Effect of St. John's wort on free radical production

St. John's wort (Hypericum perforatum) is an herbal compound used in the treatment of burns, bruises, swelling, anxiety, and most recently, mild to moderate depression. The present study was designed to evaluate the antioxidant properties of St. John's wort in both cell-free and human vascular tissue. The experiment was performed initially in a cell-free system using Krebs buffer and a combination of xanthine/xanthine oxidase to initiate the production of the superoxide radical. Additionally, human placental vein was incubated in Krebs buffer without xanthine or xanthine oxidase to study the effects of St. John's wort on human tissue in vitro. Commercially available formulations of St. John's wort, standardized to either hypericin or hyperforin, were dissolved in an alkaline solution, and the following dilutions were made: 1:1, 1:2.5, 1:5, 1:7.5, 1:10, and 1:20. Lucigenin chemiluminescence was used to measure free radical production in both systems. A pro-oxidant effect was seen at the highest concentration, 1:1. Lower concentrations revealed antioxidant properties of the compound. All dilutions below 1:1 in both systems showed a dose-related inverse relationship of superoxide inhibition. The largest suppression was seen at the most dilute concentration, 1:20. The addition of 10(-3) M tiron inhibited the chemiluminescence signal, thereby confirming the production of superoxide. The results of this study suggest that St. John's wort inhibits free radical production in both cell-free and human vascular tissue.

Determination of arginine and methylated arginines in human plasma by liquid chromatography-tandem mass spectrometry

Nitric oxide (NO) is synthesized from L-arginine (ARG) catalyzed by the enzyme nitric oxide synthase (NOS) and is important in the regulation of vascular tone, neurotransmission and host defense. N,N-Dimethyl L-arginine (asymmetric dimethylarginine, ADMA) and N-monomethyl L-arginine (MMA) are endogenous inhibitors of NOS. N,N'-Dimethyl L-arginine (symmetric dimethylarginine, SDMA), the inactive enantiomer of ADMA is also known to be present endogenously. A simple, sensitive and fast LC-MS-MS method was developed to extract and quantitate ADMA, SDMA, MMA and ARG from human plasma. 13C6-ARG was used as the internal standard for the assay. Protein precipitation using acetonitrile gave good recoveries of all the compounds from plasma. The compounds were separated by HPLC in less than 15 min using a silica column. The limits of detection for this method were found to be approximately 1 ng/ml for ARG, ADMA and SDMA and 2.5 ng/ml for MMA. The total LC-MS-MS analysis time is less than 15 min making this the fastest and most specific method reported to date. The use of an isocratic liquid chromatographic separation makes this method optimal for high sample throughput. The inter- and intra-day precision (% RSD) and accuracy (% error) for this assay were less than 15%. The average concentrations of ARG, ADMA, SDMA and MMA in plasma from 20 human subjects were found to be 10.9+/-4.1 microg/ml, 25.1+/-9.4 ng/ml, 33.2+/-13.1 ng/ml and 19.6+/-3.8 ng/ml, respectively.

Distribution of endothelin receptors in saphenous veins of African Americans: implications of racial differences

The relative density of endothelin-receptor subtypes A and B (ET(A) and ET(B), respectively) on endothelial and smooth muscle cells is the major determinant of the contractile response to endothelin-1 (ET-1). To investigate the effects of race on the distribution of ET receptors, the endothelin-receptor subtypes on membrane fractions prepared from saphenous veins obtained from African-American patients undergoing coronary bypass surgery were analyzed. Similar studies were repeated with endothelium-denuded samples to study the role of endothelium- and smooth muscle-derived ET(B) receptors. Competitive-binding experiments on membrane fractions by using [125I]-ET-1 and unlabeled ligands, ET-1, ET-3, sarafotoxin-6-c, and BQ-123 yielded two classes of binding sites on endothelium-intact vessels from both female and male subjects. In women, the maximal binding capacities were 91+/-6 and 67+/-13 fmol/mg protein for ET(A) and ET(B) receptors, respectively; the corresponding values in men were 178+/-19 and 127+/-13 fmol/mg protein. Similar experiments performed with endothelium-denuded saphenous veins indicated the presence of both receptor subtypes on vascular smooth muscle, in contrast to our earlier report on the presence of only ET(A) receptors on vessels obtained from white Americans. Our findings demonstrate that the ratio and the density of ET receptors are different in black and white Americans.

St. John's wort: a new alternative for depression?

OBJECTIVE

The primary purpose of this article is to review the existing literature concerning the therapeutic uses, adverse effects, and possible drug interactions of St. John's wort (Hypericum perforatum) as compared to other antidepressant medications.

METHODS

Reference material was obtained through database searches with time restrictions of 1985 to the present. Studies selected were those written in the English language which compared the role of St. John's wort, tricyclic antidepressants, monoamine oxidase inhibitors, and serotonin-selective reuptake inhibitors in the treatment of depression. Other studies were selected based on their evaluation of the safety and efficacy of St. John's wort as an antidepressant for a minimum of four weeks.

RESULTS

A review of existing literature recognized nine clinical trials that reported the efficacy of St. John's wort as compared to placebo and to other antidepressant medications. Of these nine, four controlled studies were chosen based upon their large patient populations and their consistency in brand and dosage of St. John's wort used. These four studies demonstrated that St. John's wort was as effective as other antidepressant medications and more effective than placebo, as the clinical symptoms of depression greatly decreased upon administration of H. perforatum. The side-effect profile of H. perforatum at this time appears to be superior to any current U.S.-approved antidepressant medication.

CONCLUSIONS

From the existing literature, St. John's wort appears to be a safe and effective alternative in the treatment of depression. Tricylic antidepressants and monoamine oxidase inhibitors can produce serious cardiac side-effects, such as tachycardia and postural hypotension, and many unwanted anticholinergic side-effects, including dry mouth and constipation. St. John's wort has proven to be free of any cardiac, as well as anticholinergic, side-effects normally seen with antidepressant medications. Based upon limited studies, St. John's wort appears to be an acceptable alternative to traditional antidepressant therapy, although trials on a larger scale are warranted in this area. Hypericum is available to the lay public as an over-the-counter preparation and may be misused if not fully understood.

Palmar digital vessel relaxation in healthy horses and in horses given carbohydrate

OBJECTIVE

To compare in vitro smooth muscle relaxation of palmar digital vessels from healthy horses with those from horses in the prodromal stage of experimentally (carbohydrate) induced laminitis.

ANIMALS

16 adult horses.

PROCEDURE

Segments of palmar digital vessels were obtained from 5 healthy horses and 6 horses given carbohydrate. Vascular rings from the palmar digital artery and vein were suspended in individual organ baths containing buffer solution and indomethacin; isometric tension was recorded, and contraction and relaxation were compared. Smooth muscle contraction in response to cumulative addition of phenylephrine was recorded in the absence and presence of 1 microM NG-nitro-L-arginine methyl ester (L -NAME). After wash out, vascular rings were preconstricted with phenylephrine (0.3 microM), and cumulative endothelium-dependent (acetylcholine-induced) and independent (nitroprusside-induced) smooth muscle relaxations were recorded in the absence or presence of L -NAME.

RESULTS

Phenylephrine increased vascular smooth muscle tone in ring preparations of palmar digital arteries and veins. Addition of acetylcholine or nitroprusside induced relaxation of palmar digital artery and vein ring preparations. Use of L-NAME (1 microM) significantly reduced maximal relaxation induced by acetylcholine, but not by nitroprusside. Maximal relaxation induced by acetylcholine, but not by nitroprusside, was reduced in vascular rings prepared from carbohydrate-overloaded horses.

CONCLUSION AND CLINICAL RELEVANCE

Reduced endothelium-dependent relaxation of palmar digital vessels may have a role in the pathophysiology of acute laminitis after carbohydrate overload in horses.

Enhanced hemodynamic response to [D-ALA2,D-MET5]-methionine enkephalin (DAME) in streptozotocin-induced diabetic rats is reversed by insulin replacement

The present study examined the alterations of hemodynamic responses to [D-ala2,D-Met5]-methionine enkephalin (DAME) in diabetic animals. Male Sprague-Dawley rats (12 weeks old) were used for this study. Diabetes was induced by a single injection of streptozotocin (65 mg/kg, i.v.). After 7 days, blood glucose levels were determined to confirm the diabetic state. Animals were anesthetized and instrumented to monitor mean arterial pressure, hindlimb bloodflow and hindlimb vascular resistance. Administration of DAME produced a significantly greater reduction in blood pressure, increase in hindlimb bloodflow and decrease in hindlimb vascular resistance in diabetic vs. control rats. These effects were blocked by naloxone. All hemodynamic changes were attenuated after pretreatment with the ganglionic blocker, hexamethonium, indicating that the responses were mediated either within the central nervous system or at the ganglia. Insulin reversed the exaggerated depressor effect of DAME on streptozotocin-treated rats. Collectively, these results suggest that diabetic rats have altered opioidergic hemodynamic responses to DAME due to mu receptor alterations in the CNS or in autonomic ganglia. These effects were reversed by replacement of insulin.

Gender differences in the expression of endothelin receptors in human saphenous veins in vitro

The contractile response to endothelin-1 (ET-1) appears to be modulated by the relative density of ETA and ETB receptors. To determine the effects of gender on the distribution of ET receptors, we analyzed the endothelin receptor subtypes on membrane fractions prepared from saphenous vein samples obtained from patients of different genders undergoing coronary artery bypass graft surgery. The contractile response to ET-1 in the presence and absence of 1 microM of the ETA receptor antagonist BQ-123 was also investigated. Similar studies were repeated with endothelium-denuded samples to study the role of endothelium- and smooth muscle-derived ETB receptors. Competitive binding experiments were performed on membrane fractions using [125I]ET-1 and unlabeled ligands ET-1, ET-3, sarafatoxin 6c and BQ-123. Analysis of the binding data with endothelium-intact samples yielded two classes of binding sites in both women and men. In women, the maximum binding capacities were 83 +/- 6 and 97 +/- 10 fmol/mg protein for ETA and ETB receptors, respectively; the corresponding values in men were 618 +/- 121 and 201 +/- 10 fmol/mg protein. In addition, ET-1-induced contractions were 2-fold greater in men than in women at high ET-1 concentrations. Competitive binding studies with endothelium-denuded saphenous veins demonstrated the presence of only ETA receptors in both female and male tissue. These results indicate that the ratio and the density of ET receptors are different in men and women, which might be an important factor in the regulation of the contractile response.

Oxidized low-density lipoprotein inhibits acetylcholine-induced vasorelaxation and increases 5-HT-induced vasoconstriction in isolated human saphenous vein

The present study determined the vasomotor effects of oxidized low-density lipoprotein (ox-LDL) in human saphenous veins and determined whether decreased availability of L-arginine was responsible for the impaired endothelial function. Human saphenous veins were obtained from white males undergoing coronary bypass surgery. We examined the effects of ox-LDL on ACh-induced endothelium-dependent relaxation, sodium nitroprusside-induced endothelium-independent relaxation and 5-HT-induced contraction. ACh-induced vasorelaxation in the presence of L-arginine and ox-LDL was also examined. In addition, we assessed the endothelial influence on the contractile response to 5-HT. ox-LDL significantly inhibited ACh-induced relaxation but did not affect sodium nitroprusside-induced relaxation. L-Arginine pretreatment did not prevent ox-LDL-induced impairment of the relaxation response to ACh. ox-LDL significantly potentiated 5-HT-induced contraction at concentrations between 3 x 10(-6) M and 10(-4) M, an effect that was endothelium-dependent. Denudation of endothelium also significantly enhanced the contractile response to 5-HT. These data suggest that ox-LDL impairs ACh-induced endothelium-dependent relaxation and enhances 5-HT-induced endothelium-dependent contraction in human saphenous vein. L-Arginine deficiency is not responsible for the endothelial dysfunction induced by ox-LDL in human saphenous vein.

Indicators of postoperative pain in cats and correlation with clinical criteria

OBJECTIVE

To identify clinical indicators that may help identity postoperative pain in cats after ovariohysterectomy.

ANIMALS

Healthy, laboratory animal source cats.

PROCEDURE

Clinical indicators of pain were identified, and relief from pain in response to butorphanol was studied in 5 groups of cats. 10 cats had 1 hour of general anesthesia only, followed by recovery without additional medication. 10 cats had general anesthesia and ovariohysterectomy, followed by recovery without additional medication. 10 cats had general anesthesia, ovariohysterectomy, and postoperative administration of 0.1 mg of butorphanol/kg of body weight. Another 10 cats had general anesthesia, ovariohysterectomy, and postoperative administration of 0.3 mg butorphanol/kg. 10 cats received 0.1 mg of butorphanol/kg, IM, only. Samples and recorded data were obtained before, during, and after the anesthesia period. Clinical variables measured included heart rate, blood pressure, respiratory rate, rectal temperature, PCV, and blood glucose concentration. Results were compared with changes in norepinephrine, epinephrine, and cortisol concentrations.

RESULTS

Cats that did not receive analgesics had higher cortisol concentration than did cats without surgery and cats that received butorphanol after surgery. Systolic blood pressure measured by ultrasonic Doppler was found to be predictive of cortisol concentration, using a multiple linear regression model.

CONCLUSIONS

Cortisol concentration increased in response to surgical stress and pain, and this increase was diminished by use of butorphanol.

CLINICAL RELEVANCE

Systolic blood pressure was the best clinical predictor of postoperative pain.

Identification of receptor binding and activation sites in endothelin-1 by use of site-directed mutagenesis

This study addresses the structural requirements for the intracellular processing and receptor binding properties of endothelin-1 (ET-1). Point mutants of preproendothelin-1 cDNA, with replacement of the codons for Lys9 of ET-1 by ones for Ala and Glu and of Ile20 and Trp21 by ones encoding Ala, were expressed in COS-7 cells. Competitive binding experiments on rat vascular smooth muscle cells (A-10), which were shown to be an ETA receptor-rich cell line, between [125I]ET-1 and synthetic ET-1, wild-type recombinant ET-1, and recombinant [Ala9]ET-1, [Glu9]ET-1, [Ala20]ET-1, and [Ala21]ET-1 yielded Ki values of 0.2 +/- 0.02, 0.2 +/- 0.02, 0.04 +/- 0.01, 1.4 +/- 0.2, 1.6 +/- 0.2, and > 50 nmol/L, respectively. In similar experiments with ETB receptor-rich human Girardi heart cells, the corresponding values were 0.2 +/- 0.03, 0.2 +/- 0.03, 0.2 +/- 0.04, 0.2 +/- 0.06, 1.4 +/- 0.4, and > 50 nmol/L. The ETA receptor-mediated contractile responses to [Glu9]ET-1 and [Ala20]ET-1, measured by using canine coronary artery rings, were decreased approximately fourfold to fivefold compared with the response produced by synthetic or wild-type recombinant ET-1, whereas [Ala9]ET-1 was found to be more potent, and [Ala21]ET-1 did not produce any contraction. These results demonstrate that Ile20 and Trp21 are involved in binding to both receptor subtypes. Of considerable interest was the observation that [Glu9]ET-1 also blunts the ETA receptor subtype-mediated contractile response to ET-1 stimulus.

Insulin attenuates alpha-adrenergic aortic contraction in normotensive but not borderline hypertensive rats

Hyperinsulinemia can alter vasoconstrictor responses in normotensive and hypertensive rats, but the effects of insulin on vascular contraction have not been evaluated in borderline hypertension. This study determined the effects of insulin on alpha-adrenoceptor mediated aortic contraction in male and female borderline hypertensive rats (BHR) and normotensive Wistar Kyoto (WKY) rats. Dose-response curves to norepinephrine (NE) and phenylephrine (PE) were performed in thoracic aorta in the absence or presence of insulin (100 microU/ml) for 2 hrs. Contraction to NE and PE was reduced in aorta from female WKY rats incubated with insulin compared to control. In aorta from 6 of 13 male WKY rats (insulin responders), an attenuated response to NE and PE was observed in the presence of insulin. However, insulin did not alter responses to NE or PE in aorta from male or female BHR. These results indicate that insulin impairs alpha-adrenoceptor mediated contraction in the normotensive female WKY rat and in a group of responder male WKY rats, but not in BHR. This study supports the presence of a resistance to the vascular effects of insulin in BHR.

Nonsuperoxide lucigenin-enhanced chemiluminescence from phospholipids and human saphenous veins

The use of lucigenin-enhanced chemiluminescence (CL) for the detection of superoxide (O2.-) has grown in popularity due to an increased demand for a simple and specific system capable of measuring superoxide. In this study we report a lucigenin-CL signal emanating from human saphenous veins (SV) that was not inhibited by superoxide dismutase (SOD) and lasted for more than 24 h. A larger CL-signal with similar properties was produced by saphenous veins that had been dehydrated. A similar, non-SOD-inhibitable lucigenin-CL was also produced with a variety of phospholipids and phosphatidic acid. The chemical moiety responsible for the phospholipid CL is oxygen dependent but remains unidentified because a variety of lipids and phosphate containing species failed to produce such a signal. These results suggest that the use of lucigenin as a specific CL enhancer for O2.- must be clearly discriminated with a specific O2.- inhibitor when used in biological systems.

Physiologically based pharmacokinetic model useful in prediction of the influence of species, dose, and exposure route on perchloroethylene pharmacokinetics

The ability of a physiologically based pharmacokinetic (PBPK) model to predict the uptake and elimination of perchloroethylene (PCE) in venous blood was evaluated by comparison of model simulations with experimental data for two species, two routes of exposure, and three dosage levels. Unanesthetized male Sprague-Dawley rats and beagle dogs were administered 1, 3, or 10 mg PCE/kg body weight in polyethylene glycol 400 as a single bolus, either by gavage or by intraarterial (ia) injection. Serial blood samples were obtained from a jugular vein cannula for up to 96 h following dosing. The PCE concentrations were analyzed by headspace gas chromatography. For each dose and route of administration, terminal elimination half-lives in rats were shorter than in dogs, and areas under the blood concentration-time curve were smaller in rats than in dogs. Over a 10-fold range of doses, PCE blood levels in the rat were well predicted by the PBPK model following ia administration, and slightly underpredicted following oral administration. The PCE concentrations in dog blood were generally overpredicted, except for fairly precise predictions for the 3 mg/kg oral dose. These studies provide experimental evidence of the utility of the PBPK model for PCE in interspecies, route-to-route, and dose extrapolations.

The use of methylene blue as an extravascular surgical marker impairs vascular responses of human saphenous veins

Methylene blue is occasionally applied to the adventitia of blood vessels during coronary artery bypass and other vascular procedures to assist in the orientation of the vessel. Inherent in this method is the assumption that extravascular application of methylene blue is innocuous with regard to vascular function. In the first part of this study, the in vitro vascular reactivity of methylene blue-labeled saphenous veins was compared with that of veins that were not marked with methylene blue. The vasoactive agents tested were designed to examine multiple pathways. They included potassium chloride, prostaglandin F2 alpha, phenylephrine, serotonin, angiotensin II, BHT-933 (alpha 2-adrenergic agonist), sodium nitroprusside, acetylcholine, isoproterenol, and verapamil. Compared with unmarked veins, those marked with methylene blue demonstrated a significant impairment of both vasoconstrictor and vasodilator function. These observations were made on a relatively small number of patients and could therefore be attributed to inherent differences between patients or surgical procedures. In the second part of this study, these variables were eliminated by dividing a single vein from one patient into three segments for a 45-minute exposure to external only methylene blue, internal and external methylene blue, or no methylene blue. The segments were then evaluated for vasoreactivity in vitro. Externally applied methylene blue reduced vasoconstriction regardless of the agonist. Further, both endothelium-dependent and -independent vasodilation was diminished by external methylene blue exposure. In veins exposed to methylene blue both internally and externally the results were similar but the magnitude of impairment greater. It is concluded that surgical marking of blood vessels with methylene blue has the potential to adversely affect vascular reactivity and therefore the use of alternative dyes should be considered.

Opioidergic receptors in the arcuate nucleus are not involved in the cardiovascular effects of clonidine

The arcuate nucleus is the bed nucleus for the pro-opiomelancortin system of the brain with important connections with other nuclei involved in cardiovascular function. Clonidine has been reported to produce its cardiovascular effects through an interaction with opioid and alpha 2-adrenergic receptors. The present study examined the arcuate nucleus as a site of action of clonidine. Male spontaneously hypertensive rats were anesthetized with pentobarbital and were instrumented for the measurement of blood pressure and heart rate. Cannulae were placed either through the cisterna magna (IC) or in the arcuate nucleus. Administration of clonidine (0.03-3.75 micrograms, IC) produced a dose-dependent hypotension and bradycardia. Pretreatment with naloxone (30 micrograms, IC) prior to clonidine administration resulted in a significant attenuation of both the clonidine-induced hypotension and bradycardia. In contrast, administration of naloxone (100 ng) into the arcuate nucleus prior to the central administration of clonidine did not alter the cardiovascular effects of clonidine. These results support the role of central opioidergic receptors in the cardiovascular effects of clonidine but do not support the arcuate nucleus as the site of action.

Use of tissue disposition data from rats and dogs to determine species differences in input parameters for a physiological model for perchloroethylene

Tissue disposition of perchloroethylene (PCE) was determined experimentally in two mammalian species of markedly different size in order to derive input parameters for the development of a physiologically based pharmacokinetic (PBPK) model, which could forecast the disposition of PCE in each species. Male Sprague-Dawley rats and male beagle dogs received a single bolus of 10 mg PCE/kg body wt in polyethylene glycol 400 by gavage. Serial samples of brain, liver, kidney, lung, heart, skeletal muscle, perirenal fat, and blood were taken for up to 72 hr following PCE administration. Blood and tissue PCE concentrations were analyzed using a gas chromatography headspace technique. Dogs exhibited considerably longer tissue and blood half-lives than did rats. The dogs also exhibited larger area under tissue concentration versus time curves for all tissues except the liver. Whole body clearance of PCE in the rat was greater than that in the dog. Model simulations indicated this could be attributed to more rapid and extensive PCE exhalation and metabolism by the rat. The in vivo blood:air partition coefficient determined for rats was similar to an in vitro value previously reported. In vivo tissue: blood partition coefficients, however, were 1.4 to 2.8 times greater than published in vitro values. The PCE in vivo blood:air partition coefficient for the dog was twice that of the rat, but tissue:blood partition coefficients were 1.5 to 3.0 times greater in the rat than in the dog. These results demonstrated the existence of significant differences in partition coefficients in two species commonly used in toxicity testing. The PBPK model was shown to have utility in predicting the impact of metabolism and exhalation on pharmacokinetics of PCE in different species of widely differing size.

Copper deficient rat heart can compensate for doxorubicin-induced oxidant stress

The hypothesis that copper (Cu) alters drug metabolizing enzymes and functions as an antioxidant nutrient in doxorubicin cardiotoxicity was tested. Male Sprague-Dawley rats were fed Cu adequate (+Cu; 5 mg Cu/kg of diet), marginally Cu deficient (MCu; 1.2 mg Cu/kg of diet), or severely Cu deficient (-Cu; 0.5 mg Cu/kg of diet) diets for 6 wk. Doxorubicin (1, 2, or 4 mg/kg body wt) or saline were administered intraperitoneally 1 time/wk for 4 wk. Compared to control hearts, Cu,Zn superoxide dismutase activity was decreased by 9% in MCu rats and by 21-40% in -Cu rats. Glutathione peroxidase activity was elevated 5-15% in -Cu rats. Doxorubicin administration increased heart Cu,Zn superoxide dismutase activity in +Cu and -Cu rats 18 h after the last of 4 injections, but not 18 h after 1 injection. There was no synergism between doxorubicin and Cu deficiency on lipid peroxidation, plasma creatine phosphokinase, cardiac hypertrophy, electrocardiographic abnormalities, or morphological changes. Heart glutathione S-transferase activity was decreased by Cu deficiency, and like Cu,Zn superoxide dismutase activity, returned to normal in -Cu rats given doxorubicin. Thus, the Cu deficient rat heart may be able to compensate for doxorubicin-induced oxidant stress by increasing the activity of Cu,Zn superoxide dismutase and glutathione S-transferase.

Copper and selenium deficiencies do not enhance the cardiotoxicity in rats due to chronic doxorubicin treatment

This study tests the hypothesis that Cu and Se deficiencies enhance doxorubicin-induced cardiotoxicity and anemia. Male Sprague-Dawley rats (n = 48) were fed Cu and Se-adequate (+Cu+Se), Cu-deficient (-Cu), Se-deficient (-Se) or Cu and Se-deficient (-Cu-Se) diets for 5.5 wk. Doxorubicin (4 mg/kg body wt) or saline was administered once weekly for the last 4 wk of the study. Copper deficiency was confirmed by 79% lower liver Cu, 67% lower liver Cu,Zn superoxide dismutase (Cu,Zn SOD) activity and 76% lower erythrocyte Cu,Zn SOD activity. Selenium deficiency was confirmed by 90% lower liver glutathione peroxidase activity. Rats fed the -Cu diet had greater reductions in hematocrit than did those fed the +Cu diet after administration of doxorubicin. Doxorubicin, Cu deficiency and Se deficiency all produced electrocardiographic abnormalities and ultrastructural anatomical lesions. However, the dietary deficiencies did not enhance doxorubicin-induced cardiotoxicity. Doxorubicin, but not Cu or Se deficiency, raised lipid peroxidation 16% in liver (P < 0.01) and 18% in heart (not significant). These data suggest that the cardiomyopathies caused by doxorubicin and Cu and Se deficiencies have some similarities, but cardiac changes may be related to mechanisms other than lipid peroxidation.

Hemodynamic effects of centrally administered, norcocaine in the rat

Norcocaine is the N-demethylated metabolite of cocaine. It is present in the CNS and is reported to be pharmacologically active. The present study was designed to evaluate the cardiovascular actions of norcocaine following central administration. Wistar Kyoto (WKY) rats were anesthetized with pentobarbital and instrumented for measurement of blood pressure and renal and hindlimb blood flow (via Doppler flowprobes). A cerebroventricular cannula was placed in the lateral ventricle for drug administration. Cocaine or norcocaine was administered centrally in a dose range of 0.025 to 4.0 mg/kg. Under the above experimental conditions, 4.0 mg/kg of norcocaine decreased blood pressure without a significant change in either hind limb or renal blood flow. Central administration of cocaine also produced a similar depressor response. In conscious, unrestrained rats, cocaine produced a pressor response while norcocaine did not significantly alter blood pressure. The depressor response to both cocaine and norcocaine in the anesthetized animal is speculated to be due to the local anesthetic properties of the drugs.

The effect of copper deficiency on adriamycin toxicity

The effect of copper (Cu) deficiency on antioxidant status of liver and heart and its effect on adriamycin toxicity was assessed. Weanling rats were fed Cu-adequate (+Cu) and Cu-deficient (-Cu) diets for 3 weeks and then injected intraperitoneally once with saline or adriamycin (8 mg/kg). All -Cu rats had increased heart/body weight ratios compared to +Cu rats. Regardless of Cu status, adriamycin decreased body weight and increased heart cytochrome c oxidase, but had no effect on Cu,Zn-superoxide dismutase, glutathione peroxidase, the thiobarbituric acid (TBA) index or liver cytochrome c oxidase activity. -Cu, adriamycin rats had significantly decreased heart weights (0.54 + 0.02 g/kg body wt.) compared to -Cu, saline rats (0.69 +/- 0.05 g/kg body wt.). Four of 6 -Cu rats given adriamycin had abnormal EKGs, with 3 showing extrasystoles and 1 having a severe bradycardia. All +Cu rats given adriamycin had normal EKGs. Pathological changes detected with electron microscopy were most severe in -Cu rats given adriamycin.

Differential routes of cocaine administration indicate a peripheral cardiotoxic action

The present study utilized different routes of administration in unanesthetized Wistar Kyoto (WKY) rats to determine whether cocaine-induced death was mediated through a peripheral or central site of action. Administration of cocaine via a route that resulted in high concentrations of cocaine reaching the heart produced arrhythmias, convulsions, a decrease in heart rate and mean arterial pressure, and death. However, administration of the same dose via a route that resulted in passage of cocaine through the liver before reaching the heart produced only a pressor response. Additionally, administration of the same dose via a route that resulted in high levels of cocaine reaching the brain did produce a pressor response that was followed by a decrease in blood pressure and heart rate, arrhythmias, convulsions and death. However, these effects were delayed in comparison to the response when high concentrations of cocaine reached the heart immediately. These results support a peripheral site of action for cocaine-induced death.

Altered responsiveness of saphenous vein grafts to norepinephrine and tyramine: relation to tissue catecholamine stores

This study was designed to investigate early changes in reactivity in relation to the adrenergic innervation of venous grafts. Saphenous vein grafts were implanted into the carotid artery by the end-to-end technique in mongrel dogs. After 1 week, the grafts were harvested and dose-response curves to norepinephrine and tyramine were determined and compared with those of nongrafted saphenous veins and carotid arteries. Tissue norepinephrine levels of the blood vessels were measured by HPLC with electrochemical detection. Grafted vessels demonstrated an enhanced sensitivity to norepinephrine and a significant attenuation to tyramine. Additionally, grafted saphenous veins exhibited a significant depletion of norepinephrine content when compared to nongrafted veins. These differences suggest that denervation of the saphenous vein produces a supersensitivity to catecholamines that could account for enhanced vascular reactivity observed following implantation of saphenous vein grafts.

In vitro responses of equine digital vessels to dopamine and fenoldopam

The in vitro responses of isolated vascular preparations of digital arteries and veins obtained from healthy anaesthetised horses were determined for dopamine and fenoldopam. The digital vessels were harvested, cut into 4 mm vascular segments, suspended in tissue baths and attached to force-displacement transducers. Dose-response studies between 10(-8) and 10(-4)M concentrations were performed for all drugs. The change in tension of each vascular ring was measured in grams of force. The reactivity between palmar and plantar digital vessels and baseline vascular responses were determined for dopamine. The vascular responses of dopamine were compared to in vitro data for other known vasoconstrictor agents. The mechanism of vasoconstriction induced by dopamine was further defined using prazosin, a specific competitive alpha-1 adrenoceptor antagonist. The vasodilating ability of fenoldopam, a dopamine-1 (DA-1) receptor agonist, was also determined using noradrenaline- preconstricted vascular segments from palmar digital vessels. The effective concentration to produce 50 per cent of the maximal response (EC50) and the maximal contraction in grams of force per milligram of the vascular ring (g/mg) were calculated. There were no differences in the reactivity between the palmar and plantar digital vessels. Dopamine produced intense constriction in arteries and veins but only at very high molar concentrations. Prazosin decreased significantly the sensitivity of the veins to dopamine (increased the mean EC50 values) but not the arteries. Prazosin had no effect on the maximal contractions of the vessels. Fenoldopam produced very little relaxation of either the arteries or veins. These results suggest that dopamine produces constriction in equine digital arteries and veins and that the constriction is only partially mediated by alpha-1 adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic cocaine treatment enhances the responsiveness of the left anterior descending coronary artery and the femoral artery to vasoactive substances

The mechanism by which cocaine produces sudden cardiac death has not been elucidated, but clinical evidence indicates that it may be due to a direct or indirect action on coronary vessels. The present study was designed to compare the responses of the isolated left anterior descending (LAD) coronary artery and femoral artery taken from untreated dogs with the response of these vessels taken from dogs administered cocaine (1 mg/kg i.v.) daily for 4 weeks. The actions of norepinephrine, U-46619 (a thromboxane A2 analog) and serotonin (5-HT) were evaluated. The direct vascular action of cocaine was also determined. Morphology of the blood vessels was evaluated by scanning electron microscopy. Chronic cocaine treatment significantly increased the sensitivity and maximum response of the femoral artery to norepinephrine, U-46619 and 5-HT. The sensitivity of the LAD coronary artery to U-46619 and 5-HT and the maximum response to U-46619 and 5-HT were also enhanced. Vasoconstriction produced by cocaine was not dose dependent and only occurred at high concentrations (10(-5)-10(-4) M). Morphology of the vessels was evaluated by scanning electron microscopy. Femoral arteries from cocaine-treated dogs exhibited loss of endothelial integrity, areas of excessive endothelial cell sloughing and thrombus formation. LAD coronary arteries exhibited only areas of enhanced endothelial cell sloughing. The results of this study indicate that the femoral artery and the LAD coronary artery are more sensitive to endogenous vasoactive substances after chronic cocaine use, which may result in enhanced peripheral vasoconstriction and cardiac ischemia. Morphological results demonstrate femoral arterial thrombosis associated with cocaine use.

Postadulticide pulmonary hypertension of canine heartworm disease: successful treatment with oxygen and failure of antihistamines

Postadulticide pulmonary hypertension mechanisms and treatment with antihistamines and supplemental oxygen were studied in eight dogs with heartworm disease. To ensure severe postadulticide thromboembolism, additional heartworms (either 20 or 40 into 4 dogs each) were transplanted into naturally infected dogs before thiacetarsamide treatment. During pentobarbital anesthesia, 2 pulmonary hemodynamic studies were conducted on each dog with a sequence of baseline, hypoxia with FlO2 = 10%, hyperoxia with FlO2 = 100%, a second baseline, treatment with either diphenhydramine (D) or cimetidine (C), and another hypoxia. All dogs were pulmonary hypertensive, with each dog having a mean pulmonary arterial pressure (PPA) greater than 20 mm of Hg. Mean PPA increased from baseline conditions (25.0 +/- 4.5 SD for D and 24.3 +/- 4.4 for C) to hypoxia (28.5 +/- 4.7 for D and 28.4 +/- 3.7 for C), and decreased during hyperoxia (16.9 +/- 3.0 for D and 17.4 +/- 3.0 for C), respectively. Neither antihistamine reduced PPA at normoxia. The degree of pulmonary hypertension when breathing room air increased even more during hypoxia, and this increase was not attenuated by either antihistamine. Histamine did not appear to mediate pulmonary hypertension during postadulticide thromboembolism, nor to modify the hypoxia-mediated pulmonary hypertension at this disease stage. Because baseline PO2 was low (66.6 +/- 11.7 mm of Hg for D and 69.4 +/- 14.2 for C) and because PPA decreased during administration of oxygen, the pulmonary hypertension was mostly hypoxia-induced. In addition to the arterial lesions, much of the pulmonary hypertensive mechanism was an active and reversible vasoconstriction in response to hypoxia caused by the secondary lung disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Central mechanisms of action involved in cocaine-induced tachycardia

The present study was designed to determine the central effects of cocaine on heart rate and blood pressure in Wistar Kyoto rats (WKY) and to evaluate mechanisms involved in the response. Cocaine (0.025-4 mg/kg) was administered to unanesthetized, unrestrained rats via a cannula placed into the lateral ventricle. Procaine (0.1 and 4 mg/kg) was also administered centrally. Cocaine did not significantly alter blood pressure at doses of 0.025, 0.1, or 0.5 mg/kg, icv. Only the highest dose, 4 mg/kg, icv produced a significant pressor response. Cocaine produced significant dose-dependent tachycardia, with the maximum increase in heart rate occurring within 5 min. Procaine (4 mg/kg, icv) produced tachycardia, but the effect was significantly less than that produced by cocaine (4 mg/kg, icv). Cocaine also produced tachycardia at a dose of 0.1 mg/kg, but procaine did not significantly alter heart rate at the same dose. Central phentolamine pretreatment (0.1 mg/kg, icv) significantly attenuated the increase in heart rate produced by cocaine. These results indicate that the centrally mediated tachycardia produced by cocaine is partly due to its local anesthetic activity and to indirect stimulation of alpha receptors.

Cardiovascular function and serum catecholamine concentrations after anesthesia and surgery in the dog

Peripheral vasoconstriction and plasma catecholamine concentrations were studied in 37 dogs after cervical disc fenestration and salivary gland excision, laparotomy for intestinal anastomoses and cystotomy, or laparotomy for repair of diaphragmatic rupture, gastrotomy, and pyloromyotomy. Meperidine (4.4 mg/kg) was administered before extubation of 12 dogs undergoing laparotomy. Heart rate, respiratory frequency, indirect blood pressure, rectal temperature, toe web temperature, and plasma concentrations of epinephrine and norepinephrine were determined before induction of anesthesia, after intubation, after extubation, at sternal recumbency, and at standing. All dogs were hypothermic during surgery. After surgery, peripheral hypothermia (large rectal-toe web temperature gradients) increased from a mean of 4.6 degrees C after intubation to a mean of 10.4 degrees C when the dogs initially stood. Heart and respiratory rates and blood pressures during recovery were similar to those before anesthesia. Mean plasma catecholamine concentrations were neither significantly higher during recovery than before surgery nor were they increased in any surgical group, including the dogs not treated with meperidine. After anesthesia, 15% of the epinephrine and 12% of the norepinephrine samples were more than two standard deviations above the mean of the preanesthetic concentrations of all dogs. The ratio of all dogs with an epinephrine concentration more than two standard deviations above the mean of baseline epinephrine concentrations was greater at sternal recumbency than before anesthesia and the ratio of dogs with an increased epinephrine concentration at sternal recumbency was greater in the laparotomy dogs (9 of 24) than in the cervical surgery dogs (0 of 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Catecholaminergic and opioidergic mechanisms involved in the hypotensive response of pindolol

The present study evaluated the involvement of opioidergic and catecholaminergic mechanisms in the hypotensive action of pindolol. Pindolol (1 mg/kg i.a.) was administered to unanesthetized spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats instrumented for direct arterial pressure monitoring. Peripheral administration of pindolol produced a significant decrease in blood pressure in both SHR and WKY rats with SHR animals having a greater response. Heart rate was reduced in SHR; however, a tachycardia was observed in WKY rats. Pretreatment with naloxone (100 micrograms/kg i.a.) 10 min prior to pindolol administration prevented the hypotensive response. Similar pretreatment with yohimbine, an alpha 2-receptor antagonist, also prevented the pindolol-induced hypotensive response in both SHR and WKY rats. Neither naloxone nor yohimbine alone significantly affected blood pressure or heart rate. These results suggest that opioidergic and catecholaminergic mechanisms are involved in the hypotensive action of pindolol.

Relationship of plasma clonidine to growth hormone concentrations in children and adolescents

The relationships of plasma clonidine to increases in growth hormone, blood pressure response and degree of sedation were studied in nine children with short stature after a 0.15 mg/m2 oral clonidine tolerance test. A direct correlation was found between the peak plasma clonidine concentration and the increase in plasma growth hormone (P = 0.055). The patients experienced varying degrees of blood pressure reduction and sedation that were unrelated to age. No correlation was found between the plasma clonidine concentration and degree of blood pressure reduction or degree of sedation. The half-life of oral clonidine ranged from 1.75 to 2.38 hours, which is significantly less than that for adults.

Reactivity of equine palmar digital arteries and veins to vasodilating agents

Palmar digital arteries and veins removed surgically from healthy horses under general anesthesia were cut into 4 mm vascular rings, suspended in tissue baths, and attached to force displacement transducers for continuous measurement of vascular tension. In vitro vascular responses were determined for acetylcholine, acepromazine, isoxsuprine hydrochloride (isoxsuprine), prostaglandin E2 (PGE2), and prostaglandin I2 (prostacyclin). After preconstriction with norepinephrine hydrochloride (norepinephrine), or prostaglandin F2 alpha (PGF2 alpha), the concentrations needed to produce 50% maximum relaxation (EC50) and the maximum percentage of relaxation were determined for each drug. Acetylcholine was the most potent arterial vasodilator (smallest EC50 value) and PGE2 was the least potent. Prostacyclin was the least potent venodilator (highest EC50 value); there were no differences between acetylcholine, acepromazine, isoxsuprine, and PGE2. Isoxsuprine produced greater arterial relaxation than all other agents. Isoxsuprine and acepromazine produced significantly greater venous relaxation than did acetylcholine and PGE2. Prostacyclin produced minimal vasodilation of arteries or veins. Acepromazine and isoxsuprine relaxed the veins significantly more than the arteries. When PGF2 alpha was used instead of norepinephrine to preconstrict the arteries and veins, the potency and effectiveness of acepromazine and isoxsuprine to produce vasodilation were significantly decreased. Results indicate that acepromazine and isoxsuprine can relax the equine digital vasculature but their effectiveness varies depending on the origin of the constriction.

In vitro reactivity of digital arteries and veins to vasoconstrictive mediators in healthy horses and in horses with early laminitis

The in vitro reactivity of vasoconstrictive mediators that are implicated in acute laminitis was determined in palmar and plantar digital arteries and veins obtained from healthy horses and in palmar digital vessels of horses with early laminitis (Obel grade I). To obtain baseline reactivity data, 3 experiments were conducted, using healthy horses: (1) the reactivity of palmar and plantar digital arteries and veins to angiotensin II, norepinephrine, and 5-hydroxytryptamine (serotonin) were compared; (2) the direct effects of bacterial endotoxin on vascular reactivity were assessed; and (3) the reactivity of palmar digital arteries and veins to angiotensin II, norepinephrine, prostaglandin F2 alpha (PGF2 alpha), serotonin, and a thromboxane-endoperoxide analog (U46619) were determined. The vascular reactivity of these same 5 vasoconstrictors then was determined in horses with early laminitis and was compared with data from healthy (control) horses. Obel grade-I laminitis was experimentally induced in horses, using carbohydrate overload. Dose responses were conducted for each agent at concentrations between 10(-8)M and 10(-4)M. The potency of a drug was defined as the mean effective concentration necessary to induce 50% of maximal contraction (EC50). There were no differences in EC50 concentrations and in maximal contractions between forelimb and hind limb arteries and veins for angiotensin II, norepinephrine, and serotonin. Incubation with endotoxin had no effect on the reactivity of arteries and veins to angiotensin II, norepinephrine, and serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone pretreatment blocks the hypotensive effects of atenolol in SHR and WKY rats

The present study examined the interaction of opioidergic systems in the hypotensive action of atenolol. Anesthetized, adult spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats were instrumented to monitor blood pressure and heart rate. Atenolol (100 micrograms/kg, i.v.) produced a decrease in blood pressure of similar magnitude in both SHR and WKY rats. However, the bradycardia was greater in SHR. Pretreatment with naloxone (0.1 mg/kg, i.v.) 15 min prior to the administration of atenolol completely prevented the hypotensive response in SHR and decreased the maximum hypotensive response by approximately 50% in WKY rats. The atenolol-induced bradycardia was unaffected in both groups of animals. Additionally, pretreatment with yohimbine, an alpha 2-receptor antagonist, inhibited the hypotensive response of atenolol in SHR. Collectively, these results suggest an interaction between opioidergic and catecholaminergic systems as a possible site of action of antihypertensive drugs.

Interaction of propranolol with central serotonergic neurons

Central monoaminergic mechanisms are believed to be involved in cardiovascular regulation. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol in pentobarbital anesthetized mongrel dogs. Ventriculocisternal perfusion of propranolol (25 ug/kg/min for 30 min) decreased serotonin turnover as indicated by a significant decrease in cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (5-HIAA). This effect was accompanied by a significant reduction in mean arterial pressure and heart rate. These results indicate that propranolol decreases central serotonergic activity and suggests a possible role for central serotonergic pathways in the antihypertensive action of propranolol. Several studies have indicated that central serotonergic pathways participate in the regulation of blood pressure. Brainstem regions including the nucleus tractus solitarius, the raphe nucleus and the anterior hypothalamic preoptic region are involved in cardiovascular control and contain a dense population of serotonergic neurons. A centrally-mediated hypotensive effect of propranolol has been demonstrated. Centrally administered propranolol increases cerebrospinal fluid (CSF) levels of norepinephrine and reduces blood pressure possibly due to decreased peripheral sympathetic nerve activity. Central serotonergic pathways may also be involved in the antihypertensive action of some beta-adrenoceptor antagonists. Destruction of central serotonergic neurons with 5,7-dihydroxytryptamine and desipramine abolished the antihypertensive effect of intracisternal propranolol in sinoaortic denervated dogs. Acute administrations of (-)-propranolol and (-)-pindolol decreased the synthesis rate of serotonin, while acute administration of salbutamol, a beta 2-adrenoceptor agonist, increased 5-HIAA levels in rat brain structures. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol.

Naloxone inhibits the centrally-mediated hypotensive actions of BHT-933 (azepexole)

Central administration of BHT 933, a highly selective alpha 2 agonist, to pentobarbital-anesthetized, normotensive dogs resulted in a rapid, significant decrease in blood pressure. The maximal response occurred at 30 min and remained significantly decreased for 60 min. Concomitant with the hypotensive response was a decrease in heart rate. Pretreatment with naloxone 15 min prior to the administration of BHT 933 completely abolished the hypotensive response and significantly inhibited the bradycardia. These results suggest a role for central opioidergic systems in the control of blood pressure which may serve as important sites of antihypertensive drug action. The central regulation of sympathetic tone by catecholaminergic systems plays an important role in the control of cardiovascular function in both normal and pathological states. A high density of catecholaminergic nerve terminals is found in regions of the brainstem involved in cardiovascular control. Stimulation of the alpha receptors in these areas decreases peripheral sympathetic tone and subsequently lowers blood pressure. Recent histochemical evidence has demonstrated the presence of opioid peptides in the nucleus tractus solitarii, nucleus ambiguous and hypothalamus as well as other discrete brain areas associated with cardiovascular control. Activation of the opiate receptors in these brain areas decreases sympathetic tone and blood pressure. Additionally, both catecholaminergic and opioidergic systems have been implicated in the reaction to certain stimuli (i.e., pain, stress) which entail important hemodynamic adaptations. The similarity between the central opiate and catecholaminergic systems suggests a relationship between the two systems in blood pressure control and a potential site of antihypertensive drug action. The purpose of the present study was to determine if an opioidergic component is involved in the hypotensive action of BHT 933 (azepexole). BHT 933 is a relatively new hypotensive agent which is a much more specific alpha 2 agonist than clonidine.

Increases in CSF norepinephrine associated with the onset of digoxin-induced arrhythmias

The present study utilized a time course of CSF catecholamine concentrations during peripheral digoxin administration in vivo to provide a clarification of central catecholaminergic mechanisms involved in digitalis cardiotoxicity. A continuous peripheral digoxin infusion produced significant increases in CSF norepinephrine just prior to arrhythmogenesis in anesthetized dogs. Phentolamine pretreatment significantly increased the arrhythmogenic and lethal doses of digoxin. These results suggest that activation of central noradrenergic neurons may be the initiating factor in digoxin-induced arrhythmogenesis.

Pharmacokinetics of tiazofurin in dogs

The pharmacokinetics of total and free tiazofurin, an antineoplastic agent, was studied in healthy mongrel dogs following intravenous and oral administration of the drug. The free fraction of tiazofurin was obtained from plasma by an ultrafiltration technique using a micropartition system. Total and free tiazofurin levels were determined by a sensitive high performance liquid chromatographic method. The percentage of tiazofurin bound to plasma proteins remained constant at approximately 15 per cent following administration to healthy mongrel dogs. The mean pharmacokinetic parameters of elimination rate constant (K), effective half-life (t 1/2), mean residence time (MRT) and the time to reach peak plasma level (tmax--after oral administration) were 0.32 +/- 0.04 h-1, 2.24 +/- 0.25 h, 3.23 +/- 0.36 h, and 1.78 +/- 0.50 h, respectively. The apparent volume of distribution at steady state was 0.98 +/- 0.30 1 kg-1 and the plasma clearance was 5.24 +/- 2.39 ml min-1 kg-1. About 90 per cent of tiazofurin was absorbed following oral administration. The pharmacokinetic parameters did not differ significantly between the total and free drug levels, indicating that the pharmacokinetics of total tiazofurin levels reflect those of the free tiazofurin in plasma.

Central dopamine receptors and their role in digoxin-induced cardiotoxicity in the dog

The role of the dopaminergic system in digoxin-induced cardiotoxicity has been examined. Specific dopaminergic agonists and antagonists were administered into the ventriculocisternal system of pentobarbitone-anaesthetized dogs before systemic administration of digoxin. Pretreatment with apomorphine, a specific dopamine agonist, did not significantly alter the arrhythmogenic or lethal doses of digoxin. However, the digoxin-induce increase in CSF noradrenaline was decreased significantly in apomorphine-pretreated animals. Pretreatment with pimozide, a specific dopamine antagonist, significantly decreased the arrhythmogenic dose of digoxin but did not alter the lethal dose. As with apomorphine, pimozide-pretreated animals accumulated significantly less noradrenaline in CSF compared with control dogs. These results suggest that dopamine receptors are not directly related to the cardiotoxic actions of digoxin. However, dopaminergic receptors may influence the balance of central catecholaminergic systems that influence the peripheral cardiovascular system.

Enhanced sympathetic activity as a mechanism for cardiac glycoside toxicity in hypomagnesemia

Hypomagnesemia was induced in dogs by administration of furosemide (20 mg/kg/day) for 7 days. Following replacement of a concomitant loss of potassium, ouabain was infused at 1 microgram/kg/min until death. The inotropic response, sustained ventricular arrhythmias, and death all occurred at significantly lower ouabain doses in hypomagnesemic animals than in normomagnesemic dogs. Additionally, hypomagnesemic dogs exhibited an abrupt sympathetic discharge which could be interrupted by bilateral ablation of the stellate ganglia. These results suggest that the neuroexcitatory effects of cardiac glycosides are enhanced during hypomagnesemia.

Age-related changes in the pharmacodynamics of verapamil

Age-related differences in the pharmacodynamics of numerous drugs are well documented. In this study, changes in the pharmacodynamics of verapamil were examined in anesthetized and unanesthetized puppies (6 weeks old) and adult dogs. Unanesthetized puppies and adult dogs were given verapamil (0.5 mg/kg intravenously and 2.5 mg/kg orally), and anesthetized dogs received only intravenous infusions. Plasma samples and ECG tracings were taken over a 4- to 7-hour period. Plasma verapamil was determined by high-pressure liquid chromatography with spectrofluorometric detection. Puppies exhibited a shorter t1/2 than adults after intravenous administration (62.3 +/- 10.8 minutes compared to 87.3 +/- 7.8 minutes). Puppies had a greater area under concentration (AUC) after oral dosing than adults, indicating a greater amount of the dose reaching the systemic circulation. The PR interval was significantly prolonged in adults but not in puppies, even though the blood levels in the puppies were greater than those in the adults. Therefore, even though there is little variation in the pharmacokinetic parameters between adult dogs and puppies, there is a marked reduction in the sensitivity of the puppies to the ECG effects of verapamil.

Site and mechanism of the centrally mediated hypotensive action of propranolol

Ventriculocisternal perfusion of propranolol (25 micrograms/kg/min for 30 min) throughout the entire brain ventricular system in anesthetized dogs decreased arterial pressure and increased cerebrospinal fluid (CSF) norepinephrine. Localized perfusion of propranolol into the fourth ventricle produced increased CSF norepinephrine levels and a hypotensive response comparable to that seen with whole-brain ventriculocisternal perfusion. In comparison, perfusion of propranolol through the forebrain (lateral-third) ventricles resulted in changes in CSF norepinephrine comparable to those observed with the administration of the drug into the fourth ventricle but resulted in a reduced hypotensive response. Increased CSF norepinephrine levels and a hypotensive response were also observed after peripheral i.v. infusion of propranolol (100 micrograms/kg/min for 45 min). Collectively, these results support the hypothesis that an interaction of propranolol at noradrenergic nerve terminals in the hindbrain area results in a hypotensive effect which may contribute to the antihypertensive action of the drug.

Pharmacodynamics of acute intranasal administration of verapamil: comparison with i.v. and oral administration

The present study was conducted in order to examine the intranasal administration of verapamil and compare this route to oral and intravenous administration in a 3 way crossover protocol in five dogs. Unanesthetized, adult mongrel dogs were given verapamil intravenously (0.5 mg/kg), orally (2.5 mg/kg) and intranasally (0.75 mg/kg) with at least a 3-4 day washout period between each administration. Blood samples were collected over a 10 hour period and analyzed for verapamil using HPLC with fluorescence detection. A lead II ECG was monitored to determine the effects of verapamil on heart rate and P-R interval. Following intravenous administration, verapamil was distributed according to a two compartment model. Bioavailability (corrected for dose and elimination rate constant) following intranasal administration (36% +/- 7%) was approximately 3 fold that after oral administration (13% +/- 3%). Absorption from the nasal cavity appeared instantaneous compared to an absorption half-life of 50 +/- 6 min after oral administration. All three routes of administration resulted in significant increases in heart rate and increases in the P-R interval. Maximal P-R interval prolongation occurred after peak plasma concentrations of verapamil. The results of this study suggest that the intranasal route is a viable alternative route of administration for verapamil.

The effects of central beta-receptor antagonism on digoxin cardiotoxicity

Pentobarbital-anesthetized dogs were pretreated centrally with either metoprolol, a selective beta1 antagonist, or ICI 118,551, a selective beta2 antagonist, prior to a continuous digoxin infusion (2.5 micrograms/kg/min, i.v.). Both agents produced depressant effects on hemodynamic parameters upon central administration. Metoprolol (1, 3 and 5 mg, i.c.v.) significantly increased the lethal dose of digoxin in a non-dose dependent manner, but these protective effects were not evident upon peripheral administration of the drug (5 mg). This protective effect was not due to local anesthetic properties of metoprolol since pretreatment with lidocaine did not alter the toxicity of digoxin. Central beta2 blockade by ICI 118,551 did not alter the doses of digoxin to ventricular arrhythmias or death. These results suggest that beta receptors are not directly involved in the cardiotoxic actions of digoxin. Metoprolol's effects may be attributed to non-specific central depressant activity.

Prazosin alters free and total plasma digoxin levels in dogs

Digoxin has been associated with a variety of drug interactions. Studies in our laboratory indicate that single doses of prazosin may alter plasma steady-state digoxin levels. Adult mongrel dogs were given digoxin tablets orally (0.008 mg/kg, twice daily) until steady-state levels of digoxin were reached. Dogs were then tested in a cross-over study with prazosin and saline. Plasma samples were assayed for both free and total plasma digoxin via radioimmunoassay. Free drug was separated from protein bound drug utilizing a micropartition system. Plasma binding of digoxin as well as nonspecific tissue binding was reduced by prazosin and resulted in an increased fraction of free digoxin in plasma. The increase in free digoxin leads to an increase in the pool of digoxin available for pharmacologic activity. Acute experiments demonstrated such an increased pharmacologic effect of digoxin in the presence of prazosin. Prazosin pretreatment prior to a continuous digoxin infusion significantly increased the positive inotropic effect of digoxin compared to dogs receiving only digoxin.

Effects of magnesium on the action of vasodilatory agents

Studies have shown that alterations of the magnesium (Mg2+) concentration can alter the response of certain vasoactive compounds. Responses of rabbit thoracic aorta to verapamil, diltiazem, nitroglycerin and isoproterenol were examined in zero Mg2+ (0.0 M), N Mg2+ (1.2 mM), 2 N Mg2+ (2.4 mM) and 4 N Mg2+ (4.8 mM). Preconstriction was induced with norepinephrine and cumulative dose-response curves were obtained for the vasodilators. The dose-response curve for isoproterenol was shifted to the right in zero Mg2+ while there was no effect on the other vasodilators. Elevation of the Mg2+ concentration to 4.8 mM produced a shift to the left in the dose-response curves for diltiazem, nitroglycerin and isoproterenol with no effect on verapamil. Therefore, Mg2+ deficiency does not appear to affect the vasodilatory actions of the agents tested except for the beta-receptor agonist, isoproterenol. Elevated Mg2+, however, potentiated the actions of isoproterenol, nitroglycerin and diltiazem, but not verapamil.

Altered vascular reactivity in the rabbit during hypercholesterolemia

Rabbits were fed a 0.6% cholesterol diet (HC) for 4, 6 or 8 weeks. Age- and sex-matched controls (C) were fed rabbit chow. Isometric contractions with cumulative doses of norepinephrine (NE), histamine (HIS) and serotonin (5-HT) were measured in isolated aortic rings. Serum cholesterol levels in female HC animals increased approximately 19, 20 and 25 times control values, and male HC animals increased approximately 17, 20 and 18 times control values after 4, 6 and 8 weeks, respectively. In the present study, altered vascular responsiveness during hypercholesterolemia was demonstrated in isolated vessels with major changes occurring after 4 weeks. Female HC rabbits exhibited increased maximal responses to all agents tested and were greater than male HC rabbits. Male HC rabbits had increased maximal responses to NE and HIS but not 5-HT. Significant changes were also observed in both male and female HC animals at 6 and 8 weeks, however these were somewhat attenuated when compared to 4-week HC animals. These results suggest that changes in vascular responsiveness due to hypercholesterolemia can be demonstrated very early after the onset of hypercholesterolemia and can be demonstrated in isolated vessels bathed in cholesterol-free media.

Central alpha receptors and their role in digoxin cardiotoxicity

The purpose of this study was to determine the role of alpha receptors in the inotropic and cardiotoxic actions of digoxin. Pentobarbital-anesthetized dogs were pretreated centrally with either prazosin, a selective alpha-1 antagonist, or yohimbine, a selective alpha-2 antagonist. Cardiac rhythm, blood pressure and contractile force were monitored during a 60-min period after pretreatment and during a continuous i.v. infusion of digoxin (2.5 micrograms/kg/min). Both agents, when administered into the lateral ventricle, produced depressant effects on hemodynamic parameters. Yohimbine (100 and 200 micrograms/kg i.c.v.) significantly increased the arrhythmogenic and lethal doses of digoxin in a dose-dependent manner. The protective effects of yohimbine (200 micrograms/kg) were not evident when the drug was given peripherally. Central alpha-1 blockade with prazosin increased the lethal dose of digoxin only at the largest dose (100 micrograms/kg i.c.v.) and its effects may be attributed to nonspecific central activity. BHT-933 (5 micrograms/kg i.c.v.), a selective alpha-2 agonist, enhanced the toxic effects of digoxin by decreasing both the arrhythmogenic and lethal dose of digoxin. These results suggest a central alpha-2 receptor mediation of the cardiotoxic actions of digoxin.

Increased automaticity and decreased inotropism of ouabain in dogs with furosemide-induced hypomagnesemia

Unusual cardiac glycoside toxicity has been reported during hypomagnesemia, but the underlying cause remains unexplained. Therefore, we characterized and compared the effects of ouabain in normo- and hypomagnesemic dogs with and without induced heart block. Hypomagnesemia (30% reduction in plasma magnesium) was induced with 7 days of furosemide treatment (20 mg/kg, p.o.). Following potassium replacement, ouabain was infused at 1 microgram/kg/min and changes in contractile force (strain gauge), electrocardiogram (ECG), ventricular automaticity, and cardiac conduction were monitored. The inotropic response, sustained ventricular arrhythmias, and death all occurred at significantly lower ouabain doses in hypomagnesemic dogs with and without heart block. The changes observed were specifically related to magnesium depletion, and not to potassium depletion. Ouabain activity was not different from control when both magnesium and potassium were replaced. Ouabain-induced increases in automaticity were enhanced during hypomagnesemia and contributed to the development of sustained ventricular arrhythmias at lower ouabain doses. Although the inotropic response occurred at smaller ouabain doses in hypomagnesemia, the overall increase in contractile force was diminished. Thus, increased glycoside toxicity appears related to enhanced automaticity development at doses which are normally subtoxic.

Cerebroventricular propranolol elevates cerebrospinal fluid norepinephrine and lowers blood pressure

Ventriculocisternal administration of dl- and d-propranolol produced dose-dependent increases in cerebrospinal fluid norepinephrine and reductions in blood pressure. A highly significant correlation was found between the increase in norepinephrine and the hypotensive effect. The propranolol-induced hypotension was prevented by intracisternal phentolamine. These data indicate that the hypotensive effect of centrally administered propranolol results from a drug-induced release of norepinephrine, which stimulates central alpha receptors to lower arterial pressure.