Dietary magnesium deficiency increases Gi alpha levels in the rat heart after myocardial infarction.

OBJECTIVE: Magnesium (Mg) is crucial for the function of G proteins which play important roles in mediating the inotropic effects of beta adrenergic agonists in the heart and are altered in heart failure. This study was performed to determine whether or not dietary Mg deficiency alters functional activity and levels of the two major ventricular G proteins, Gi alpha and Gs alpha in the heart after myocardial infarction (MI). METHODS: Six week old rats were fed an Mg adequate or deficient diet for 6 weeks. At the end of week 3, MI was induced by coronary artery ligation. A sham operation was performed as control. After surgery, surviving animals were maintained on their assigned diets for another 3 weeks. Then, cardiac function was measured, plasma and tissue were collected. RESULTS: Severe hypomagnesemia and increased plasma catecholamine level were observed in all animals fed the Mg deficient diet. A significant reduction of myocardial Mg concentration accompanied by elevated plasma and myocardial calcium concentrations was observed in MI animals with existing Mg deficiency vs. animals fed the Mg adequate diet. Cardiac function was impaired in MI rats and further reduced in MI rats with existing Mg deficiency. Gi alpha level was not altered by either Mg deficiency or MI alone, but was dramatically elevated in animals with combined Mg deficiency and MI (9.9 +/- 0.7 arbitrary unit.mg-1 protein) as compared to MI alone (5.8 +/- 0.6, P < 0.05) and Mg deficiency alone (6.1 +/- 0.8, P < 0.05). Gs alpha level did not differ between groups. GppNHp-, but not fluoride-stimulated adenylyl cyclase activity was slightly reduced in MI animals with existing Mg deficiency. CONCLUSION: The findings suggest that dietary Mg deficiency increases the expression of Gi alpha in the heart after MI, while levels and function of Gs alpha are not compromised during dietary Mg deficiency either with or without MI.

Alveolar dead space does not affect indirect Fick cardiac output determinations.

We examined the influence of three variables (different breathing circuits, breath selected for analysis, and alveolar dead space ventilation) on the accuracy of noninvasive cardiac output determinations with the Fick CO2 (indirect) equation. We compared noninvasive determinations with invasive thermodilution measurements over a wide range of cardiac outputs in 17 2-mo-old pigs anesthetized with halothane and nitrous oxide and paralyzed with either pancuronium or d-tubocurare. We found that rebreathing and nonrebreathing circuits provide accurate cardiac output determinations and that the optimal breath for analysis with either the rebreathing or nonrebreathing technique appears to depend on the cardiac output. When alveolar dead space was increased by using positional changes and the intracardiac administration of glass beads, there was still a good correlation between noninvasive and invasive cardiac output determinations. We conclude that both rebreathing and nonrebreathing techniques of indirect Fick cardiac output determinations correlate well with thermodilution measures over a wide range of cardiac outputs and alveolar dead space/tidal volume fractions.

Severe dietary magnesium deficiency does not alter levels and function of myocardial Gs alpha and Gi alpha.

Magnesium ions (Mg2+) play a crucial role in the activation and synthesis of guanine nucleotide-binding proteins (G proteins). However, there is no information about the influence of in vivo magnesium deficiency on the function and levels of G proteins. This study was done to investigate whether dietary magnesium deficiency alters function and levels of the two major myocardial G proteins, Gi alpha and Gs alpha. Severe hypomagnesemia and a significant reduction of myocardial magnesium occurred in rats fed a magnesium-deficient diet for 6 wk vs. rats fed a normal-magnesium diet (control). The magnesium-deficient rats developed focal myocardial lesions but their cardiac function was not impaired. Myocardial immunodetectable Gs alpha and Gi alpha levels of magnesium-deficient rats did not differ from control (Gs alpha: 2.39 +/- 0.52 vs. 2.76 +/- 0.72 arbitrary units/microgram protein, P > 0.05; Gi alpha: 1.60 +/- 0.52 vs. 1.89 +/- 0.30 arbitrary units/microgram protein, P > 0.05). Similarly, the function of Gs alpha and Gi alpha estimated by basal and ligand-stimulated adenylyl cyclase activity was not significantly different between the two groups of animals. The results show that dietary-derived magnesium deficiency sufficient to produce severe hypomagnesemia does not produce any significant change in levels or function of myocardial G proteins.

Phenotypic expression of the systemic toxicity of cocaine in genetically epilepsy-prone rats.

The purpose of the present investigation was to determine whether the sensitivity to systemic toxic effects of cocaine is altered in genetically epilepsy-prone rats (GEPRs). Moderate seizure (GEPR-3) and severe seizure (GEPR-9) rats, and the control strain, Sprague-Dawley rats, 10 weeks of age, were lightly anesthetized with halothane and nitrous oxide. Following surgical preparation and stabilization, the animals were given a constant intravenous infusion of cocaine (4 mg/kg per min) until death. Blood pressure, ECG, and EEG were monitored continuously throughout the experiment. Cocaine doses required to produce seizures (i.e., epileptiform activity on the EEG) were not significantly different between GEPRs and control rats (16.8+/-0.6 mg/kg in GEPR-3, 18.7+/-0.7 mg/kg in GEPR-9, and 14.7+/-1.3 mg/kg in Sprague-Dawley). Seizure duration, amplitude and the number of epileptiform bursts were also similar among the three strains. Additionally, there was no significant difference in cocaine doses that produced arrhythmias and cardiac asystole between GEPRs and control. The results indicate that genetically epilepsy-prone rats do not exhibit altered sensitivity to cocaine-induced seizures despite the marked susceptibility to sound-evoked seizures. Local anesthetic-induced seizures and acoustically-evoked seizures apparently have different underlying mechanisms.

Effects of nitric oxide synthesis inhibition with or without nitric oxide inhalation on responses to systemic cocaine administration in rats.

The effects of N omega-nitro-L-arginine methyl ester (L-NAME) i.v. and nitric oxide (NO) inhalation on integrated systemic responses to cocaine were studied in lightly anesthetized, paralyzed, and mechanically ventilated rats. Cocaine (4 mg/kg/min i.v.) produced seizures then isoelectric electrocephalographic (isoEEG) activity as well as an initial increase in systolic blood pressure and heart rate, then progressive cardiovascular system depression culminating in asystole. Pretreatment with L-NAME (2 mg/kg/min i.v.) for 30 min significantly reduced the incidence of seizure as compared to saline treated animals (saline 7/8; L-NAME 3/8). Doses of cocaine that produced arrhythmias, isoEEG and asystole were significantly lower in the L-NAME treated animals as compared to the saline group. L-NAME did not affect peak systolic blood pressure and heart rate responses to cocaine. No inhalation (80 ppm) did not affect CNS and cardiovascular responses to cocaine in control animals but enhanced the effects of L-NAME on cocaine toxicity. The results show that pretreatment with L-NAME reduces the central nervous system stimulatory effect of cocaine (reduced seizure incidence) and enhances its depressant effect on both the central nervous system (lower does for isoEEG) and the cardiovascular system (lower dose for arrhythmias and asystole), but does not affect the cardiovascular stimulatory action of cocaine. NO inhalation does not protect against any of the systemic effects of cocaine in animals with normal or suppressed NO production.

Haloxon-induced delayed neurotoxicity: effect of plasma A (aryl) esterase activity on severity of lesions in sheep.

A markedly different response to the delayed neurotoxic effects of a single dose of 400 or 800 mg/kg of the organophosphorus anthelmintic haloxon was observed in two populations of sheep. Animals with a gentically determined low level of activity of the plasma enzyme A (aryl) esterase developed clinical signs of delayed neurotoxicity within one month. Lesions relating to degeneration of myelinated nerve fibers were seen in brain stem, spinal cord, and on occasion peripheral nerve. The incidence of clinical signs, and severity of lesions as determined by semiquantitative morphological study of the spinal cord, were greater in animals given the higher dose. Neither clinical signs nor lesions relating to organophosphate-induced delayed neurotoxicity were seen in sheep with high plasma activity of A esterase.

Morphine self-administration in the rat during adjuvant-induced arthritis.

Rats injected with Freund's adjuvant develop a syndrome resembling human rheumatoid arthritis complete with paw swelling, edema and persistent pain. At the onset of pain, arthritic rats and their pain-free littermate controls (vehicle injection) were allowed to self-administer intravenous morphine (5.0 mg/kg/injection) in a 24 hr/day schedule. Self-injected morphine appeared to provide analgesia in arthritic rats as demonstrated by a decreased sensitivity to applied tail pressure. Arthritic rats self-inject significantly less morphine than pain-free animals. Injection of indomethacin, which alleviates the pain and inflammation of the adjuvant-induced disease, reduces, at least initially, morphine self-injection in the arthritic but not pain-free animals. As the adjuvant-induced inflammation and pain dissipated, arthritic rats rapidly began to increase opioid intake. The presence of persistent pain apparently reduces the addictive properties of morphine.

Cocaine cardiotoxicity differs markedly in isolated hearts of two strains of rats exhibiting phenotypic differences in sensitivity to seizures.

Isolated hearts from two strains of rats bred for sensitivity or resistance to amygdala kindling that also exhibit, in vivo, differential sensitivity to the cardiotoxicity of cocaine were studied. The goal was to determine if the differential cardiotoxic sensitivity was due, at least in part, to intrinsic strain-dependent differences in the heart. The Langendorff preparation was used (n=8 per strain). Hearts were perfused with increasing concentrations of cocaine (5 x 10(-6), 1 x 10(-5), 5 x 10(-5), 1 x 10(-4), and 5 x 10(-4) M) for 5 min with a 5 min washout between exposure to successive concentrations. Consistent with in vivo observations, hearts from genetically slow amygdala kindling rats (Slow) required lower cocaine doses to develop cardiac arrhythmias and arrest as compared to the hearts from genetically fast amygdala kindling rats (Fast). At 5 x 10(-5) M cocaine arrhythmias occurred in 38% (3/8) Slow and 0% Fast hearts. Five of 8 Slow hearts and none of 8 Fast hearts were arrested by 10(-4) M cocaine. Arrest in Fast hearts occurred only with 5 x 10(-4) M cocaine. Cocaine constricted coronary arteries (no significant difference between strains). On the other hand, coronary arteries of Slow but not Fast hearts dilated during cocaine washout after perfusion with all but the highest concentration of cocaine. We conclude that factors intrinsic to the heart and coronary artery influence the sensitivity or response of these structures to cocaine.

Nitric oxide modulation affects the tissue distribution and toxicity of bupivacaine.

We have previously demonstrated that inhibition of nitric oxide synthase (NOS) alters the toxicity of local anesthetics including bupivacaine. Because significant changes in blood distribution are associated with the use of nonselective NOS inhibitors, the purpose of this study was to determine whether modification of bupivacaine toxicity by nonselective NOS inhibition is due to alteration in tissue disposition of bupivacaine. Rats were anesthetized with halothane and pretreated with either: 1) a nonselective NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 2 mg/kg/min, IV for 30 min); 2) a neuronal NOS inhibitor, 7-nitroindazole (7-NI, 30 mg/kg, IP); or 3) vehicle (control). Thirty minutes later, bupivacaine 2 mg/kg/min IV was infused until onset of seizures, arrhythmias, or asystole. L-NAME caused a rapid increase in plasma bupivacaine concentrations (3-4 times faster than in the other groups), which was associated with markedly lower bupivacaine doses (mg/kg) required to produce arrhythmias in L-NAME (4.2 +/- 0.5) vs. control (26 +/- 3, p < 0.01) and 7-NI groups (17 +/- 3, p < 0.01). Myocardial bupivacaine concentrations at arrhythmia onset were slightly lower in the L-NAME group. Bupivacaine seizure doses in 7-NI and L-NAME pretreated animals were similar to control but significantly different from each other. Brain bupivacaine concentrations at seizure onset were similar among the groups. There were no significant differences between 7-NI and control groups in any parameter observed. We conclude that enhanced cardiotoxicity of bupivacaine by nonselective NOS inhibition is primarily due to rapid increases in plasma and myocardial distribution of bupivacaine.

Percutaneous epidural neuroplasty: prospective evaluation of 0.9% NaCl versus 10% NaCl with or without hyaluronidase.

BACKGROUND AND OBJECTIVES: Percutaneous epidural neuroplasty (epidural neurolysis, lysis of epidural adhesions) is an interventional pain management technique that has emerged over approximately the last 10 years as part of a multidisciplinary approach to treating radiculopathy with low back pain. In addition to local anesthetic and corticosteroid, hypertonic saline (10% NaCl) and hyaluronidase are used for the technique. The objective of this study was to determine if hypertonic saline or hyaluronidase influenced treatment outcomes. METHODS: Eighty-three subjects with radiculopathy plus low back pain were assigned to one of four epidural neuroplasty treatment groups: (a) hypertonic saline plus hyaluronidase, (b) hypertonic saline, (b) isotonic saline (0.9% NaCl), or (d) isotonic saline plus hyaluronidase. Subjects in all treatment groups received epidural corticosteroid and local anesthetic. RESULTS: Twenty-four subjects did not complete the study. Most of the other 59 subjects receiving any of the four treatments as part of their pain management obtained significant relief immediately after treatment. Visual analog scale (VAS) scores for the area of maximal pain (VASmax; back or leg) were reduced in 25% or more of subjects in all treatment groups at all post-treatment follow-up times (1, 3, 6, 9, and 12 months). A smaller fraction of subjects treated with hypertonic saline or hyaluronidase and hypertonic saline required more additional treatments than did subjects receiving the other treatments. CONCLUSIONS: Percutaneous epidural neuroplasty, as part of an overall pain management strategy, reduces pain (sometimes for over one year) in 25% or more of subjects with radiculopathy plus low back pain refractory to conventional therapies. The use of hypertonic saline may reduce the number of patients that require additional treatments.

Anesthesia update: agents, definitions, and strategies.

The trend in modern anesthesia is to "lighten up." This generally involves use of several drugs with selective and complementary actions. The pharmacokinetic properties of such drugs should allow rapid onset, rapid recovery, and rapid responses to changes in delivered doses. Peri-operative management issues also are inherent to use of modern drugs and techniques. For example, provisions must be in place for postoperative analgesia if rapid recovery is anticipated. Light anesthesia reduces morbidity and mortality, and reduces the drug, facility, and personnel costs associated with anesthesia. However, the requirements for anesthesia and the expertise of personnel administering anesthesia vary considerably. Many regulatory bodies and scientific journals require a description of how anesthesia adequacy and depth will be assessed, as well as extensive justification for the use of neuromuscular blocking agents. In environments where adequate experience and sophistication for the use of cutting edge drugs and methods are not available, older drugs and techniques may be adequate and preferable to protect animals from pain or distress.

Respiratory and cardiovascular effects of halothane, isoflurane and enflurane delivered via a Jackson-Rees breathing system in temperature controlled and uncontrolled rats.

We studied the respiratory and cardiovascular effects of 1.25 MAC halothane, isoflurane and enflurane in oxygen delivered via the Jackson-Rees breathing system in 10 rats. Mean arterial pressure, heart rate and respiratory rate were depressed significantly (P less than 0.05) in rats (n = 5) whose body temperature was not controlled after 2 hr of anesthesia regardless of the inhalational agent. Respiratory and metabolic acidosis developed. The respiratory and cardiovascular depression was most marked under enflurane anesthesia. In normothermic rats (n = 5) the initial cardiovascular depression stabilized after 30 min of halothane and isoflurane anesthesia. Moderate respiratory depression developed (PCO2 48.42 +/- 2.48 torr with halothane vs. 41.02 +/- 1.68 torr with isoflurane). Because the cardiovascular and respiratory changes caused by halothane and isoflurane were far less than changes produced by enflurane, halothane or isoflurane is preferable to enflurane for maintaining anesthesia in rats. Maintenance of constant temperature minimizes the cardiovascular and respiratory disturbances.

A simple and rapid high-pressure liquid chromatographic assay for pentobarbital and other barbiturates in serum.

A single extraction technique for measuring pentobarbital and other barbiturates in serum involved high-pressure liquid chromatography and UV detection yielding a sensitivity for pentobarbital of about 1 microgram/ml. We concluded that the assay provides a practical method for use in pharmacokinetic studies and in serum concentration monitoring in experimental animals.

Lactate dehydrogenase isoenzymes in blood and cerebrospinal fluid from healthy beagles.

Serum and CSF lactate dehydrogenase (LDH) activity and LDH isoenzyme profile, as well as total protein, were measured in samples obtained from 26 healthy Beagles. The LDH activity in serum was approximately 10 times that in the CSF. The CSF LDH isoenzyme profile was a mirror image of the serum LDH isoenzyme profile. Age was negatively correlated (ie, as age increased, the activity decreased) with LDH3 in CSF and LDH5 in serum and was positively correlated with LDH1 in serum. Seemingly, simultaneous measurement of serum and CSF LDH activity and LDH isoenzyme profile can aid in establishing status of the blood-brain barrier, whether brain damage exists and whether other organ systems are involved.

Effect of atropine on tear formation in anesthetized dogs.

Tear production (as determined by the Schirmer I tear test) in five dogs given atropine (0.02 mg/kg) subcutaneously and in five dogs given 0.9% saline solution subcutaneously was compared before and during halothane anesthesia. Fifteen minutes after the atropine injections, mean tear production was 15.0 +/- 2.9 mm/minute, as compared with 23.8 +/- 2.9 mm/minute before treatment rate. There was no change in tearing 15 minutes after injection of the saline solution. Tear production declined in both groups during anesthesia. Ten minutes after anesthetic induction, mean tear formation was 20% of pretreatment value in the atropinized dogs and 54% of the pretreatment value in the dogs given saline solution. At the end of 1 hour of anesthesia, tearing was essentially zero in both groups.

Trichlorfon-induced congenital cerebellar hypoplasia in neonatal pigs.

The neuroteratogenicity of trichlorfon was evaluated in 3 groups of pregnant sows (8/group). The treatments were: control (no trichlorfon), trichlorfon (60 mg/kg of body weight) in the feed only on day 55 of gestation, or trichlorfon (60 mg/kg of body weight) in the feed on day 55 and day 70 of gestation. One week after farrowing, all newborn pigs were removed from the sows and were euthanatized. Brain and cerebellum weights of the newborn pigs were recorded. Mean cerebellum weights and cerebellum/total brain weight ratios of the neonatal pigs were 3.780 and 0.106 (group 1, n = 26), 3.183 and 0.098 (group 2, n = 42), and 2.986 and 0.088 (group 3, n = 61). Although trichlorfon interfered with cerebellar development and the severity of the trichlorfon-induced cerebellar hypoplasia was dosage-related, ataxia did not develop in the neonatal pigs.

Legal implications of the extra-label use of drugs in food animals.

Although the Food and Drug Administration has never sanctioned the extra-label use of drugs in animals, it has not, until recently, objected when veterinarians deviated from label instructions as long as such use did not result in violative residues in food products derived from treated animals. However, because of the potential human health hazards associated with abuse of this position, the FDA has developed a less lenient policy toward the extra-label use of drugs in food animals. A brief review of the legislated responsibility and authority of the FDA with particular reference to the extra-label use of drugs in food animals is provided. Emphasis is placed on how anesthetics and anesthesia adjuncts in food animals can pose human health hazards.