EFFICACY OF TOLAZOLINE AND VATINOXAN IN REDUCING ADVERSE EFFECTS OF BUTORPHANOL-AZAPERONE-MEDETOMIDINE IMMOBILIZATION IN ROCKY MOUNTAIN ELK (CERVUS CANADENSIS)

A mixture of butorphanol, azaperone, and medetomidine (BAM) is frequently used for immobilization of North American hoofstock. Common adverse effects include respiratory depression, hypoxemia, and bradycardia. In this nonblinded crossover study the efficacy of two a-2 adrenergic antagonists, tolazoline and vatinoxan, were evaluated in alleviating adverse effects of BAM in Rocky Mountain elk (Cervus canadensis). Early administration of these antagonists was hypothesized to cause an increase in heart rate, respiratory rate, partial pressure of oxygen (PaO2) and hemoglobin oxygen saturation (SpO2), as well as reduction in mean arterial blood pressure without affecting sedation levels. Eight captive adult female elk were immobilized on three separate occasions at least 14 d apart with 0.15 mg/kg butorphanol, 0.05 mg/kg azaperone, and 0.06 mg/kg medetomidine. Tolazoline (2 mg/kg IM), vatinoxan (3 mg/mg medetomidine IV) or sterile saline (2 ml IM) were administered 20 min postinduction. The BAM caused hypoxemia, bradycardia, and moderate hypertension, and because of the severe hypoxemia observed, all animals received intratracheal oxygen throughout immobilization. Heart rate, respiratory rate, rectal temperature, SpO2, PaO2, and systolic, diastolic, and mean arterial blood pressure were monitored every 5 min throughout the immobilization. Intramuscular tolazoline caused a brief but significant drop in mean arterial pressure compared with controls and a brief but nonsignificant increase in heart rate. Vatinoxan caused a significant drop in blood pressure and a brief significant increase in heart rate. Changes in respiratory rates and PaO2 were not observed with either antagonist; however, all animals received oxygen, which may have influenced this result. The depth of sedation was unchanged after administration of either drug.

Pharmacokinetics and pharmacodynamic effects of tolazoline following intravenous administration to horses

Tolazoline is an α2-adrenergic receptor antagonist, used in veterinary medicine to antagonize the central nervous system depressant and cardiovascular effects of α2 receptor agonists. The pharmacokinetics and pharmacodynamic effects of tolazoline when administered subsequent to detomidine in the horse were recently reported, although the reversal of the sedative and cardiovascular effects following detomidine may not be complete. The current study therefore investigated the pharmacokinetics and pharmacodynamic effects of tolazoline when administered as a sole agent. Nine healthy adult horses were administered tolazoline (4mg/kgIV) and blood samples were collected at time 0 (prior to drug administration) and at various times up to 72h post drug administration. Plasma samples were analyzed using liquid chromatography-mass spectrometry and resulting data analyzed using compartmental analysis. Systemic clearance, steady state volume of distribution and terminal elimination half-life were 0.820±0.182L/h/kg, 1.68±0.379L/kg and 2.69±0.212h, respectively. Tolazoline administration had no effect on chin to ground distance, but the heart rate decreased (relative to baseline) and the percentage of atrial-ventricular block increased in all horses within 2min of administration. Packed cell volume and glucose concentrations were also increased throughout the sampling period. While not commonly used as a sole agent, caution is indicated whenever tolazoline is administered since the effects may be unpredictable.

Pharmacological characterization of a Bombyx mori alpha-adrenergic-like octopamine receptor stably expressed in a mammalian cell line

Series of agonists and antagonists were examined for their actions on a Bombyx morialpha-adrenergic-like octopamine receptor (OAR) stably expressed in HEK-293 cells. The rank order of potency of the agonists was clonidine>naphazoline>tolazoline in Ca(2+) mobilization assays, and that of the antagonists was chlorpromazine>yohimbine. These findings suggest that the B. mori OAR is more closely related to the class-1 OAR in the intact tissue than to the other classes. N'-(4-Chloro-o-tolyl)-N-methylformamidine (DMCDM) and 2-(2,6-diethylphenylimino)imidazolidine (NC-5) elevated the intracellular calcium concentration ([Ca(2+)](i)) with EC(50)s of 92.8 microM and 15.2 nM, respectively. DMCDM and NC-5 led to increases in intracellular cAMP concentration ([cAMP](i)) with EC(50)s of 234 nM and 125 nM, respectively. The difference in DMCDM potencies between the cAMP and Ca(2+) assays might be due to "functional selectivity." The Ca(2+) and cAMP assay results for DMCDM suggest that the elevation of [cAMP](i), but not that of [Ca(2+)](i), might account for the insecticidal effect of formamidine insecticides.

Antagonism of detomidine sedation in the horse using intravenous tolazoline or atipamezole

REASONS FOR PERFORMING STUDY

The ability to shorten the duration of sedation would potentially improve safety and utility of detomidine.

OBJECTIVES

To determine the effects of tolazoline and atipamezole after detomidine sedation.

HYPOTHESIS

Administration of tolazoline or atipamezole would not affect detomidine sedation.

METHODS

In a randomised, placebo-controlled, double-blind, descriptive study, detomidine (0.02 mg/kg bwt i.v.) was administered to 6 mature horses on 4 separate occasions. Twenty-five mins later, each horse received one of 4 treatments: Group 1 saline (0.9% i.v.) as a placebo control; Group 2 atipamezole (0.05 mg/kg bwt i.v.); Group 3 atipamezole (0.1 mg/kg bwt i.v.); and Group 4 tolazoline (4.0 mg/kg bwt i.v.). Sedation, muscle relaxation and ataxia were scored by 3 independent observers at 9 time points. Horses were led through an obstacle course at 7 time points. Course completion time was recorded and the ability of the horse to traverse the course was scored by 3 independent observers. Horses were videotaped before, during and after each trip through the obstacle course.

RESULTS

Atipamezole and tolazoline administration incompletely antagonised the effects of detomidine, but the time course to recovery was shortened.

CONCLUSIONS AND POTENTIAL RELEVANCE

Single bolus administration of atipamezole or tolazoline produced partial reversal of detomidine sedation and may be useful for minimising detomidine sedation.

Comparative Effects of Tolazoline and Nitroprusside on Human Isolated Radial Artery

BACKGROUND

The radial artery is increasingly being used in coronary revascularization as an alternative conduit to a saphenous vein graft. Its perfect endothelial capacity provides a high patency rate comparable with the internal mammary artery (IMA). However, its spastic characteristics cause difficulties during its intraoperative preparation and may lead to early postoperative graft failure. Thus, treatment and/or prevention of radial artery spasm with an effective vasodilator agent is essential for its longevity. Endogenous vasoconstrictors, including noradrenaline, endothelin-1, and thromboxane A2, are likely to play a role in the pathogenesis of graft spasm. In the present study, we evaluated the vasorelaxant effect of tolazoline, a nonselective alpha-adrenoceptor blocker, against the contractions induced by various spasmogenic agents in an isolated human radial artery.

METHODS

Tolazoline (10(-9)-10(-4) M) or sodium nitroprusside (SNP, 10(-9)-10(-4) M) were cumulatively applied on radial artery rings precontracted submaximally with noradrenaline, endothelin-1, thromboxane analogue, U46619, or potassium chloride. In addition, some rings were pretreated with tolazoline (4 x 10(-6) M) for 30 minutes and the contractile response curve to noradrenaline was assessed in its presence.

RESULTS

Tolazoline effectively reversed noradrenaline-induced contractions in the radial artery, whereas it failed to produce remarkable relaxations on rings contracted with other spasmogenic agents, while SNP overcame the contractions induced by all spasmogens to a similar extent. In addition, brief pretreatment of radial artery rings with tolazoline significantly inhibited the contractions to noradrenaline.

CONCLUSIONS

Tolazoline is not as broadly effective as SNP against all spasmogens investigated; however, it may be effective in counteracting alpha-adrenoceptor-mediated vasospasm in human radial arteries.

EFFECTIVENESS OF ANTAGONISTS FOR TILETAMINE-ZOLAZEPAM/XYLAZINE IMMOBILIZATION IN FEMALE WHITE-TAILED DEER

A combination of tiletamine-zolazepam/xylazine (TZ/X) is effective in the chemical immobilization of white-tailed deer (Odocoileus virginianus); however, the lengthy duration of immobilization may limit its usefulness. From October to November 2002, 21 captive female deer were assigned randomly to an alpha(2) antagonist treatment to reverse xylazine-induced sedation (seven does per group). All deer were given 220 mg of TZ (4.5+/-0.4 mg/kg) and 110 mg of X (2.2+/-0.2 mg/kg) intramuscularly (IM). Antagonist treatments were either 200 mg of tolazoline (4.0+/-0.4 mg/kg), 11 mg of atipamezole (0.23+/-0.02 mg/kg), or 15 mg of yohimbine (0.30+/-0.02 mg/kg) injected, half intravenously and half subcutaneously, 45 min after the IM TZ/X injection. In addition, 10 other deer (five per group) were immobilized as before and then given tolazoline (200 mg) after 45 min, with either a carrier (dimethyl sulfoxide [DMSO]) or carrier (DMSO) plus flumazenil (5 mg) to reverse the zolazepam portion of TZ. Mean times from antagonist injection until a deer raised its head were different for alpha(2) antagonist treatments (P=0.02). Times were longer for yohimbine (62.3+/-42.7 min) than for either atipamezole (24.3+/-17.1 min) or tolazoline (21.3+/-14.3 min). Mean times from antagonist injection until standing were not different (P=0.15) among yohimbine (112.0+/-56.4 min), atipamezole (89.7+/-62.8 min), or tolazoline (52.6+/-37.2 min). A sedation score based on behavioral criteria was assigned to each deer every 30 min for 5 hr. On the basis of sedation scores, tolazoline resulted in a faster and more complete reversal of immobilization. Flumazenil treatment did not affect recovery.

A comparison of two intramuscular doses of xylazine–ketamine combination and tolazoline reversal in llamas

OBJECTIVE

To evaluate the anesthetic and cardiorespiratory effects of two doses of intramuscular xylazine/ketamine in llamas, and to determine if an intramuscular injection of tolazoline would shorten the anesthesia recovery time.

STUDY DESIGN

Prospective randomized study.

ANIMALS

Six castrated male llamas.

METHODS

Each llama received a low dose (LD) (0.4 mg kg(-1) xylazine and 4 mg kg(-1) ketamine) and high dose (HD) (0.8 mg kg(-1) xylazine and 8 mg kg(-1) ketamine). Time to sedation, duration of lateral recumbency and analgesia, pulse, respiratory rate, hemoglobin oxygen saturation, arterial blood pressure, blood gases, and the electrocardiogram were monitored and recorded during anesthesia. Three llamas in each treatment were randomized to receive intramuscular tolazoline (2 mg kg(-1)) after 30 minutes of lateral recumbency.

RESULTS

Onset of sedation, lateral recumbency, and analgesia was rapid with both treatments. The HD was able to provide at least 30 minutes of anesthesia in all six llamas. The LD provided only 30 minutes of anesthesia in two out of six llamas. Respiratory depression and hypoxemia were seen in the HD treatment during the first 10 minutes of lateral recumbency. Two llamas were severely hypoxemic during this period and were given nasal oxygen for five minutes. Heart rate decreased, but there were no significant changes in blood pressure. Tolazoline significantly shortened the duration of recumbency in the HD treatment.

CONCLUSIONS

The HD provided more consistent clinical effects in llamas than did the LD. Intramuscular tolazoline shortens the duration of lateral recumbency in llamas anesthetized with this combination.

CLINICAL RELEVANCE

Both doses appear to be very effective in providing restraint in llamas. The LD may be used for procedures requiring a short period of anesthesia or restraint. The HD could be used when a longer duration of anesthesia is desired. Supplemental oxygen should be available if using the HD. Tolazoline (IM) shortened the recovery time with this combination in llamas.

Octopaminergic modulation of synaptic transmission between an identified sensory afferent and flight motoneuron in the locust

The role of the biogenic amine octopamine in modulating cholinergic synaptic transmission between the locust forewing stretch receptor neuron (fSR) and the first basalar motoneuron (BA1) was investigated. The amines 5-hydroxytryptamine (5-HT, serotonin) and dopamine were also studied. Bath application of octopamine, 5-HT, and dopamine at concentrations of 10(-4) M reversibly decreased the amplitude of monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in BA1 by electrically stimulating the fSR axon. These effects occurred without any detectable change in either input resistance or membrane potential of BA1. The amines also reversibly decreased the amplitude of responses to acetylcholine (ACh) pressure-applied to the soma of BA1. The muscarinic antagonist scopolamine (10(-6) M) had no significant effect on the octopamine-induced decrease in ACh responses. These observations suggest that these amines potentially could physiologically depress cholinergic transmission between fSR and BA1, at least in part, by altering nicotinic rather than muscarinic cholinergic receptor function. Although the octopaminergic agonists naphazoline and tolazoline both mimicked the actions of octopamine, the receptor responsible for octopamine-mediated modulation could not be characterized since amine receptor antagonists tested on the preparation had complex actions. Confocal immunocytochemistry revealed intense octopamine immunoreactivity in the anterior lateral association center, thus confirming the presence of octopamine in neuropil regions containing fSR/BA1 synapses and therefore supporting a role for this amine in the modulation of synaptic transmission between the fSR and BA1. 5-HT-immunoreactivity, conversely, was concentrated within the ventral association centers; very little staining was observed in the dorsal neuropil regions in which fSR/BA1 synapses are located.

Comparison of the vasodilating effect of nitroglycerin, verapamil, and tolazoline in hand angiography

PURPOSE

To compare the vasodilating effect and safety of intraarterial verapamil with the long-accepted standard vasodilators nitroglycerin and tolazoline in hand angiography.

MATERIALS AND METHODS

The authors studied 25 patients who underwent brachial artery angiography. In 22 cases, there was poor or moderate visualization of the forearm and hand vasculature. To improve blood flow to the periphery, subsequent angiograms with intraarterial vasodilating agents were obtained. First, nitroglycerin was administered (n = 22). In cases of continuous poor or moderate visualization of the forearm and hand vasculature, another angiogram was obtained with verapamil (n = 21). If opacification remained poor or moderate, eventually tolazoline was injected (n = 20). To avoid pharmacologic interactions of the different vasodilating drugs, a minimum 15-minute interval between series was observed. The degree of opacification of the forearm and hand arteries was graded on a scale from 1 to 5: visualization of the forearm arteries only was defined as 1, of the forearm arteries and superficial/deep palmar arch as 2, of the forearm arteries, superficial/deep palmar arch, and digital arteries to the level of the metacarpophalangeal joints as 3, to the level of the proximal interphalangeal joints as 4, and to the distal interphalangeal joints as 5.

RESULTS

All three vasodilating agents demonstrated highly significant improvement in blood flow; verapamil and tolazoline showed statistically greater effects than nitroglycerin. Verapamil caused the fewest and least severe adverse effects.

CONCLUSION

Intraarterial verapamil and tolazoline are comparable in terms of vasodilatory efficacy in hand arteries. However, because of its favorable adverse effect profile, verapamil is recommended for optimizing visualization of the peripheral arterial vascular system.

Butorphanol/xylazine/ketamine immobilization of free-ranging Baird's tapirs in Costa Rica

Cardiopulmonary effects and the utility of a butorphanol/xylazine/ketamine combination were evaluated during twenty immobilizations of sixteen Baird's tapirs (Tapirus bairdii) between March 1996 and January of 1998 in Corcovado National Park (Costa Rica). The animals were attracted to a bait site and darted from tree platforms. The tapirs were estimated to weigh between 200 to 300 kg. Actual weights of three tapirs taken at later dates fell within the estimated range. A butorphanol, 48+/-1.84 (x +/- SE) mg/animal IM, and xylazine, 101+/-2.72 mg/animal IM, combination was used to immobilize the animals. In some instances, ketamine was used either IM or IV at 187+/-40.86 mg/animal to prolong the immobilization period in addition to the butorphanol/xylazine combination. Naltrexone was used IM to reverse butorphanol at 257+/-16.19 mg/animal. Either yohimbine, 34+/-0.61 or tolazoline at 12+/-10.27 mg/animal, was used to reverse xylazine. The mean time from dart impact to first visible effect was 4.63+/-0.50 min (x +/- SE). Mean time to sternal recumbency was 12.21+/-1.08 min. Mean time the tapirs were immobilized was 45.63+/-3.6 min. Mean time to return to sternal recumbency and standing in animals that received yohimbine and naltrexone was 3.16+/-1.06 and 5.33+/-1.45 min, respectively. Mean time to return to sternal recumbency and standing in animals that received tolazoline and naltrexone was 1.57+/-0.39 and 3.14+/-0.51 min, respectively. Cardiopulmonary parameters including heart rate, respiratory rate, body temperature, electrocardiogram, percent oxygen satoration, and indirect blood pressure were recorded. Arterial blood gas analysis was performed on four animals. A mild degree of hypoxemia was evidenced by low arterial oxygen saturations. Five of 14 (36%) animals measured had oxygen saturations below 90%. Bradycardia (heart rates <45 BPM) was an expected finding in 11 (55%) immobilizations. Induction, recovery and muscle relaxation of each immobilization was graded. Premature arousal, which occurred in six (30%) animals, was the only problem associated with the immobilizations. Butorphanol/xylazine is a recommended protocol for immobilization of calm, free-ranging tapirs lasting less than 30 min. Supplemental intravenous administration of ketamine is recommended for longer procedures. Nasal insufflation of oxygen is recommended.

Bidirectional allosteric effects of agonists and GTP at alpha(2A/D)-adrenoceptors

Agonists and GTP exert reciprocal effects on the stability of the G protein-coupled receptor/G protein complex, implying bidirectional control over the receptor/G protein interface. To investigate this relationship, we compared the ability of a series of hydroxyl-substituted phenethylamine and imidazoline agonists to stimulate [(35)S]guanosine 5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) binding in membranes from alpha(2A/D)-adrenergic receptor-transfected PC12 cells with the magnitude of the GTP-induced reduction in agonist affinity in [(3)H]rauwolscine-binding studies. Agents previously described as full and partial agonists in functional studies showed similar relative efficacies in promoting GTP binding (r = 0.97) as well as similar relative potencies (r = 0.94). Efficacy among agonists for promotion of [(35)S]GTPgammaS binding was closely correlated with the relative influence of GTPgammaS on agonist binding (r = 0.97), consistent with a bidirectional allosteric influence by agonists and GTP on receptor/G protein complexation. In an additional series of tolazoline derivatives, a range in efficacy from full agonism to strong inverse agonism was observed, depending on the presence or absence of hydroxyl substituents. Together these results suggest that agonist-induced repositioning of transmembrane helices via their hydroxyl interactions is a critical determinant of the stability of the receptor/G protein complex and therefore of agonist efficacy.

Imidazoline derivatives and agmatine induce histamine release from the rat stomach

Strips from rat glandular stomach were used to investigate whether the imidazoline derivatives clonidine, tolazoline and BDF 6143 (4-chloro-2(2-imidazolin-2-ylamino)-isoindoline) and the guanidine derivative agmatine are able to release histamine from histamine-storing cells. Histamine was detected and quantified by HPLC. Clonidine, tolazoline and agmatine concentration-dependently induced a release of histamine from the gastric strips, whereas BDF 6143 was ineffective. However, BDF 6143 abolished the release-inducing effect of clonidine and agmatine. It is concluded that imidazoline and guanidine derivatives can induce histamine release from histamine-storing cells in the stomach by a specific mechanism (possibly via imidazoline receptors) which in turn might lead to a relevant increase in gastric acid secretion.

Depolarization of membrane potential and the smooth muscle contraction by isothiocyanatobenzyl imidazoline in guinea-pig stomach circular muscle

Isothiocyanatobenzyl imidazoline (IBI) produces characteristic slowly developing contraction of many smooth muscle preparations including the circular smooth muscle of the guinea-pig stomach. Changes in the membrane potential were recorded intracellularly, and the muscle contraction induced by IBI was investigated. IBI at 100 micromol/l slowly produced a sustained depolarization of the membrane with a maximum change of approximately 15 mV. This depolarization could not be blocked by 1-hyoscyamine, 100 nmol/l. An imidazoline analogue, oxymetazoline at 1 micromol/l, did not change the resting membrane potential as observed after IBI. Significant membrane depolarization after IBI still occurred in Ca2+-free medium. During IBI-induced depolarization, sudden reduction of Na+ to 30 mmol/l in the medium reduced the depolarization slightly. IBI-induced depolarization was additive with that produced by 20 mmol/l K+ in the medium. In the presence of tetraethylammonium chloride or levcromakalim or nifedipine, IBI continued to depolarize the membrane although functional pharmacological experiments showed that the contractile effects of IBI were significantly inhibited by 30 micromol/l levcromakalim and abolished by 100 nmol/l nifedipine. At 100 micromol/l phentolamine (reported by others as an inhibitor of ATP-sensitive potassium channels) completely blocked IBI-induced contraction. Phentolamine (30 micromol/l) blocked the contractile effects of IBI by 50%. On the other hand, S(-)-Bay K 8644, a voltage-dependent calcium channel activator, was additive with the contractile response of IBI. These results indicated that IBI produced membrane depolarization and contraction of the guinea-pig stomach circular muscle, by a mechanism not involving muscarinic receptors or alpha-adrenoceptors. Even though levcromakalim, an ATP-sensitive potassium channel opener, could not inhibit IBI-induced depolarization, the ATP-sensitive potassium channel and the voltage-dependent calcium channel may be intrinsically linked with the action of IBI.

Endotracheal tolazoline: pharmacokinetics and pharmacodynamics in dogs

Tolazoline is a potent vasodilator of both arteries and veins and has a powerful effect on the pulmonary vasculature, reducing hypoxic pulmonary vasoconstriction and lowering pulmonary artery pressure. Intravenous tolazoline lowers the mean pulmonary arterial pressure and resistance and increases the cardiac index when given to infants with persistent pulmonary hypertension of the newborn (PPHN). Endotracheally administered tolazoline decreases mean pulmonary arterial pressure and pulmonary vascular resistance, and improves oxygenation without the harmful decline in systemic arterial pressure. The purpose of our study was to examine the pharmacokinetic and pharmacodynamic characteristics of endotracheal tolazoline in order to determine the relationship between endotracheal tolazoline administration, plasma concentration and its effects on the cardiovascular and respiratory systems. Tolazoline was administered endotracheally to 7 newborn dogs, and its serum concentration and the haemodynamic parameters were monitored for 270 min post-delivery. Results are expressed as median and quartiles. It was found that 15 s after dosing, tolazoline plasma concentrations started to increase significantly above baseline levels, reaching a maximum of 2.64 (1.36; 13.16) microg/ml. The extent of tolazoline absorption was 305 (148;453) microg/ min/ml. The volume of distribution was 3.4 (1.6;7.4) 1/kg. The total body clearance was 12.1 (10.9;23.9) ml/min/kg and the elimination half-life was 225 (171;303) min. Endotracheal tolazoline produced an initial short-lived decrease in mean blood pressure in all the dogs, but thereafter the blood pressure increased gradually above baseline levels. Immediately following endotracheal tolazoline significant tachycardia developed, peaking at 90 min. Subsequently, the heart rate gradually decreased and stabilized at values above baseline for 200 min. A single endotracheal dose of tolazoline is effectively absorbed and produces measurable pharmacological effects. Determining the optimal endotracheal dose of tolazoline in the clinical setting requires additional evaluation.

Sedation with xylazine and lumbosacral epidural administration of lidocaine and xylazine for umbilical surgery in calves

OBJECTIVE

To determine whether anesthesia consisting of sedation induced by intramuscular administration of xylazine hydrochloride and lumbosacral analgesia induced by epidural administration of lidocaine and xylazine is useful for umbilical surgery in neonatal calves.

DESIGN

Prospective study.

ANIMALS

6 neonatal male dairy calves.

PROCEDURE

Calves were sedated with xylazine (0.1 mg/kg [0.045 mg/lb] of body weight, i.m.), and 5 minutes later a 2% solution of lidocaine (0.18 to 0.24 ml/kg [0.08 to 0.11 ml/lb]) and xylazine (0.05 mg/kg [0.022 mg/lb]) were administered into the lumbosacral epidural space. Calves were positioned in dorsal recumbency, and the umbilical structures were resected. Local infusion of lidocaine, cranial to the umbilicus, was required in 5 of 6 calves to provide adequate analgesia. Xylazine sedation was reversed with tolazoline (1 mg/kg [0.45 mg/lb], i.v.).

RESULTS

Calves maintained adequate cardiac output and oxygen delivery throughout the procedure but were hypotensive. Reversal of xylazine-induced sedation with tolazoline caused transient sinus bradycardia and sinus arrest, accompanied by severe systemic arterial hypotension. All calves regained a suckle reflex within 10 minutes and were able to stand within 90 minutes.

CLINICAL IMPLICATIONS

Intramuscular administration of xylazine for sedation and epidural administration of lidocaine and xylazine for analgesia failed to provide satisfactory analgesia for umbilical resection without supplemental local infiltration of lidocaine. The anesthetic protocol is most useful when respiratory compromise or cost are concerns and the surgical procedure can be completed in < 1 hour. Caution should be exercised when tolazoline is administered intravenously to reverse xylazine-induced sedation in calves.

Selectivity of atipamezole, yohimbine and tolazoline for alpha-2 adrenergic receptor subtypes: implications for clinical reversal of alpha-2 adrenergic receptor mediated sedation in sheep

The alpha2-adrenergic receptor antagonists, yohimbine, atipamezole and tolazoline, are used in veterinary medicine as reversal agents for the sedative/hypnotic effects of alpha2-agonists. Ruminants have increased sensitivity to the sedative/hypnotic effects of alpha2-agonists compared to other species. The receptors mediating the sedative effects of alpha2-agonists are located primarily on locus coeruleus neurons in the pons of the lower brainstem. Four pharmacological subtypes of the alpha2-adrenergic receptor (A,B, C and D) have been identified based on differences in ligand affinity. The aim of this study was to: 1) determine the pharmacological profile of atipamezole, yohimbine and tolazoline at the four alpha2-adrenergic receptor subtypes and; 2) determine whether these agents differ in their affinities at the alpha2-adrenergic receptor present in the sheep brainstem. In inhibition binding studies against the selective alpha2-adrenergic receptor ligand [3H]-MK-912, tolazoline showed the lowest affinity for all four alpha2-adrenergic receptor subtypes compared to yohimbine and atipamezole. The affinities of yohimbine and atipamezole were similar at the alpha2A-, alpha2B- and alpha2C-adrenergic receptors but differed by approximately 100 fold at the alpha2D-adrenergic receptor. Atipamezole had a 100 fold higher affinity at the alpha2D-adrenergic receptor when compared to yohimbine. To determine the ligand binding characteristics of these agents at the alpha2-adrenergic receptor in sheep brainstem, membranes were labelled with [3H]-MK-912 and inhibition competition curves were performed. Atipamezole showed approximately a 100 fold higher affinity for the sheep brainstem alpha2-adrenergic receptor compared to yohimbine which was similar to what was observed for the alpha2D-adrenergic receptor in PC12 cells transfected with RG-20. The results from these studies suggest that atipamezole has a high affinity for the alpha2D-adrenergic receptor that appears to be the receptor subtype in sheep brainstem.

Effects of nebulized nitroprusside on pulmonary and systemic hemodynamics during pulmonary hypertension in piglets

We tested the effects of nebulized nitroprusside (Neb-NP) on pulmonary and systemic hemodynamics during pulmonary hypertension induced by hypoxia or group B streptococci infusion in piglets. Twenty-three anesthetized and mechanically ventilated piglets received Neb-NP under four experimental conditions: 1) normoxia; 2) 15 and 60 min of pulmonary hypertension induced by hypoxia; 3) after pretreatment with dipyridamole; 4) pulmonary hypertension induced by infusion of group B streptococci. In addition, Neb-NP was contrasted to nebulization of tolazoline. During hypoxia-induced pulmonary hypertension, Neb-NP significantly reduced pulmonary artery pressure [PAP; -8.4+/-0.9 (SEM) mm Hg] and pulmonary vascular resistance (-25+/-2.1%) (both p < 0.001), whereas neither systemic arterial pressure nor cardiac output changed significantly. Selective pulmonary vasodilation began within 2 min of the onset of Neb-NP, and did not wane over 1 h. In contrast, within 5 min after Neb-NP was discontinued while hypoxia persisted, PAP rose significantly. Pretreatment with dipyridamole did not enhance the pulmonary vasodilation induced by Neb-NP, but did reduce systemic arterial pressure. Nebulized tolazoline did not reduce PAP significantly, but did lower systemic arterial pressure. Selective pulmonary vasodilation induced by Neb-NP was significantly smaller during group B streptococci-induced versus hypoxia-induced pulmonary hypertension. In sum, Neb-NP produced prompt, significant, selective reduction of PAP in piglets with pulmonary hypertension. Cautious extrapolation of these findings to selected clinical conditions in human infants may be warranted.

Activation of L-type calcium channel by tolazoline derivatives: role of isothiocyanate moiety

Studies have investigated the pharmacologic mechanism of 2-(4'-isothiocyanatobenzyl) imidazoline (IBI) and analogs for interaction with imidazoline receptors (IRs), alpha-adrenergic receptors (alpha-ARs), and calcium channels in cardiovascular muscle systems. IBI differs from tolazoline by substitution of an electrophilic isothiocyanato (NCS) group. Unlike tolazoline, which is a partial alpha-AR agonist, IBI produced an irreversible, slow-onset, and sustained contraction of rat aorta with an median effective concentration (EC50) value of 5 microM, and a maximal contraction (116%) greater than that of phenylephrine (100%) and tolazoline (59%). The IBI-induced contractions were dependent on calcium channels and independent of alpha-ARs or IRs. Similarly, structure-activity relation studies in rat aortic smooth muscles on a series of synthesized IBI analogs indicated that NCS analogs, but not those without the NCS group, exhibited effects by a non-alpha-AR, non-IR, but a calcium channel-dependent mechanism. Thus the presence of an intact IBI ring in these analogs is not a requirement for these activities. Further, IBI inhibited dihydropyridine (DHP, [3H]PN 200-110 and [3H]Bay K 8644) binding to L-type calcium channels of T-tubule membranes in rabbit skeletal muscle. In contrast to nifedipine, IBI and NCS derivatives (nifedipine-NCS, naphazoline-NCS) only partially (50-88%) displaced specific binding of these radioligands. A single site of noncooperative interaction was observed for nifedipine (nH = 0.97), whereas tolazoline-NCS (IBI, nH = 1.46) and nifedipine-NCS (nH = 1.37) exhibited a positive cooperativity in binding to DHP sites. These receptor-binding data indicate that NCS derivatives bind to L-type calcium channels and interact allosterically with DHP-binding sites. Direct binding of the NCS group to specific nucleophilic protein sites of the calcium channel may be responsible for its activation and the subsequent contractile effects of IBI.

Isothiocyanatobenzyl imidazoline is an alkylating agent for I2-imidazoline binding sites in rat and rabbit tissues

Isothiocyanatobenzyl imidazoline (IBI), the 4'-NCS analogue of tolazoline, has been used to alkylate several receptor sites in rabbit iris muscles. Because of the high affinity of tolazoline for the I2-imidazoline binding sites (Ki = 16-130 nM), this study was designed to assess whether IBI is also an alkylating agent for these sites. In competition studies, IBI displayed moderate affinity (Ki approximately 2-3 microM) against I2A-imidazoline sites in the rabbit cerebral cortex and I2B-imidazoline sites in the rat cerebral cortex labelled by [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI). However, preincubation (30 min at 25 degrees C) of rat cortical and liver membranes with IBI (10(-7) M to 10(-3) M), followed by extensive washing, markedly decreased (17% to 96%) the specific binding of [3H]2-BFI to I2B-imidazoline sites. IBI (10(-5) M to 10(-3) M) also bound irreversibly to I2A-imidazoline sites in rabbit cerebral cortex but with a lesser efficacy (27% to 83% reduction of [3H]2-BFI binding). Saturation curves of [3H]2-BFI binding in the rat cerebral cortex indicated that preincubation with 10(-6) M IBI reduced the total density (Bmax) without affecting the affinity (Kd) of I2B-imidazoline sites for IBI. Acute treatments (6 h) with IBI (10 and 30 mg/kg, i.p.) also dose-dependently reduced (26% and 41%; respectively) the total density of I2B-imidazoline sites. These results demonstrate the ability of IBI to alkylate I2-imidazoline binding sites in vitro and in vivo and provide evidence for the use of IBI as a new tool for the study of the functional implications of imidazoline binding sites.

Effectiveness of tolazoline in reversing xylazine-induced sedation in calves

OBJECTIVE

To test effectiveness of IV administration of tolazoline hydrochloride in reversing xylazine hydrochloride-induced sedation in calves.

DESIGN

Prospective study.

ANIMALS

12 female and 12 male Friesian-cross calves from 5 to 7 months old.

PROCEDURE

Calves were assigned to 1 of 4 treatment groups. Calves were given xylazine (0.3 mg/kg [0.14 mg/lb] of body weight, IM). Twenty minutes later, calves were treated with saline (0.9% NaCl) solution (1 ml/50 kg [1 ml/110 lb], IV) or tolazoline (1, 2, or 4 mg/kg [0.45, 0.9, or 1.8 mg/lb], IV). Behavioral and physiologic measurements included elapsed time from xylazine administration to recumbency, arousal and standing times after reversal drug administration, heart rate, and respiratory rate.

RESULTS

Mean (+/- SD) recumbency time for all calves was 5.4 +/- 1.8 minutes. Compared with administration of saline solution, all 3 doses of tolazoline significantly decreased arousal and standing times. Mean arousal time for calves receiving saline solution was 27.8 +/- 11.5 minutes. Administration of tolazoline at 1, 2, and 4 mg/kg resulted in mean arousal times of 4.7 +/- 3.8, 0.9 +/- 0.5, and 0.7 +/- 0.3 minutes, respectively. Mean standing time for calves receiving saline solution was 38.8 +/- 2.8 minutes. Administration of tolazoline at 1, 2, and 4 mg/kg resulted in mean standing times of 14.0 +/- 11.0, 3.0 +/- 1.2, and 2.4 +/- 1.1 minutes, respectively.

CLINICAL IMPLICATIONS

For routine use, tolazoline doses of 1 to 2 mg/kg should suffice. In cattle, IV administration of tolazoline reverses pharmacologic effects of xylazine, thereby hastening recovery from xylazine-induced sedation.

The effect of detomidine and its antagonism with tolazoline on stress-related hormones, metabolites, physiologic responses, and behavior in awake ponies

Six ponies were used to investigate the effect of tolazoline antagonism of detomidine on physiological responses, behavior, epinephrine, norepinephrine, cortisol, glucose, and free fatty acids in awake ponies. Each pony had a catheter inserted into a jugular vein 1 hour before beginning the study. Awake ponies were administered detomidine (0.04 mg/kg intravenously [i.v.]) followed 20 minutes later by either tolazoline (4.0 mg/kg i.v.) or saline. Blood samples were drawn from the catheter 5 minutes before detomidine administration (baseline), 5 minutes after detomidine administration, 20 minutes before detomidine administration which was immediately before the administration of tolazoline or saline (time [T] = 0), and at 5, 30, and 60 minutes after injections of tolazoline or saline (T = 5, 30, and 60 minutes, respectively). Compared with heart rate at T = 0, tolazoline antagonism increased heart rate 45% at 5 minutes. There was no difference in heart rate between treatments at 30 minutes. Blood pressure remained stable after tolazoline, while it decreased over time after saline. Compared with concentrations at T = 0, tolazoline antagonism of detomidine in awake ponies resulted in a 55% increase in cortisol at 30 minutes and a 52% increase in glucose at 5 minutes. The change in free fatty acids was different for tolazoline and saline over time. Free fatty acids decreased after detomidine administration. Free fatty acids did not change after saline administration. After tolazoline administration, free fatty acids increased transiently. Tolazoline tended to decrease sedation and analgesia at 15 and 60 minutes postantagonism. Antagonism of detomidine-induced physiological and behavioral effects with tolazoline in awake ponies that were not experiencing pain appears to precipitate a stress response as measured by cortisol, glucose, and free fatty acids. If antagonism of an alpha-agonist is contemplated, the potential effect on hormones and metabolites should be considered.

Irreversible agonist and antagonist properties of isothiocyanatobenzyl imidazoline in albino rabbit iris muscles

The alkylating agent isothiocyanatobenzyl imidazoline (IBI) was synthesized to investigate the unique receptor interacting properties of imidazolines. On the isolated rabbit iris sphincter, IBI produced concentration-dependent responses with an EC50 of 18 mumol/1, and at the highest concentration tested the maximum contraction of the tissue was 50% of the carbachol maximum. At equiactive concentrations with the similar washing procedure, the total duration of responses to IBI and carbachol was 24 and 3 min, respectively. After repeated washing, the sphineter relaxes to the control baseline of tone but, after reexposure to IBI for 6 h, failed to contract, indicating that desensitization or irreversible block has developed. Unlike with carbachol, the sphincter contraction to IBI was not affected by atropine 1 mumol/1, indomethacin 1 mumol/1, verapamil 10 mumol/1, or nifedipine 10 mumol/1. At a higher concentration of nifedipine and papaverine 100 mumol/1, the response to IBI was blocked. Furthermore, the contractile response to IBI was abolished by Ca++ removal from the medium. Under similar conditions, 26 +/- 8% of the maximum response to carbachol was preserved. Thus influx of extracellular as well as rise in intracellular Ca++ appears vital for the contractile response to IBI. IBI did not contract the iris dilator, but shifted the concentration-response curve to the alpha-adrenoceptor activator, phenylephrine, to the right with a reduction in the maximum response. The tissue failed to regain the sensitivity to phenylephrine after 6 h of repeated washing. Phentolamine and nifedipine provided a small but significant protection of the response to phenylephrine against the irreversible block by IBI. Based on chemical and pharmacological properties of IBI, it is concluded that the molecule acts in the rabbit as an irreversible agonist on unidentified receptors of the iris sphineter and an irreversible antagonist of multiple receptors on the iris dilator. These molecular properties of IBI are clearly different from that of the parent imidazoline molecule tolazoline.

Autonomic mediation of glucagon secretion during insulin-induced hypoglycemia in rhesus monkeys

Autonomic activation mediates the majority of the increase of glucagon secretion during insulin-induced hypoglycemia in several species including dogs, mice, and rats. However, the role of the autonomic nervous system to increase glucagon during hypoglycemia in humans remains controversial, and investigations in nonhuman primates have not been previously conducted. The autonomic contribution to glucagon secretion during hypoglycemia in a nonhuman primate was examined by two independent pharmacological approaches. Glucagon responses to clamped insulin-induced hypoglycemia were compared in conscious rhesus monkeys in the presence or absence of ganglionic blockade with trimethaphan, or during combined muscarinic and adrenergic receptor blockade with atropine, propranolol, and tolazoline. Insulin-induced hypoglycemia (plasma glucose = 1.9 +/- 0.1 mmol/l) activated parasympathetic nerves to the pancreas as assessed by increased plasma pancreatic polypeptide (PP) levels (delta = 135.0 +/- 36.8 pmol/l, P < 0.01), produced sympathoadrenal activation as assessed by elevations of plasma epinephrine (EPI) (delta = 22.3 +/- 2.95 nmol/l, P < 0.0005) and norepinephrine (NE) (delta = 3.72 +/- 0.77 mmol/l, P < 0.0025) and increased plasma immunoreactive glucagon (IRG) (delta = 920 +/- 294 ng/l, P < 0.025). Nicotinic ganglionic blockade with trimethaphan prevented parasympathetic (deltaPP = 16.5 +/- 16.3 pmol/l, P < 0.01 vs. control) and sympathoadrenal (deltaEPI = 1.52 +/- 0.98 nmol/l; deltaNE = -0.62 +/- 0.24 mmol/l, both P < 0.0025 vs. control) activation during hypoglycemia and inhibited the IRG response by 70% (delta = 278 +/- 67 ng/l, P < 0.025 vs. control). Combined muscarinic and adrenergic receptor blockade reduced parasympathetic activation (deltaPP = 48.3 +/- 16.3 pmol/l, P < 0.01 vs. control) and inhibited the IRG response by a similar degree to ganglionic blockade (deltaIRG = 284 +/- 60 ng/l, P < 0.025 vs. control). These results demonstrate by two independent pharmacological approaches that autonomic activation makes a substantial contribution to increased glucagon secretion during hypoglycemia of approximately 2.0 mmol/l in a species of nonhuman primate.

Dual vasoactive effects of tolazoline on rabbit pulmonary arteries

OBJECTIVES

To determine the vasoactive effects of tolazoline on isolated rabbit pulmonary arteries.

DESIGN

Prospective, in vitro, randomized, controlled trial.

SETTING

Experimental laboratory in a university-affiliated hospital.

PARTICIPANTS

New Zealand White Rabbits.

INTERVENTIONS

The pulmonary artery rings were obtained via thoracotomy. Their vasoactive responses were assessed in the presence and absence of intact endothelium and with or without precontraction by norepinephrine (NE, 3 x 10(-6) M) or potassium chloride (KCl, 3 x 10(-2) M). Using a tissue bath preparation, cumulative concentration response curves of tolazoline were obtained at different concentrations (10(-9) to 10(-4) M) after a period of stabilization.

MEASUREMENTS AND MAIN RESULTS

Tolazoline caused vasoconstriction of isolated pulmonary arteries without any pretreatment. The magnitude of the constriction was dose related and reached 300 g/g wet tissue at a concentration of 10(-4) M. On KCl-precontracted pulmonary arteries, tolazoline caused significant dose-related vasoconstriction. On the NE-precontracted vessel rings, it elicited significant dose-dependent vasodilation up to 60% relaxation at 10(-5) M. All the above effects were endothelium independent.

CONCLUSIONS

Tolazoline has dual endothelium-independent vasoactive effects, causing vasoconstriction on isolated rabbit pulmonary arteries, either untreated or precontracted with KCl, and vasodilation on those precontracted with NE. Tolazoline may act as a competitive alpha-adrenoceptor blocking agent.

Tolazoline decreases survival time during microwave-induced lethal heat stress in anesthetized rats

Effects of alpha-adrenergic antagonists have been studied during environmental heating but not during microwave-induced heating. Tolazoline may exert some of its effects via alpha-adrenegic blockade. In the present study, ketamine-anesthetized Sprague-Dawley rats were exposed to 2450-MHz microwaves at an average power density of 60 mW/cm2 (whole-body specific absorption rate of approximately 14 W/kg) until lethal temperatures were attained. The effects of tolazoline (10 mg/kg body weight) on physiological responses (including changes in body temperature, heart rate, blood pressure, and respiratory rate) were examined. Survival time was significantly shorter in the tolazoline group than in saline-treated animals. In general, heart rate and blood pressure responses were similar to those that occur during environmental heat stress. Heart rate, however, was significantly elevated in animals that received tolazoline, both before and during terminal microwave exposure. It is possible that changes associated with the elevated heart rate (e.g., less cardiac filling) in tolazoline-treated animals resulted in greater susceptibility to microwave-induced heating and the lower survival time.

Production of pulmonary vasodilation by tolazoline, independent of nitric oxide production in neonatal lambs

OBJECTIVE

To determine whether tolazoline reduces pulmonary vascular resistance (PVR) by means of endogenous nitric oxide production.

DESIGN

Thirty newborn lambs (2 to 7 days of age) were anesthetized with pentobarbital, and their lungs were ventilated through an endotracheal tube. Intravascular catheters were placed in the left ventricle, descending aorta, right atrium, and pulmonary artery for continuous monitoring of intravascular pressures. Cardiac output was measured with radiolabeled microspheres. Arterial carbon dioxide pressure and pH were maintained in a normal range throughout the experiments. Animals were randomly assigned to the following groups: group 1, lungs ventilated with a hypoxic gas mixture and administered tolazoline; group 2, given N omega-nitro-L-arginine (L-NA) (5 mg/min intravenously for 60 minutes) and tolazoline; group 3, given L-NA with hypoxia and tolazoline. Acetylcholine (0.5 microgram/kg) was injected into the right atrium to assess pulmonary nitric oxide synthase activity before and after the L-NA infusion. Data were analyzed by analysis of variance.

RESULTS

L-NA inhibited the acetylcholine-induced reduction in mean pulmonary artery pressure (MPAP) by more than 75%. Hypoxia and L-NA increased both MPAP and PVR. Tolazoline produced immediate reductions in both MPAP and PVR in all three groups (group 1, 27% +/- 3% and 50% +/- 5%; group 2, 34% +/- 5% and 50% +/- 6%; and group 3, 31% +/- 4% and 46% +/- 5%, respectively).

CONCLUSIONS

These results suggest that tolazoline produces vasodilation independent of nitric oxide production. Understanding the mechanism by which tolazoline produces pulmonary vasodilation may provide insight into the clinical use of this drug and information regarding other potential endogenous mediators of pulmonary vasomotor tone in the neonate.

Effects of alpha 2-adrenergic receptor agonist and antagonist drugs on cholinergic contraction in bovine tracheal smooth muscle in vitro

Effects of alpha 2-adrenergic receptor stimulation on the cholinergic contractile response of bovine tracheal smooth muscle were studied. To determine the presence and function of alpha2-adrenergic receptors on cholinergic nerves innervating bovine tracheal muscle, effects of 2 alpha 2-adrenoceptor agonists and an antagonist were determined. Muscular contractions were elicited by either electrical field stimulation (EFS) or exogenous acetylcholine (ACH). The contractile response to EFS and exogenous ACH was examined for each tissue. Electrical field stimulation of bovine tracheal smooth muscle caused contractions that were completely abolished by atropine, indicating that predominant excitatory innervation of bovine trachea is cholinergic. The alpha 2 adrenoreceptor agonists clonidine and medetomidine (10(-6) M to 10 (-4) M) concentration-dependently inhibited the contractile response to EFS but not the response to exogenous ACH. Contractions induced by EFS were significantly (P < 0.05) inhibited in clonidine (10(-4) M) - treated tissues at low frequencies (0.01 to 10 Hz), whereas medetomidine (10(-5) M, 10(-4) M) inhibitory effects of the alpha 2-adrenoreceptor agonists clonidine and medetomidine were attenuated by the a2-adrenoreceptor antagonist tolazoline. The alpha 2-agonists used in this study appear to cause prejunctional inhibition of cholinergic nerves, because the smooth muscle contractions elicited by EFS, but not exogenous ACH, were inhibited, compared with controls.

Hydrops-induced changes in cochlear blood flow

Laser Doppler flowmetry was used to assess cochlear blood flow (CBF) in the hydropic ear in four experiments. 1) The increase in CBF elicited by local electrical stimulation of the cochlea in the hydropic ear was compared to that observed in normal controls. The magnitude of the evoked CBF change was reduced by approximately 30% in the hydropic ear compared to the normal ear. 2) The reduction in CBF evoked by direct electrical stimulation of the superior cervical ganglion was reduced by approximately one third in the hydropic ear compared to a normal ear. 3) Rhythmic (flux motion or vasomotion) variations in CBF, observed in association with lower blood pressure and thought to extend the autoregulatory range in an organ system, were reduced or eliminated in the hydropic ear. 4) The autoregulatory response to a decreased perfusion pressure, produced by decreased cardiac output, was clearly reduced relative to control in the hydropic ear. These findings represent the first report of significant CBF changes with hydrops. They are consistent with reports of increased sensitivity of the hydropic ear to trauma and stress and may be relevant considerations in the treatment of hydrops in humans.

The role of alpha-adrenergic mechanisms within the area postrema in dopamine-induced emesis

Intracerebroventricular injection of dopamine (0.5-4.0 mg) produced dose-dependent and short-lasting emesis (1-8 min) in cats, which was abolished after ablation of the area postrema. Relatively selective alpha 2-adrenoceptor antagonists (yohimbine and idazoxan) and a mixed alpha 1- and alpha 2-adrenoceptor antagonist (tolazoline), but not a non-selective alpha 1-adrenoceptor antagonist (prazosin), injected intracerebroventricularly inhibited the emesis induced by intracerebroventricular dopamine. However, dopamine receptor antagonists (chlorpromazine, droperidol, spiperone, domperidone, triflupromazine, sulpiride and metoclopramide), an antimuscarinic drug (atropine), a ganglionic blocking agent (mecamylamine), an opioid receptor antagonist (naloxone) and a 5-HT receptor antagonist (methysergide), all injected intracerebroventricularly, had no significant effect on emesis evoked by intracerebroventricular dopamine. The emetic response to intracerebroventricular dopamine was attenuated in cats pretreated with intracerebroventricular reserpine, 6-hydroxydopamine, alpha-methyl-p-tyrosine and hemicholinium-3. It is postulated that dopamine-induced emesis is mediated through the release of noradrenaline acting at alpha 2-adrenoceptors and that it depends on the integrity of monoaminergic and possibly cholinergic structures within the area postrema. It appears, therefore, that the emetic effect of intracerebroventricular dopamine is mediated by adrenergic rather than dopaminergic mechanisms in the area postrema, at least in the cat.

Pharmacologic modulation of the cutaneous vasculature in the isolated perfused porcine skin flap

The isolated perfused porcine skin flap (IPPSF), an ex vivo model system used in cutaneous toxicology and pharmacology, is capable of assessing the percutaneous absorption of vasoactive compounds. However, the vascular responses of the IPPSF to classical pharmacologic agents have not been calibrated. The ability of acetylcholine, nitroglycerin, tolazoline, and norepinephrine to affect vasculature resistance and glucose utilization was investigated in the IPPSF. Norepinephrine infusions between 10(-7) and 10(-5) M increased vascular resistance in a dose-dependent manner; half-maximal (EC50) and maximal responses occurred at 3.18 x 10(-6) and 10(-5) M, respectively. In non-preconstricted flaps, neither acetylcholine, nitroglycerin, nor tolazoline vasodilated the IPPSF; however, acetylcholine, nitroglycerin, and tolazoline each lowered vascular resistance in a dose-dependent manner in norepinephrine-preconstricted flaps. Maximal relaxation was induced at 10(-4), 10(-6), and 5 x 10(-5) M, by tolazoline, acetylcholine, and nitroglycerin, respectively, whereas the EC50 values were 2.88 x 10(-7), 1.35 x 10(-8), and 1.72 x 10(-7) M, respectively. In flaps pretreated with norepinephrine and methylene blue (a potential blocker of edothelium-derived relaxing factor), no concentration of acetylcholine, and only the highest concentration of nitroglycerin, lowered vascular resistance. In non-preconstricted flaps, glucose utilization decreased in norepinephrine-infused flaps, increased in nitroglycerin- and tolazoline-infused flaps, and was biphasic in acetylcholine-infused flaps. These results indicate that the IPPSF responds to pharmacologic agents in a manner similar to classic in vitro and in vivo models. Thus, the IPPSF would be a relevant model for investigating the delivery and/or toxicity of pharmacologically active compounds.

The effect of alpha 2-adrenoceptor antagonists in isolated globally ischemic rat hearts

The alpha 2-adrenoceptor antagonist, yohimbine, has been reported to protect hypoxic myocardium. Yohimbine has several other activities, including 5-HT receptor antagonism, at the concentrations at which protection was found. Therefore we designed a study to determine if yohimbine was protecting ischemic myocardium via antagonism of alpha 2-adrenoceptors. In isolated globally ischemic rat hearts, the effects of two structurally distinct classes of alpha 2-adrenoceptor antagonists, the indole alkaloids (yohimbine and rauwolscine) and the imidazolines (idozoxan and tolazoline) were investigated. Pretreatment with yohimbine (1-10 microM) caused a concentration-dependent increase in reperfusion left ventricular developed pressure and a reduction in end diastolic pressure and lactate dehydrogenase release. The structurally similar compound rauwolscine (10 microM) also protected the ischemic myocardium. In contrast, idozoxan (0.3-10 microM) or tolazoline (10 microM) had no protective effects. The cardioprotective effects of yohimbine were partially reversed by 30 microM 5-HT. These results indicate that the mechanism for the cardioprotective activity of yohimbine may involve 5-HT receptor antagonistic activity.

Synthesis and biological activities of a new set of irreversibly acting 2-(4'-isothiocyanatobenzyl)imidazoline analogs in rat thoracic aorta

IBI [2-(4'-isothiocyanatobenzyl)imidazoline, 3] has been shown to cause slow-onset, long-lasting contractions of rat thoracic aorta through a non-alpha-adrenergic receptor (non-alpha-AR) mediated mechanism. A series of IBI-related anlogs 7-14 and 16 was prepared to determine the structural requirements for the interaction with non-alpha-AR in rat aortic strips. All IBI analogs produced concentration-dependent contractile responses on rat thoracic aorta. Whereas the actions of analogs 7, 14, and 16 were partly mediated by alpha-ARs, the stimulatory activities of the remaining IBI analogs were unaffected by phenoxybenzamine pretreatment, suggesting that a non-alpha-adrenergic mechanism is involved. We have shown that the contractile actions of IBI and analogs 10-13 were not blocked with the imidazoline/guanidinium receptive site (IGRS) ligands idazoxan, cirazoline, or clonidine. However, the calcium channel blockers nifedipine or verapamil shifted the concentration-response curve of IBI and its analogs 10-13 to the right and reduced the maximal contractile responses. The action of IBI on rat thoracic aorta was reduced by the omission of extracellular calcium in the medium. These results suggest that the stimulatory activities of IBI and analogs 10-13 are not related to the activation of alpha-AR or IGRS receptors and are likely coupled to the voltage-dependent Ca2+ channels.

[Effect of intracerebroventricular injection of norepinephrine and serotonin on somatosensory evoked potentials in rabbits]

The components of somatosensory evoked potentials (SEP), P7, N9, P12, and N15, were recorded from dura mater before and after intracerebroventricular injection (icv) of monoamines. The peak latencies of P12 and N15 were prolonged and the amplitudes of P12 and N15 were decreased following icv norepinephrine (NE) 1 microgram.microliter-1 and serotonin (5-HT) 1 microgram.microliter-1. Generally, P12 and N15 restored to normal 1-2 h after icv NE and 5-HT. Dopamine (DA) 1 microgram.microliter-1, however, did not show any significant effect on SEP. Tolazoline (Tol) 2.5 micrograms.microliters-1 markedly potentiated the inhibitory effect of NE, but propranolol (Pro) 1 microgram.microliter-1 weakened the effect of NE on SEP. These results indicated that icv NE and 5-HT exerted an inhibitory effect on the somatosensory afferent function from thalamus to cortex and/or intercortical areas and alpha-adrenoceptor had an influence on the inhibitory effect of NE.

Persistent pulmonary hypertension in high-risk congenital diaphragmatic hernia patients: incidence and vasodilator therapy

Survival of congenital diaphragmatic hernia patients depends on the gravity of pulmonary hypoplasia and persistent pulmonary hypertension (PPH). Many vasoactive drugs have been used in the treatment of PPH, but often they also lower peripheral resistance, leading to a significant drop in arterial blood pressure. The incidence of PPH in 52 high-risk diaphragmatic hernia patients and the results of treatment with tolazoline and prostacyclin were evaluated in a study lasting 52 months and involving 52 patients. High-risk patients require ventilatory support within 6 hours after birth. Study parameters were alveolar-arterial oxygenation difference (AaDO2), oxygenation index (OI), and mean arterial blood pressure (MABP), measured at set times before and after administration of tolazoline or prostacyclin. Twenty-one patients had documented episodes of PPH (46%), and 18 of them died. Tolazoline did not lower AaDO2 and OI values, but MABP dropped significantly. Prostacyclin caused a significant decrease of AaDO2 and OI values without an effect on MABP. We concluded: (1) PPH presented in 46% of our patients, associated with a high mortality rate; (2) tolazoline is not an effective dilator of the pulmonary vascular bed and lowers MABP; and (3) prostacyclin is an effective pulmonary vasodilator as reflected by ventilation parameters without systemic side effects; it does not affect overall outcome but can used as a "bridge" to extracorporeal membrane oxygenation.

Model describing transdermal iontophoretic delivery of lidocaine incorporating consideration of cutaneous microvascular state

A three-compartment pharmacokinetic model describing percutaneous absorption of iontophoretically driven topically applied lidocaine in the isolated perfused porcine skin flap is presented. Delivery from the active (drug-dosed) electrode to skin is estimated as a ramp input profile. Model parameters were estimated separately for dosing (4 h current-on) and washout (4 h current-off) periods in experiments with coadministered vasoactive drugs [tolazoline (vasodilator) and norepinephrine (vasoconstrictor)] and controls (lidocaine alone). The model presented was able to predict 8-h lidocaine absorptions and compartmental mass profiles for each of the three treatments, was able to document vascular effects of co-iontophoresed vasoactive compounds, and gives insight into the factors that modulate cutaneous disposition of iontophoretically delivered lidocaine in a biologically relevant model approximating in vivo delivery.

Optimization of distal artery opacification in peripheral arteriography: comparison between nitroglycerin, tolazoline and buflomedyl

A prospective double blind randomized comparative study was performed in patients with insufficient distal artery opacification during peripheral arteriography, in order to compare the vasodilating capabilities of intraarterially injected nitroglycerin, tolazoline and buflomedyl versus placebo. All three vasodilating agents showed statistically significant improvement of distal artery opacification as compared to the placebo (p < 0.0001). Overall results were better for buflomedyl (p = 0.231) then for tolazoline (p = 0.142) and nitroglycerin, but these differences were statistically not significant. It is concluded that buflomedyl is an excellent vasodilating agent in peripheral arteriography.

Influence of yohimbine and tolazoline on the cardiovascular, respiratory, and sedative effects of xylazine in the horse

To determine the effects of yohimbine and tolazoline on the cardiovascular, respiratory and sedative effects of xylazine, four horses were sedated with xylazine and treated with either yohimbine, tolazoline or saline. Xylazine was administered as an intravenous (i.v.) bolus (1.0 mg/kg) followed by a continuous infusion at the rate of 12 micrograms/kg/min. Heart rate, respiratory rate, mean arterial pressure, arterial blood gases, and the chin-to-floor distance were recorded throughout the experiment. After 60 min, either yohimbine or tolazoline was administered i.v. in incremental doses until reversal of sedation (defined as the return of the chin-to-floor distance to baseline values) was achieved. A control group in which a saline bolus was administered instead of an antagonist drug was included for comparison. The average dose of yohimbine administered was 0.12 +/- 0.02 (SEM) mg/kg. While the average dose of tolazoline was 7.5 +/- 1.1 mg/kg. Both tolazoline and yohimbine antagonized the ventricular bradycardia and A-V conduction disturbances observed with xylazine administration. No change in mean arterial pressure was observed with xylazine or yohimbine administration, but tolazoline caused persistent mild systemic hypertension. There were no clinically significant changes in respiratory rate or arterial blood gas values with administration of either xylazine, yohimbine or tolazoline. The chin-to-floor distance decreased significantly with xylazine administration and increased significantly with administration of either yohimbine or tolazoline. In conclusion, both yohimbine and tolazoline successfully antagonized the cardiovascular and CNS depression associated with xylazine administration.

Intracranial pressure reduction by a central alpha-2 adrenoreceptor agonist after subarachnoid hemorrhage

Pharmacological manipulation of cerebral venous blood volume is a theoretical approach to reduce elevated intracranial pressure (ICP). Microapplication of alpha-2 adrenoreceptor agonists has been shown to constrict pial veins selectively. This report explores the physiological effects of the intravenous alpha-2 agonist xylazine in a canine model of raised ICP after subarachnoid hemorrhage (mean arterial pressure, heart rate, and ICP were measured and compared in five groups: normal saline [n = 4], xylazine [0.05-1.00 mg/kg] [n = 28], tolazoline [a semiselective alpha-2 antagonist, 5 mg/kg] [n = 6], tolazoline [5 mg/kg] plus xylazine [1.0 mg/kg] [n = 7], and phenylephrine [0.008 mg/kg], a selective alpha-1 agonist [n = 3]). Treatment with xylazine produced a significant (P < 0.01), transient, dose-dependent reduction in ICP that was blocked by pretreatment with tolazoline. Treatment with tolazoline alone produced significant (P < 0.01) increases in ICP and mean arterial pressure. Treatment with phenylephrine produced significant (P < 0.01) increases in mean arterial pressure but had no affect on ICP. These results raise the possibility of using an alpha-2 adrenoreceptor agonist for the treatment of elevated ICP after brain injury.

High-frequency ventilation in newborn lambs after intra-uterine creation of diaphragmatic hernia

Infants with congenital diaphragmatic hernia (CDH) die, because their lungs are hypoplastic and their pulmonary vascular resistance remains elevated after birth. In human newborns, it is difficult to appreciate the benefit of new therapeutic approaches, because the pathological findings are not uniform, the disease is rare and the clinical criteria for poor prognosis with conventional therapy are uncertain. To study the benefit of high-frequency ventilation (HFV) the use of Tolazoline in CDH, we created a diaphragmatic defect in sheep fetuses at 0.6 gestation and studied full-term newborns after a caesarian section. A sternotomy was performed to place catheters and flow probes on the aorta and pulmonary artery and to clamp the ductus arteriosus and the left pulmonary artery. Twins were used as control, and the CDH lambs were either ventilated with conventional ventilation (CV) or HFV. 23 ewes were operated upon with a 22% abortion rate and 31 newborn lambs (10 controls and 21 CDH) were studied. A complete gasometric and hemodynamic study was performed in 23 lambs (7 controls, 8 CDH with CV and 8 CDH with HFV). Clinical and pathological findings of the lambs with CDH were very similar to severe CDH in humans with bilateral lung hypoplasia, severe respiratory distress, high pulmonary vascular resistance and severe hypoxemia. HFV dramatically improved CO2 elimination, allowed less aggressive ventilation, and was associated with higher flows and lower systemic and pulmonary vascular resistance. However, HFV did not improve oxygenation leaving the newborn with severe hypoxemia associated with massive intrapulmonary foramen ovale shunting from right to left.

Stellate ganglion drives sympathetic regulation of cochlear blood flow

The functional properties of the sympathetic fibers innervating the cochlea are not well understood. Adrenergic fibers supplying lateral wall structures of the cochlea have been observed terminating on radiating arterioles and collecting venules. Adrenergic fibers also terminate as 'free' endings in the spiral osseous lamina. Stimulation or transection of sympathetic fibers originating from superior cervical chain and supplying the cochlea have yielded mixed results concerning many aspects of cochlea physiology. In order to clarify the origin of sympathetic fibers and their role in control of cochlear blood flow (CBF), we examined the effect of electrical stimulation of the stellate ganglion (ESS) and transection of postganglionic fibers from the stellate on CBF measured by laser Doppler flowmetry and on systemic blood pressure (BP) in the guinea pig. ESS produced a 20-35% increase in BP and 10-15% decrease in CBF. The decrease in CBF presumably reflects the net result of increased perfusion pressure, local autoregulatory mechanisms, and a direct sympathetic-induced vasoconstriction. Section of the immediate postganglionic sympathetic trunk had little or no effect on the ESS-related change in BP; however, it eliminated the CBF reduction. Intravenously infused beta 1-blocker diminished the BP increase due to ESS, while the electrically-evoked reduction in CBF remained. Local application of an alpha-blocker on the round window blocked ESS evoked CBF reductions without altering the BP increase. These data confirm the functional role of sympathetic projections from the stellate ganglion in CBF regulation in guinea pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary bladder serosal chemoreceptor induced cardio-respiratory responses: possible pathway

There is limited experimental information about pain originating from the urinary bladder. In the present study application of 3-5 ml of 1% ammonium oxalate, 1% potassium chloride, 100m M citric acid, IM ammonium chloride, 1% oxalic acid, 0.5% sodium hydroxide, or 2 micrograms/ml bradykinin, to the serosal surface of the urinary bladder in anaesthetized dogs, resulted in an increase in heart rate, rise of both systolic and diastolic blood pressures and increase in respiratory rate and depth. These facilitatory cardio-respiratory responses were coupled with powerful contractions of the urinary bladder wall. By contrast, mucosal application of the chemicals did not bring about any significant change. The cardio-respiratory responses obtained were completely abolished on serosal application of procaine (1%), section of the hypogastric nerves or by spinalectomy at T8. Bilateral cervical vagotomy and pelvic nerve section did not modify the responses. However, the blood pressure responses were abolished by the administration of tolazoline hydrochloride, indicating a major role of sympathetics in this nociceptive reflex.

Imidazoline antagonists of alpha 2-adrenoceptors increase insulin release in vitro by inhibiting ATP-sensitive K+ channels in pancreatic beta-cells

1. Islets from normal mice were used to study the mechanisms by which imidazoline antagonists of alpha 2-adrenoceptors increase insulin release in vitro. 2. Alinidine, antazoline, phentolamine and tolazoline inhibited 86Rb efflux from islets perifused with a medium containing 3 mM glucose, i.e. under conditions where many adenosine 5'-triphosphate (ATP)-sensitive K+ channels are open in the beta-cell membrane. They also reduced the acceleration of 86Rb efflux caused by diazoxide, an opener of ATP-sensitive K+ channels. 3. ATP-sensitive and voltage-sensitive K+ currents were measured in single beta-cells by the whole-cell mode of the patch-clamp technique. Antazoline more markedly inhibited the ATP-sensitive than the voltage-sensitive current, an effect previously observed with phentolamine. Alinidine and tolazoline partially decreased the ATP-sensitive K+ current. 4. The four imidazolines reversed the inhibition of insulin release caused by diazoxide (through opening of ATP-sensitive K+ channels) or by clonidine (through activation of alpha 2-adrenoceptors) in a concentration-dependent manner. Only the former effect correlated with the ability of each drug to increase control insulin release stimulated by 15 mM glucose alone. 5. It is concluded that the ability of imidazoline antagonists of alpha 2-adrenoceptors to increase insulin release in vitro can be ascribed to their blockade of ATP-sensitive K+ channels in beta-cells rather than to their interaction with the adrenoceptor.

Relaxation effects of diltiazem, verapamil, and tolazoline on isolated cat ophthalmociliary artery

This study investigates the ability of an alpha 1-adrenergic blocker, tolazoline, two Ca2+ entry channel blockers, verapamil and diltiazem, acetylcholine and Ca(2+)-free solutions to relax alpha 1-adrenergically activated cat ophthalmociliary artery ring segments. The form of a contraction elicited with adrenaline, noradrenaline or phenylephrine was shown to depend on the passive tension set. As passive tension was increased, which is equivalent to increasing blood pressure in vivo, the alpha 1-adrenergic elicited contraction became more sustained. Incubation with the Ca2+ channel blockers diltiazem and verapamil was ineffective in reducing the phasic response to the alpha 1-adrenergic agonists whereas addition of these blockers in increasing concentrations during a sustained adrenergic contraction caused a graded and significant relaxation. This implies that therapeutic use of Ca2+ channel blockers may act directly on the ophthalmociliary artery to inhibit the autonomic control of sustained tone in this vessel. Removal of exogenous Ca2+ from the bathing medium initially reduced the phasic component and eliminated the tonic component of the alpha 1-adrenergic contraction. Repeated exposure of the ring segment to Ca(2+)-free medium further reduced the phasic component elicited with alpha 1-adrenergic contraction. Pre-incubation with the alpha 1-adrenergic blocker Tolazoline reduced the alpha 1-adrenergic antagonist phasic response only at high concentrations (10(-4) M), whereas Tolazoline applied during the tonic phase was an effective relaxant at lower concentrations. Acetylcholine added to the bath during an alpha 1-adrenergic contraction induced relaxation at low concentrations and contraction for higher concentrations. The relaxation was eliminated by removal of the endothelial cell layer. These data support the notion that the phasic component of an alpha 1-adrenergic contraction is mainly dependent on intracellular Ca2+ stores whereas the tonic component relies almost exclusively on extracellular Ca2+. It is suggested that Ca2+ channel blockers and alpha 1-adrenergic antagonists may potentially play a role in relaxing the ophthalmociliary artery to improve the ocular circulation in vascular diseases in which there is considerable vascular tone present.

Effects of imidazole compounds on catecholamine release in adrenal chromaffin cells

1. Effects of imidazole compounds and guanabenz on the stimulus-evoked release of catecholamine (CA) were studied in cultured bovine adrenal chromaffin cells. 2. Clonidine, oxymetazoline, phentolamine, chlorpheniramine, and guanabenz inhibited acetylcholine (ACh)-evoked CA release in a dose-dependent manner, but not high K(+)-evoked release. 3. The inhibition by these compounds was not antagonized by nonimidazole and nonguanidine alpha 2-antagonists (yohimbine and phenoxybenzamine) but was significantly antagonized by tolazoline (imidazole alpha 2-antagonist) and cimetidine (imidazole H2-antagonist). Moreover, tolazoline by itself augmented the ACh-evoked, but not the high K(+)-evoked, CA release. 4. Although chlorpheniramine and cimetidine are antagonists for H1 and H2 histaminergic receptors, the site of action for these compounds in our results seemed to differ from the histamine receptors. 5. These results suggest that the inhibitory action of imidazole compounds and guanabenz on ACh-evoked CA release in adrenal chromaffin cells is mediated through an imidazole receptor. Adrenal chromaffin cells may contain an endogenous clonidine-displacing substance (CDS) which has been found in adrenal gland and brain as an endogenous ligand for imidazole receptors. Thus, CDS may have a regulatory role in the stimulus-secretion coupling in these cells.

Determination of lidocaine concentrations in skin after transdermal iontophoresis: effects of vasoactive drugs

The purpose of this study was to investigate the effect of vasoactive drugs on transdermal lidocaine inotophoresis by measuring the concentrations of radiolabeled lidocaine which has penetrated the skin. Previous studies had demonstrated that coinotophoresis of vasoactive drugs could modulate the transcutaneous flux of lidocaine and suggested that a dermal depot of lidocaine was involved. To address this, lidocaine hydrochloride (14C) was iontophoresed in vivo in anesthetized weanling pigs either alone or with the vasodilator tolazoline or the vasoconstrictor norepinephrine. Tissue cores under the active electrode were then collected, quick-frozen, and sectioned on a cryostat, and then the radioactivity was determined in each 40-microns section. Coiontophoresis with norepinephrine resulted in increased concentrations of lidocaine in skin up to a depth of 3 mm. These concentrations decreased to lidocaine-alone levels after a 4-hr washout. Tolazoline decreased tissue concentrations of lidocaine. Concentrations were intermediate when lidocaine alone was administered. These studies support the hypothesis that coiontophoresis of vasoactive drugs modulates the transdermal delivery of lidocaine, in part by altering the cutaneous "depot."

[Respiratory effects of microinjection of three kinds of neurotransmitters in ventromedial region of nucleus facialis]

The effects of microinjection of three kinds of neurotransmitters in ventromedial region of nucleus facialis (VMNF) on respiration were observed in vagotomized, spontaneously breathing rabbits anesthetized with urethane. Microinjection of adrenaline in VMNF induced a marked increase in respiratory rate and amplitude of integrated phrenic activity associated with an increase in the initial rate of rise of inspiratory activity. However, alpha-receptor antagonist tolazoline elicited marked decreases in respiratory rate accompanied by little or no changes in amplitude of integrated phrenic activity and initial rate of rise of inspiratory activity. The respiratory effects of microinjection of adrenaline were blocked by previous injection of tolazoline. Microinjection of GABA and glycine resulted in a decrease of respiratory frequency. These results suggest that adrenaline, GABA and glycine may modulate respiration by acting on VMNF neurons as neurotransmitters.