion, this study demonstrates that endothelium-derived NO modulates the response to vasoconstrictors such as 5-HT and is likely to be an important regulator of blood flow in the digital resistance vascular bed. Other factor(s) released by the endothelium are also important in regulating blood flow, whose identity remains to be established.

Effects of an endothelin receptor antagonist and nitroglycerin on digital vascular function in horses during the prodromal stages of carbohydrate overload-induced laminitis

OBJECTIVE

To evaluate changes in digital vascular function in horses with carbohydrate overload (CHO)-induced laminitis and determine the effects of an endothelin (ET) receptor antagonist and nitroglycerin on laminitis-associated vascular dysfunction.

ANIMALS

20 adult horses without abnormalities of the digit.

PROCEDURES

Hemodynamic variables were recorded before (baseline) and hourly after all horses were administered a CHO ration via nasogastric tube. In 4 groups of 5 horses each, saline (0.9% NaCl) solution or ET receptor antagonist (10(5)M in digital blood) was administered into the digital arterial circulation according to 1 of 2 schedules. During anesthesia, blood flow; arterial, venous, and capillary pressures; and total, precapillary, and postcapillary resistances were measured in an isolated perfused digit of each horse. In all groups, nitroglycerin was infused (10(5)M in digital blood), and digital microvascular assessments were repeated.

RESULTS

The CHO caused a significant decrease in right atrial pressure by 14 hours that was not affected by administration of saline solution or ET receptor antagonist. In isolated digits of anesthetized horses, CHO resulted in a significant decrease in digital blood flow associated with a significant increase in total and postcapillary resistances. Treatment with the ET receptor antagonist and nitroglycerin caused a significant decrease in total resistance. Postcapillary resistance was significantly decreased following treatment with the ET receptor antagonist but was not altered by treatment with nitroglycerin.

CONCLUSIONS AND CLINICAL RELEVANCE

Treatment with an ET receptor antagonist and nitroglycerin resulted in significant improvement in vascular resistance in isolated perfused digits of anesthetized horses with CHO-induced laminitis.

Gastrointestinal derived factors are potential triggers for the development of acute equine laminitis

Equine laminitis is the painful and debilitating condition resulting from cellular damage and inflammation of the tissues comprising the bonds supporting the pedal bone within the hoof capsule. One of the reasons why this condition is so complicated and enigmatic is its association with gastrointestinal disturbances, particularly a diet of lush grass at certain times of the year. Determining the link between disturbances to the hindgut flora and pathophysiology in the foot is one of the keys to preventing this condition. Therefore, one of the biggest challenges in equine laminitis research is to determine those events that precede the onset of the classical clinical signs of lameness, bounding digital pulses, and warm feet. A main theory for the pathogenesis of acute laminitis involves the development of vasoconstriction within the foot, which leads to ischemic and/or inflammatory tissue damage. Serotonin is an important constrictor mediator controlling digital blood flow. Certain equine hindgut bacteria produce amino acid decarboxylase enzymes that convert free amino acids into monoamines. Therefore, amines formed and released from the gastrointestinal tract are hypothesized to act as the link between the ingestion of lush grass and the digital ischemia thought to precede laminitis. Equine cecal contents contain a range of amines that are present in micromolar concentrations. Tryptamine is the most potent cecum-derived amine, causing vasoconstriction in vitro and in vivo through direct activation of serotonin receptors and displacing serotonin from platelets. Monoamines found in the cecum of the horse could potentially induce hemodynamic disturbances in the digit that result in laminar ischaemia, and so trigger laminitis.

Pharmacological characterization of alpha1-adrenoceptors in equine digital veins

Alpha-adrenoceptors mediate contractile responses in equine digital veins (EDVs) and arteries. Vascular smooth muscle alpha(1)-adrenoceptor subtypes have been implicated in a number of conditions, such as acute equine laminitis, and are therapeutic targets for the treatment of this condition. Digital veins, rather than arteries, were investigated in the present study because they have been specifically implicated in the pathophysiology of acute laminitis. The order of potency of a series of alpha(1)-adrenoceptor-selective agonists and antagonists was determined in isolated rings of EDVs under conditions of isometric tension. A61603 was the most potent agonist, with a higher potency (76-fold greater) than phenylephrine (PHE), suggesting the presence of the alpha(1A)-adrenoceptor subtype. Prazosin (30 nm) caused competitive inhibition of the responses to A61603 and PHE, with pK(b) values of 8.05 +/- 0.28 and 8.20 +/- 0.27, respectively. In addition, the alpha(1A)-adrenoceptor antagonist, WB4101 (10 nm), also caused competitive inhibition of the responses to the two agonists, with pK(b) values of 8.37 +/- 0.16 and 8.54 +/- 0.23, respectively, confirming the presence of the alpha(1A)-adrenoceptor subtype in EDVs. The selective alpha(1D)-adrenoceptor antagonist, BMY7378 (100 nm) did not cause a significant change in the response to the agonists, giving lower pK(b) values (6.97 +/- 0.27 and 6.88 +/- 0.17 vs. A61603 and PHE, respectively). Chloroethylclonidine dihydrochloride (45 microm, 30 min), used to produce selective inactivation of alpha(1B)-adrenoceptors, caused noncompetitive inhibition of the response to PHE, but was without effect on the response to A61603. These findings indicate that EDVs possess at least two different alpha(1)-adrenoceptor populations, which are predominantly of the alpha(1A) and alpha(1B) subtypes. These data may assist in the development of more selective antagonists for therapeutic use in horses.

Principles of pharmacodynamics and their applications in veterinary pharmacology

Pharmacodynamics (PDs) is the science of drug action on the body or on microorganisms and other parasites within or on the body. It may be studied at many organizational levels--sub-molecular, molecular, cellular, tissue/organ and whole body--using in vivo, ex vivo and in vitro methods and utilizing a wide range of techniques. A few drugs owe their PD properties to some physico-chemical property or action and, in such cases, detailed molecular drug structure plays little or no role in the response elicited. For the great majority of drugs, however, action on the body is crucially dependent on chemical structure, so that a very small change, e.g. substitution of a proton by a methyl group, can markedly alter the potency of the drug, even to the point of loss of activity. In the late 19th century and first half of the 20th century recognition of these facts by Langley, Ehrlich, Dale, Clarke and others provided the foundation for the receptor site hypothesis of drug action. According to these early ideas the drug, in order to elicit its effect, had to first combine with a specific 'target molecule' on either the cell surface or an intracellular organelle. It was soon realized that the 'right' chemical structure was required for drug-target site interaction (and the subsequent pharmacological response). In addition, from this requirement, for specificity of chemical structure requirement, developed not only the modern science of pharmacology but also that of toxicology. In relation to drug actions on microbes and parasites, for example, the early work of Ehrlich led to the introduction of molecules selectively toxic for them and relatively safe for the animal host. In the whole animal drugs may act on many target molecules in many tissues. These actions may lead to primary responses which, in turn, may induce secondary responses, that may either enhance or diminish the primary response. Therefore, it is common to investigate drug pharmacodynamics (PDs) in the first instance at molecular, cellular and tissue levels in vitro, so that the primary effects can be better understood without interference from the complexities involved in whole animal studies. When a drug, hormone or neurotransmitter combines with a target molecule, it is described as a ligand. Ligands are classified into two groups, agonists (which initiate a chain of reactions leading, usually via the release or formation of secondary messengers, to the response) and antagonists (which fail to initiate the transduction pathways but nevertheless compete with agonists for occupancy of receptor sites and thereby inhibit their actions). The parameters which characterize drug receptor interaction are affinity, efficacy, potency and sensitivity, each of which can be elucidated quantitatively for a particular drug acting on a particular receptor in a particular tissue. The most fundamental objective of PDs is to use the derived numerical values for these parameters to classify and sub-classify receptors and to compare and classify drugs on the basis of their affinity, efficacy, potency and sensitivity. This review introduces and summarizes the principles of PDs and illustrates them with examples drawn from both basic and veterinary pharmacology. Drugs acting on adrenoceptors and cardiovascular, non-steroidal anti-inflammatory and antimicrobial drugs are considered briefly to provide a foundation for subsequent reviews in this issue which deal with pharmacokinetic (PK)-PD modelling and integration of these drug classes. Drug action on receptors has many features in common with enzyme kinetics and gas adsorption onto surfaces, as defined by Michaelis-Menten and Langmuir absorption equations, respectively. These and other derived equations are outlined in this review. There is, however, no single theory which adequately explains all aspects of drug-receptor interaction. The early 'occupation' and 'rate' theories each explain some, but not all, experimental observations. From these basic theories the operational model and the two-state theory have been developed. For a discussion of more advanced theories see Kenakin (1997).

Current research and theories on the pathogenesis of acute laminitis in the horse

A large number of studies have been undertaken in recent years aimed at furthering our understanding of the complex mechanisms underlying the common and debilitating condition of acute laminitis in the horse. Many of these studies have either reinforced or cast doubt on previously held theories on the pathogenesis of this disease, while others have suggested new mechanisms which may play a key role in its development. This review seeks to put the current hypotheses into the context of this recent body of evidence. While a unifying theory may not yet seem to be achievable, this review demonstrates that most of the current theories are not mutually exclusive. Studies utilising in vitro and in vivo models of the disease, particularly addressing the areas of inflammation, haemodynamic disturbances and enzyme activation in the hoof, as well as the preceding events occurring in the hindgut, have helped to explain many clinical observations of the disease and may possibly lead to more effective therapies and means of prevention in the future.

Effects of monoamines formed in the cecum of horses on equine digital blood vessels and platelets

OBJECTIVE

To determine in vitro vasoactive potency of monoamines formed in the cecum and found in the systemic circulation of horses.

SAMPLE POPULATION

Segments of digital blood vessels obtained from 6 healthy mixed-breed horses and ponies euthanatized at an abattoir and platelets isolated from 4 healthy ponies.

PROCEDURE

Paired rings of digital artery and vein from the same horse were examined, and isometric tension was recorded. Concentration-response curves for tryptamine (TRP), tyramine (TYR), phenylethylamine (PEA), isoamylamine (IAA), and isobutylamine (IBA) were obtained. Vasoconstrictor mechanisms were investigated for TRP and TYR by the use of antagonists. Washed platelets loaded with [3H]-5-hydroxytryptamine (5-HT) were incubated with monoamines; the amount of radioactivity displaced after 30 minutes was estimated.

RESULTS

TRP, TYR, and PEA were potent constrictors of arteries and veins, with TRP and TYR being more potent in veins than arteries. Constrictions induced by TYR were inhibited by benextramine (alpha-antagonist) and nisoxetine (neuronal-uptake blocker), whereas TRP responses were inhibited by ketanserin (5-HT receptor antagonist). All 5 amines displaced 5-HT from platelets with the order of potency being TYR > TRP > PEA > IAA > IBA.

CONCLUSIONS AND CLINICAL RELEVANCE

Amines from the equine cecum cause digital vasoconstriction. The most potent (TRP and TYR) cause selective venoconstriction. Tyrosine activates predominantly alpha-adrenoceptors through the release of neuronal norepinephrine, whereas TRP activates 5-HT receptors. All amines tested released 5-HT from platelets. Amines formed in the cecum and released into the systemic circulation warrant additional investigation as trigger factors for digital ischemia and subsequent laminitis.

Seasonal changes in plasma concentrations of cecum-derived amines in clinically normal ponies and ponies predisposed to laminitis

OBJECTIVE

To measure concentrations of amines formed in the cecum of clinically normal ponies, determine amine concentrations in plasma samples collected in spring and winter, and compare concentrations of amines and serotonin in plasma samples obtained from clinically normal ponies and ponies predisposed to laminitis.

SAMPLE POPULATION

Cecal contents obtained from 10 ponies euthanatized at an abattoir and blood samples obtained from 42 adult ponies.

PROCEDURE

Cecal contents were assayed for amines by high-performance liquid chromatography (HPLC). Blood samples were collected at various times of the year from 20 ponies predisposed to acute laminitis and 22 clinically normal ponies. Plasma serotonin concentration was measured by HPLC, and tryptamine (TRP), tyramine (TYR), phenylethylamine (PEA), and isoamylamine (IAA) were measured by liquid chromatography-mass spectrometry.

RESULTS

15 amines were identified in cecal contents. Plasma TRP, TYR, PEA, and IAA concentrations ranged from 10pM to 100nM in both groups of ponies. Plasma concentrations of serotonin or other amines did not differ between clinically normal ponies and those predisposed to laminitis; however, significantly higher concentrations of TRP, PEA, and IAA were found in samples obtained in the spring, compared with winter samples.

CONCLUSIONS AND CLINICAL RELEVANCE

Various amines are found in the cecum of ponies, several of which can be detected in the plasma. Concentrations increase significantly in the spring and may reach concentrations close to the threshold for causing vasoconstriction. Release of amines from the cecum into the systemic circulation may contribute to hemodynamic disturbances in horses and ponies with acute laminitis.

Characterization and comparison of the responses of equine digital arteries and veins to endothelin-1

OBJECTIVE

To compare the responses of equine digital arteries (EDAs) and equine digital veins (EDVs) to endothelin-1 (ET-1) and determine the role of the endothelium and type of receptors involved in the modulation and mediation of those responses, respectively.

SAMPLE POPULATION

5 to 9 palmar digital vessels/experiment from 28 healthy horses.

PROCEDURE

Rings of dissected vessels were mounted under tension between force transducer wires in organ baths containing Krebs-Henseleit solution at 30 degrees C. Responses of EDAs and EDVs (with intact [+e] or denuded [-e] endothelium) to cumulative concentrations of ET-1 (10(-10) to 3 X 10(-7) M) were compared. For (+e)EDAs and (+e)EDVs precontracted with a thromboxane-mimetic (U44069; 10(-8) M) and (-e)EDAs and (-e)EDVs, responses to an ETB receptor agonist (S6c; 10(-10) to 3 X 10(-7) M) were evaluated. Responses to ET-1 (10(-7) M) in (-e)EDAs and (-e)EDVs were evaluated after incubation with an ETA receptor antagonist (BQ-123; 3 X 10(-7) M), an ETB receptor antagonist (BQ-788; 3 X 10(-7) M), or vehicle solution.

RESULTS

Endothelin-1 induced a concentration-dependent contraction of endothelium-intact and -denuded EDAs and EDVs; EDVs were more sensitive. Neither vessel type relaxed in response to S6c, although 2 of the (-e)EDAs contracted mildly. Whereas BQ-123 inhibited the (-e)EDA and (-e)EDV responses to ET-1, BQ-788 had no effect.

CONCLUSIONS AND CLINICAL RELEVANCE

Endothelin-1 induced digital vasoconstriction (marked constriction in veins). This action was unaffected by endothelium and mediated predominantly by ETA receptors. These findings suggest ET-1 can induce selective digital venoconstriction.

Endotoxin and dietary amines may increase plasma 5-hydroxytryptamine in the horse

Uptake of 5-hydroxytryptamine (5-HT) into platelets is an important mechanism by which low plasma concentrations are maintained, and platelet activation may therefore result in significant release of this vasoconstrictor. The present study examined the kinetics of active uptake of radiolabelled [3H]5-HT by washed equine platelets in vitro, and investigated the effects on this process of 4 other naturally occurring monoamines which may be released from the caecum in conditions of carbohydrate overload. The release of [3H]5-HT by platelets was also studied, since platelet accumulation and activation has been associated with acute laminitis. Release of [3H]5-HT was measured in response to platelet activating factor (PAF), unlabelled 5-HT and the indirect activation of platelets by endotoxin in the presence of blood leucocytes. Km value for the uptake of 5-HT by equine platelets was 2.4 +/- 0.6 micromol/l and the Vmax was 8.3 +/- 0.6 pmol [3H]5-HT/10(7) platelets/min. The rate of uptake of 5 micromol/l [3H]5-HT was significantly decreased by the uptake inhibitors fluvoxamine and clomipramine. The 4 other monoamines examined all inhibited the uptake of [3H]5-HT in a noncompetitive manner, decreasing Vmax by between 17 and 82%. Incubation of platelets with LPS (0.1 mg/ml) in the absence of leucocytes did not result in significant release of [3H]5-HT; however, in the presence of leucocytes 3.8 +/- 1.7 pmol [3H]5-HT/10(7) platelets (mean +/- s.e.) were released. This release was significantly inhibited by parthenolide and WEB2086, but not by aspirin. This suggests that PAF from activated leucocytes was responsible for the 5-HT release. These data show that 5-HT uptake by equine platelets is a saturable process operating most efficiently at substrate concentrations in the low micromolar range. The noncompetitive inhibition of 5-HT uptake by other naturally occurring monoamines may result in increased plasma concentrations of 5-HT, as would its release by endotoxin. Such a rise in plasma 5-HT concentrations may contribute to selective vasoconstriction in the equine digital circulation.