Comparative genotoxicity of quinolone and quinolonyl-lactam antibacterials in the in vitro micronucleus assay in Chinese hamster ovary cells.

The in vitro micronucleus assay is gaining increased attention as a potential alternative to the standard in vitro metaphase analysis assay. In particular, the in vitro micronucleus assay has been proposed as a useful method for chemicals that induce both structural and numerical chromosome alterations, such as DNA gyrase/topoisomerase inhibitors. In this study, we compared the micronucleus-inducing activity of quinolonyl-lactam antibacterials that inhibit DNA-gyrase and bind to penicillin-binding proteins relative to the activity of structurally related quinolone antibacterials that also inhibit DNA-gyrase. All of the quinolones that were structurally related to the quinolonyl-lactams were cytotoxic and induced large increases in the frequency of micronucleated binucleated cells (MNBC) at concentrations between 0.02 and 0.16 mM. These changes were larger than those seen with the commercial quinolones, ciprofloxacin (cytotoxic at > or = 0.57 mM and MNBC at > or = 0.3 mM) and nalidixic acid (cytotoxic at 1.8 mM and no MNBC up to this dose). In contrast, the quinolonyl-lactams were not cytotoxic up to 1.0 mM concentrations and induced either no MNBC or a low frequency of MNBC at higher concentrations compared to the quinolones. Quinolonyl-lactams appear to be less cytotoxic and genotoxic than structurally related quinolones. These results add to the growing database on the in vitro micronucleus assay in general, and more specifically to the relatively small database for the in vitro micronucleus assay in Chinese hamster ovary cells.

Effects of doxylamine succinate on thyroid hormone balance and enzyme induction in mice.

The effects of doxylamine (as the succinate salt) on microsomal enzyme activity and serum thyroid hormone levels were examined in B6C3F1 mice following dietary exposure for 7 or 15 days (0, 40, 375, 750, or 1500 ppm in diet, expressed as free base doxylamine). In addition, the hepatic P450 enzyme inducer sodium phenobarbital (375 ppm, expressed as free acid phenobarbital) was used as a positive control for CYP2B induction. Exposure of mice to doxylamine produced dose-related increases in liver weight at both time points. Liver weights were also increased in the phenobarbital-treated mice. Doxylamine treatment caused a dose-dependent increase (up to 2.6-fold) in liver microsomal cytochrome P450 in both male and female mice, at both time points. Analyses of the activities of various hepatic microsomal cytochromes P450 indicated that doxylamine caused a marked induction of CYP2B enzymes. This was demonstrated by a large increase in the O-dealkylation of 7-pentoxyresorufin (up to 38-fold) and the 16beta-hydroxylation of testosterone (up to 6.9-fold), both of which are indicative of CYP2B induction. In addition, like phenobarbital, doxylamine treatment resulted in a modest induction of CYP3A and CYP2A enzymes and approximately a 50% increase in thyroxine-glucuronosyltransferase activity. Doxylamine did not appear to induce P450 enzymes in the CYP1A, CYP2E, or CYP4A enzyme subfamilies. None of the enzyme-inducing effects of doxylamine could be distinguished from those of phenobarbital. These results suggest that doxylamine is a phenobarbital-type inducer of liver microsomal cytochrome P450 in B6C3F1 mice. Exposure to either doxylamine or phenobarbital also resulted in decreases in serum thyroxine (T4) levels (approximately 80% of control) with compensatory increases in serum thyroid-stimulating hormone levels (approximately 4-fold). No clear changes in serum triiodothyronine levels were apparent. These findings are consistent with the hypothesis that doxylamine increases the activity of those hepatic enzymes involved in T4 metabolism.

Ethacrynic acid and furosemide alter Cl, K, and Na distribution between blood, choroid plexus, CSF, and brain.

Can loop diuretics like ethacrynic acid and furosemide, when administered intravenously, significantly alter ion transport and fluid dynamics in CNS? To shed light on this unresolved issue, we tested the ability of these agents to effect redistribution of Na, K and Cl in adult rat brain. Cl penetration into various CNS regions was assessed as the volume of distribution, i.e., uptake, of 36Cl from blood. Ethacrynic acid and furosemide (50 mg/kg IV) reduced by 20-30% the rate of permeation of 36Cl across the blood-CSF barrier, and they elevated [K] and [Cl] in choroid plexus (CP) by 15-25%. The loop diuretic-induced buildup of K and Cl in CP (lateral and 4th ventricle) was likely a reflection of decreased movement of these ions across the apical membrane into CSF. 36Cl activity in parietal cortex and pons-medulla decreased in treatment with furosemide and ethacrynic acid, due to slowing of Cl transport across blood-brain and/or blood-CSF barriers. Our inhibitory findings in intact rats are consistent with those from previous in vitro experiments demonstrating diminution by loop diuretics of Na, K and Cl transport across isolated CP membranes.

Brain transfer coefficients for 67Ga: comparison to 55Fe and effect of calcium deficiency.

The transfer coefficients (Kin) for the uptake of gallium-67 (67Ga) into brain and CSF were determined in unanesthetized male Fischer-344 rats fed either a normal or a low-Ca diet. Kin for 67Ga was also compared with transfer coefficients for the uptake of iron-55 (55Fe) and 125I-albumin in control animals. The value of CSF 67Ga Kin was 3 x 10(-7) ml.g-1.s-1 and was 50% larger in low-Ca animals. Brain regional Kin values for 67Ga were 3-9 x 10(-7) ml.g-1.s-1 with no differences in Kin between normal and low-Ca rats. CSF Kin values for 55Fe were 40% and those for albumin were 15% of Kin for 67Ga. For brain, Kin values for 55Fe were 15-40% smaller than for 67Ga, but for albumin the Kin values were 85% less than for 67Ga. 67Ga was found to be 99% bound to plasma proteins, whereas 55Fe was 99.9% bound. The results indicate that metals that are primarily bound to transferrin enter the CSF and brain very slowly. Uptake of both metals was faster than albumin, which may indicate that metal bound to small chelates contributes significantly to brain uptake. In addition, Ca deficiency does not enhance entry of Ga into the brain.

Saturable transport of manganese(II) across blood-nerve barrier of rat peripheral nerve.

To determine whether the blood-nerve barrier of the rat peripheral nerve transports manganese(II) (Mn) by a saturable mechanism similar to that found at the blood-brain barrier, we measured the uptake of 54Mn from blood into desheathed sciatic nerve and into cerebral cortex of awake rats at different plasma concentrations of unlabeled Mn using an intravenous infusion technique. The unidirectional influx (Jin) of Mn into sciatic nerve was facilitated and saturable, when steady-state plasma Mn ranged from 4 to 4,312 ng/ml (0.073-78.4 microM), as was the unidirectional influx of Mn into the cerebral cortex. Michaelis-Menten constants (Km and Vmax) and the passive diffusion constant (Kd), determined by nonlinear least squares, were as follows: for the blood-nerve barrier (sciatic nerve) Km = 4.7 microM, Vmax = 0.56 x 10(-3) nmol.s-1.g wet wt-1, and Kd = 6.3 x 10(-6) ml.s-1.g wet wt-1; for the blood-brain barrier (cerebral cortex) Km = 1.0 microM, Vmax = 0.40 x 10(-3) nmol.s-1.g wet wt-1, and Kd = 0.3 x 10(-6) ml.s-1.g wet wt-1. The results demonstrate facilitated concentration-dependent mechanisms of transport of Mn at the blood-nerve and blood-brain barriers.

Saturable transport of Ca into CSF in chronic hypo- and hypercalcemia.

To further characterize possible saturable transport of Ca into CSF during chronic plasma [Ca] changes, weanling rats were fed diets differing in Ca for 10 weeks. Transfer coefficients for unidirectional uptake of 45Ca and 36Cl into CSF (Kcsf) were determined 3 or 10 min after intravenous tracer injection in unanesthetized animals. In rats fed low Ca diet, Kcsfs for 45Ca and 36Cl were elevated above control, but the 45Ca/36Cl ratio for Kcsf, a more specific measure of Ca transport, was also increased. In animals fed high Ca diet, Kcsfs of both radiotracers were not statistically different from control, but the 45Ca/36Cl ratio was decreased. Injection of CaCl2 into hypocalcemic rats elevated plasma [Ca], depressed 45Ca Kcsf, and returned the 45Ca/36Cl ratio to the control value. The inverse relationship between plasma ionized [Ca] and 45Ca Kcsf was fitted to saturation kinetics with Km less than or equal to 0.53 mumol/ml, maximal Ca influx for the saturable component between 27 and 67 x 10(-5) mumol.g-1.s-1, and the passive component of Kcsf less than or equal to 15 x 10(-5) ml.g-1.s-1. We conclude that Ca transport into CSF is saturable and this transport is important in the regulation of CSF [Ca].

Saturable transport of manganese(II) across the rat blood-brain barrier.

Unanesthetized adult male rats were infused intravenously with solutions containing 54Mn (II) and one of six concentrations of stable Mn(II). The infusion was timed to produce a near constant [Mn] in plasma for up to 20 min. Plasma was collected serially and on termination of the experiment, samples of CSF, eight brain regions, and choroid plexus (CP) were obtained. Influx of Mn (JMn) was calculated from uptake of 54Mn into tissues and CSF at two different times. Plasma [Mn] was varied 1,000-fold (0.076-78 nmol/ml). Over this plasma concentration range, JMn increased 123 times into CP, 18-120 times into brain, and 706 times into CSF. CP and brain JMn values fit saturation kinetics with Km (nmol/ml) equal to 15 for CP and 0.7-2.1 for brain, and Vmax (10(-2) nmol.g-1.s-1) of 27 for CP and 0.025-0.054 for brain. Brain JMn except at cerebral cortex had a nonsaturable component. CSF JMn varied linearly with plasma [Mn]. These findings suggest that Mn transport into brain and CP is saturable, but transport into CSF is nonsaturable.

Calcium deficiency enhances cadmium accumulation in the central nervous system.

Weanling male rats were administered 1 of 4 diets for 40 days: control (CONT), low Ca (LOCA), control plus Cd (CONT + Cd) or low Ca plus Cd (LOCA + Cd). After 40 days, Cd was analyzed in 7 brain regions, spinal cord, serum, liver, kidney, muscle and femur by atomic absorption spectrophotometry with Zeeman background correction. No significant difference in Cd between CONT and LOCA was found except in femur, where it was increased. In CONT + Cd rats, peripheral tissues showed an increase in Cd of 30-71 fold above CONT rats. Brain regions exhibited a more modest 7-10 fold change, and serum Cd was 8.5 times above control. LOCA + Cd rats showed a 25-fold increase of Cd above LOCA in serum, 25-100 fold in peripheral tissues, and a 14-20 times in brain. These findings show that brain Cd is increased during Ca deficiency, but that central nervous system Cd changes less than peripheral tissue Cd. This increase in brain Cd could alter brain function.

Transfer of 45Ca and 36Cl at the blood-nerve barrier of the sciatic nerve in rats fed low or high calcium diets.

Unidirectional fluxes of 45Ca, 36Cl, and of [3H]mannitol from blood into the sciatic nerve and cerebral cortex were determined from 5- and 15-min uptakes of these tracers after an intravenous (i.v.) bolus injection in awake rats. Rats were fed diets for 8 wk, that had either a low (0.01% wt/wt), normal (0.67%), or high (3%) Ca content. Plasma [Ca] was 32% less and 11% more in rats fed low (LOCA) and high Ca diets (HICA), respectively, than in rats fed a normal Ca diet (CONT). The mean permeability-surface area product (PA) of 45Ca at the blood-nerve barrier was about eightfold higher than at the blood-brain barrier in the same animals and did not differ significantly between groups (greater than 0.05). Mean PA ratios of 45Ca/36Cl for the blood-nerve and blood-brain barriers in CONT rats, 0.52 +/- 0.04 and 0.40 +/- 0.02, respectively, were not significantly different from corresponding ratios in LOCA and HICA groups, and corresponded to the aqueous limiting diffusion ratio (0.45). Our results show no evidence for concentration-dependent transport of Ca over a plasma [Ca] range of 0.8-1.4 mmol/liter at the blood-nerve barrier of the rat peripheral nerve, and suggest that Ca and Cl exchange slowly between nerve and blood via paracellular pathways.

Elevation of brain manganese in calcium-deficient rats.

Rats, 3 weeks of age, consumed diets low or normal in calcium (Ca) with or without supplemental manganese (Mn) as Mn (II) acetate in drinking water. After 6 weeks, the animals were killed and [Mn] was determined in 8 brain regions, spinal cord, liver, serum, kidney, femur, and skeletal muscle. Serum [Mn] increased 1.5-, 4-, and 40-fold respectively, in normal Ca-supplemented Mn rats, low Ca rats, and low Ca-supplemented Mn rats. Elevation of tissue [Mn] occurred in all experimental groups with the greatest changes in the low Ca-extra Mn group: 6 - 12 fold in brain and spinal cord, and 2.5 - 140 fold in muscle, liver, kidney, and femur. Ratios of serum [Mn]/tissue [Mn] decreased as serum [Mn] increased suggesting saturable distribution. The findings suggest that Ca deficiency may cause Mn neurotoxicity by increasing dietary Mn absorption and brain [Mn].

Na(+)-H+ exchange in choroid plexus and CSF in acute metabolic acidosis or alkalosis.

Basolateral Na(+)-H+ exchange was analyzed with an in vivo model of choroid plexus (CP) epithelium in nephrectomized adult rats anesthetized with ketamine. Acid-base balance in blood was altered for 1 h over a pH continuum of 7.19 to 7.53 by equimolar intraperitoneal injections of HCl, NH4Cl, NaCl, or NaHCO3. Compartmental analysis enabled determination of CP intracellular pH (pHi) [dimethadione (DMO) method] and the choroid cellular concentration of 23Na (stable) and 22Na (tracer). HCl acidosis reduced the outwardly directed transmembrane basolateral H+ gradient, lowered the [23Na]i by 25%, and decreased the influx coefficient (Kin) for 22Na from blood into CP parenchyma (by 45% from 0.211 to 0.117 ml.g-1.h-1) and into cerebrospinal fluid (CSF) (by 43%, from 0.897 to 0.516). Compared with acid-loaded rats (HCl or NH4Cl), the NaHCO3-alkalotic animals had significantly enhanced uptake of 22Na into the CP-CSF system. This pH-dependent transport of Na+ from blood to CP was abolished by pretreatment with amiloride, an inhibitor of Na(+)-H+ exchange. Except in severe acidosis (HCl), the choroid cell pHi (7.05 +/- 0.02 in NaCl controls) and [HCO3-] (11-12 mM) remained stable in the face of acidemic and alkalemic challenges. With respect to reaction of the blood-CSF barrier to plasma acid-base perturbations, the responses of the fourth ventricle plexus pHi, [Na+]i, and 22Na uptake were similar to corresponding ones in lateral plexuses. We conclude that in the choroidal epithelium there is a Na(+)-H+ exchange activity capable of modulating Na+ flux into the CSF by approximately 50% as arterial pH is varied from 7.2 to 7.5.

Acetazolamide and insulin alter choroid plexus epithelial cell [Na+], pH, and volume.

Agents that inhibit or stimulate Na+ transport were tested for their effects on the ionic composition and volume of the in vivo choroid plexus (CP) epithelium. Ketamine-anesthetized adult Sprague-Dawley rats treated 1 h with acetazolamide or insulin were analyzed for choroid cell [Na+]i, [HCO3-]i, and pHi (dimethadione method); for transmembrane Na+ and H+ gradients; and for the kinetics of penetration of 22Na from plasma to plexus epithelium to CSF. Acetazolamide (25 mg/kg) reduced [Na+]i by 5-10 mmol/l and substantially elevated [HCO3-]i and pHi; the concurrent 22Na uptake by the in vivo choroid plexus and CSF, as quantified by the transfer coefficient, Kin (ml.g-1.h-1), was curtailed by 55-60%. Such effects on Na+ transport and distribution are likely secondary to the alkalinization of pHi induced by carbonic anhydrase inhibition. Conversely, insulin (3 U/kg ip) stimulated Na+ transport, i.e., manifested as enhanced uptake of 22Na from plasma to choroid cell and increased [Na+]i. For various treatments altering the basolateral membrane H+ gradient, the regression analysis of the 22Na Kin vs. log [H+]i/[H+]ISF (where ISF is interstitial fluid) was significant at P less than 0.01. This is consistent with effects mediated by Na(+)-H+ exchange. K+ and Cl- redistribution phenomena were coincident with altered Na+ transport, as choroidal cells retained K+, Cl-, and H2O after acetazolamide but lost K+, Cl-, and H2O with insulin treatment. A model is presented relating alterations in CP Na+ transport, KCl content, and cell volume. Overall, the findings encourage the postulate for effects of these drugs on Na+ transport basolaterally, either indirectly by attenuating [H+]i/[H+]ISF (acetazolamide) or directly by accelerating Na+ transport (insulin).

Prevention of nerve edema and increased blood-nerve barrier permeability-surface area product in galactosemic rats by aldose reductase or thromboxane synthetase inhibitors.

Nerve water content and the permeability-surface area product (PA) to [3H]-or [14C]sucrose at the blood-nerve barrier were determined in unanesthetized control rats fed a normal diet and in rats fed galactose with or without an aldose reductase inhibitor (Statil or AL 1576) or a thromboxane synthetase inhibitor (CGS 12970). Nerve water content was determined by taking the difference between dry and wet weights of whole tibial nerves. PA was determined by an intravenous bolus injection of radiotracer with multiple-time-point graphic and quantitative autoradiographic methods. The mean nerve water content in galactosemic rats was 15% higher than in control rats after 7-11 mo on the diet. Statil and AL 1576 prevented nerve edema, but CGS 12970 was only partially effective in preventing an increase in nerve water content in galactose-fed rats. In galactosemic rats, the mean PA to sucrose at the blood-nerve barrier, calculated from nerve dry weight, was twofold higher than in control rats. Treatment with Statil, AL 1576, or CGS 12970 prevented increased PA. Our results suggest that nerve edema and increased blood-nerve barrier PA are secondary to polyol production and can be prevented by inhibiting aldose reductase.

Uptake and concentrations of calcium in rat choroid plexus during chronic hypo- and hypercalcemia.

The choroid plexus has been implicated in the regulation of cerebrospinal fluid (CSF) [Ca], but little information is available concerning Ca transport by this epithelium. We determined the transfer coefficients for 45Ca uptake into choroid plexus from blood, as well as tissue [Ca], in weanling Fischer-344 rats fed low, normal, or high Ca diets for 8 weeks. Plasma [Ca] decreased by 45% with low Ca diet and increased by 25% with high Ca diet. Choroid plexus 45Ca uptake varied inversely with plasma [Ca]. This relation was due largely to changes in extracellular Ca binding rather than to entry from blood, as the transfer coefficient was independent of plasma [Ca]. The extracellular Ca distribution in choroid plexus, the intercept of a plot of tissue 45Ca distribution against time, was reciprocally related to plasma [Ca]. Changes in total cell [Ca] during hypercalcemia were equivalent to those in plasma, and in hypocalcemia were 70% of those in plasma. These findings indicate that regulation of CSF [Ca] does not involve saturable transport of Ca into the choroid epithelium from blood, and that the apical membrane of the choroid epithelium is involved in homeostasis of CSF [Ca].

Acidosis, acetazolamide, and amiloride: effects on 22Na transfer across the blood-brain and blood-CSF barriers.

Sprague-Dawley rats were given treatments, known to decrease 22Na movement into choroid plexus and CSF, to investigate their effect on 22Na transfer across the cerebral capillaries. Acidic salts, acetazolamide, or amiloride was injected intraperitoneally into bilaterally nephrectomized rats, and the rate of 22Na uptake into parietal cortex, pons-medulla, and CSF was determined at 12, 18, and 24 min. Severe acidosis (arterial pH 7.2), produced by HCl injection, decreased the rate of 22Na entry into both brain regions and CSF by 25%, whereas mild acidosis (pH 7.3) from NH4Cl injection reduced brain entry by 18%, but CSF entry by only 10%. Like HCl acidosis, amiloride reduced transport into both brain and CSF by 22%. Penetration of 22Na into parietal cortex was unchanged by acetazolamide, but that into CSF was slowed 30%. Since uptake of 22Na into cortical regions is primarily movement of tracer across the cerebral capillaries when tracer uptake time is less than 30 min, the results indicate that both metabolic acidosis and amiloride decrease Na+ permeativity at the cerebral capillaries as well as at the choroid plexus. Acetazolamide, on the other hand, alters Na+ movement only across the choroidal epithelium.

Alteration of sodium transport by the choroid plexus with amiloride.

Cerebrospinal fluid (CSF) production results from active transport of Na+ from blood to CSF, which is followed by H2O and anions. Amiloride reduces Na+ movement in epithelial tissues. To ascertain if amiloride alters transport of Na+ in the choroid plexus, the drug was administered either i.p. to male Sprague-Dawley rats that were bilaterally nephrectomized to determine in vivo effects, or added to artificial CSF to incubate the choroid plexus in vitro. Choroid cell [Na+] was reduced after amiloride treatment both in vivo and in vitro. In addition, the rate of 22Na uptake into the CSF and choroid plexus (CP) was decreased after amiloride. Alterations in choroid cell [Na+] and 22Na penetration into CSF and CP occurred at relatively high doses of drug (1 mumol/ml, in vitro and 100 micrograms/g in vivo), but lower doses were less effective (0.1 mumol/ml in vitro and 10 micrograms/g in vivo). It is concluded that the effects of amiloride on Na+ distribution and transport in the CP are due to inhibition of basolateral Na+-H+ exchange.

Regulation of brain and cerebrospinal fluid calcium by brain barrier membranes following vitamin D-related chronic hypo- and hypercalcemia in rats.

Male Fischer-344 rats, 21 days old, were fed diets containing 0 (LOD), 2,200 (CONT), or 440,000 (HID) international units of vitamin D3 per kilogram for 12 weeks. [Ca] was measured in plasma, CSF, brain, and choroid plexus. In addition, 45Ca and 36Cl transfer coefficients (KCa and KCl) for uptake from blood into CSF and brain were determined. Although plasma ionized [Ca]s in LOD and HID rats were 50% and 136%, respectively, of values in CONT animals, CSF and brain [Ca]s ranged from only 85% to 110% of respective CONT values. Choroid plexus [Ca] was increased by 37% after HID diet, but was decreased only 10% after LOD. KCa values at CSF, parietal cortex, and pons-medulla were negatively correlated with plasma ionized [Ca], whereas KCl values at CSF and brain were not different between the diet groups. The findings demonstrate that central nervous system [Ca] is maintained during chronic hypo- or hypercalcemia by saturable transport of Ca at brain barrier membranes. This transport does not seem to involve modulation by 1,25-dihydroxyvitamin D3.

Increased transfer of 45Ca into brain and cerebrospinal fluid from plasma during chronic hypocalcemia in rats.

Recent studies have shown regulation of central nervous system [Ca] after chronic hypo- and hypercalcemia. To investigate the mechanism of this regulation, 3-week-old rats were fed diets for 8 weeks that contained low or normal levels of Ca. Plasma [Ca] was 40% less in rats fed the low Ca diet than in animals fed normal diet. Unidirectional transfer coefficients for Ca (KCa) and Cl (KCl) into cerebrospinal fluid (CSF) and brain were determined from the 10 min uptake of intravenously injected 45Ca and 36Cl in awake animals. KCa for CSF was 68% greater in low-Ca rats than in normal rats. Likewise, the values of KCa for brain regions with areas adjacent to the ventricles like the hippocampus and pons-medulla were 50% higher than in normal animals. On the other hand, KCas for parietal cortex, a brain region distant from the choroid plexus and not expected to be influenced by Ca entry into CSF, were similar between the groups. Comparison of the regional ratios of KCa/KCl revealed that a selective increase of Ca transport occurred into CSF and all brain regions except the parietal cortex in Ca-deficient rats. The results suggest that Ca homeostasis of CSF and brain [Ca] during chronic hypocalcemia is due to increased transfer of Ca from blood to brain, and that the regulation occurs via the CSF, possibly at the choroid plexus, but not via the cerebral capillaries.

Calcium in rat peripheral nerve during chronic alterations in plasma calcium.

The calcium content in desheathed tibial nerve was compared to that in cerebellum in rats fed diets containing either 0.01% (low), 0.67% (control) or 3.0% (high) Ca, for 8 weeks. For changes in concentration of plasma ionized Ca, 48% below and 35% above the control mean, percent change in endoneurial Ca content is linearly related, with a slope of 0.80, to percent change in plasma ionized Ca. A line with a slope of 0.21 describes the relation between percent change in cerebellum Ca and percent change in plasma ionized Ca. Plasma, cerebellum and nerve concentrations of Na, K and Cl were similar in the control compared with the two experimental groups of animals. The concentration of plasma Mg varied 20% below and 17% above the control mean, inversely with plasma Ca, but nerve and cerebellum Mg did not change from control values. The results of this study fail to demonstrate Ca homeostasis in rat peripheral nerve endoneurium during chronic hypo- and hypercalcemia. Endoneurial Mg, however, appears to be regulated.