Biomedical subjects
Publications and source records attributed to B M Altura.
Using a novel ion-selective electrode, we have shown that there is over a 100% increase in venous serum levels of ionized Mg during intravenous MgSO4 therapy of preeclamptic patients. By 6 hours of therapy, the range is 0.92-1.65 mmol/l (95% C.I.). There are strong correlations between levels of ionized and total Mg throughout therapy (r = 0.92-0.95, p < 0.01).
In several previous experiments to determine the composition of interstitial fluid, the results varied depending on the collecting technique, and the electrolyte concentrations differed from those of a hypothetical ultrafiltrate of plasma. In our approach, since a change of position from standing to supine is accompanied by hemodilution with interstitial fluid, we used the changes in hematocrit and composition of plasma in 20 subjects before and after lying down to calculate the composition of added interstitial fluid. The estimated protein concentration was 20.6 g/L, and the concentrations of total calcium and magnesium were low, in accord with a lower concentration of protein-bound calcium and magnesium. The activity of free cations was also lower, in agreement with a Donnan equilibrium potential of 1 mV across the endothelium. The concentration of leukocytes and platelets decreased according to the hemodilution, implying no escape or mobilization of these elements.
31P-NMR spectroscopic studies were performed in vivo on brains of rats administered cocaine. Cocaine.HCl (1-5 mg/kg) administered systemically to lightly anesthetized rats resulted in significant and progressive deficits in whole brain intracellular free Mg ([Mg2+]i). Intracellular pH (pHi) also fell in a progressive manner but only after a significant fall in brain [Mg2+]i was noted. Both [Mg2+]i and pHi returned to normal in most rats. Brains of rats that exhibited stroke-like events, however, demonstrated continued intracellular acidosis associated with progressive loss of phosphocreatine and elevation of Pi up until death. These observations are consistent with the tenet that injection of cocaine can result in severe cerebral vasospasm, ischemia and rupture of cerebral blood vessels as a consequence of depletion of brain [Mg2+]i.
Regulatory effects of extracellular magnesium ions ([Mg2+]o) on intracellular free ionized calcium ([Ca2+]i) were studied in cultured vascular smooth muscle cells (VSMCs) from rat aorta by use of the fluorescent indicator fura-2 and digital imaging microscopy. With normal Mg2+ (1.2 mM)-containing incubation media, [Ca2+]i in VSMCs was 93.6 +/- 7.93 nM with a heterogeneous cellular distribution. Lowering [Mg2+]o to 0 mM or 0.3 mM (the lowest physiological range) resulted in 5.8-fold (579.5 +/- 39.99 nM) and 3.5-fold (348.0 +/- 31.52 nM) increments of [Ca2+]i, respectively, without influencing the cellular distribution of [Ca2+]i. Surprisingly, [Mg2+]o withdrawal induced changes of cell geometry in many VSMCs, i.e., the cells rounded up. However, elevation of [Mg2+]o up to 4.8 mM only induced slight decrements of [Ca2+]i (mean = 72.0 +/- 4.55 nM). The large increment of [Ca2+]i induced by [Mg2+]o withdrawal was totally inhibited when [Ca2+]o was removed. The data suggest that: (1) [Mg2+]o regulates the level of [Ca2+]i in rat aortic smooth muscle cells, and (2) [Mg2+] acts as an important regulatory ion by modulating cell shapes in cultured VSMc and their metabolism to control vascular contractile activities.
Regulatory effects of extracellular magnesium ions ([Mg2+]o) on intracellular free ionized magnesium ([Mg2+]i) were examined in cultured vascular smooth muscle cells (VSMCs) from rat aorta by digital imaging microscopy using the Mg2+ fluorescent probe, Mag-fura-2. With normal Mg2+ (1.2 mM)-containing incubation media, [Mg2+]i in VSMCs was 0.63 +/- 0.09 mM. The ratio of [Mg2+]i/[Mg2+]o was 0.52 +/- 0.07. Elevation of [Mg2+]o up to 4.8 mM induced consistent increments in [Mg2+]i (to a mean values of 1.63 +/- 0.08 mM) in 5 min and lowered the ratio of [Mg2+]i/[Mg2+]o to 0.34 +/- 0.02. Our data suggest that [Mg2+]o can regulate [Mg2+]i, which may be related to its effects on intracellular Ca2+ ([Ca2+]i) and tone of VSMCs.
1. In isolated aortae of the male rat [Mg2+]o withdrawal and concomitant reduction in [Na+]o (to 84 mM) induced significant increases of basal tone, but, surprisingly, this did not occur in intact aortae removed from female rats. Such tension development, however, was observed in endothelium-denuded aortic preparations from both sexes. These observed gender-related differences were not dependent on animal strain or types of tissue preparations. 2. No tension development was observed in aortae obtained from castrated males treated with oestradiol. Aortic tissues of sexually-immature male and female rats exhibited marked tension development when exposed to 0 mM [Mg2+]o and low [Na+]o. 3. Tension development in Mg(2+)-free, low-Na+ media was not tachyphylactic and completely dependent on extracellular Ca2+; addition of 1.2 mM Mg2+ to the Mg2+ and Na(+)-deficient incubation media relaxed the increase in tension to a normal basal level. 4. Two known endothelial-derived relaxant factor (EDRF) inhibitors, methylene blue and haemoglobin, induced tension development in female aortae with intact endothelium exposed to Mg(2+)-Na+ deficient media, while use of a specific inhibitor of EDRF-derived nitric oxide, viz., NG-monomethyl-L-arginine (L-NMMA), resulted in potentiation of tension development in male, but not in female, aortae. This effect of L-NMMA was antagonized by L-arginine. 5. The Ca ionophore, A23187, partially relaxed contractile responses in male aortae (with intact endothelium) which were followed by potentiated contractions. Endothelium-dependent vasodilator responses to A23187 (10(-10)-10(-6) M) of aortic rings from male or female rats in normal Krebs-Ringer bicarbonate solution were not different.6. These results suggest that: (a) as in vascular smooth muscle cells, Mg2+ plays an important role in Ca2 + homeostasis in endothelial cells, probably via Na+-Ca2+ exchange; and (b) sex steroid hormones, probably the female sex hormone, 17-beta-oestradiol, may regulate contractile responses of intact vascular smooth muscle by modifying endothelium functions through such Mg2 '-regulated internal Natdependent Ca2+ entry. These data may help to explain why female subjects, despite Mg deficiency, unlike male subjects, are protected against ischaemic heart disease and cerebrovascular disease until menopause.
Effects of ethanol on intracellular-free Ca2+ concentration in cultured rat aortic smooth muscle cells were examined by digital imaging fluorescence microscopy using the Ca2+ fluorescence indicator, fura-2. Ethanol induced dose-dependent decrements in cytosolic-free Ca2+ concentration at 45 mM and 90 mM, which was consistent with previously reported observations of relaxation in intact rat aortic tissues. However, ethanol at high pharmacological concentrations (e.g., 450 mM) failed to induce any further inhibition in cytosolic-free Ca2+ concentration. Our results suggest that the vasodilator effects of ethanol, observed on intact blood vessels, may result in part from an interference with the availability of Ca2+ for excitation-contraction coupling in vascular smooth muscle cells.
It has been demonstrated that audiogenic stress (AS) can induce elevation of arterial blood pressure (ABP) in animals and humans and that noise-induced hearing loss may be associated with alterations in Mg metabolism. Experiments were designed to determine whether 1) there is a causal relationship among environmental noise stress, serum and vascular tissue (aortas and portal veins) Mg contents, and development of hypertension and 2) such noise-induced hypertension has a microcirculatory basis and what the mechanism may be. Rats maintained on normal Mg-containing diets for 12 wk (plasma [Mg] = 0.96 +/- 0.02 mM) and subjected to AS (85 dB(A), 12 h/day for 8 wk; 95 dB(A), 16 h/day for 4 wk) demonstrated significant elevation in systolic and diastolic ABP; plasma [Mg] showed a 15% deficit, whereas aortic and portal vein muscle exhibited slight reductions in Mg content and elevation in Ca. Moderate and more severely Mg-deficient animals not subjected to AS also exhibited significant elevations in systolic and diastolic ABP; vascular tissue Mg content decreased, whereas Ca content rose. Animals subjected to combined Mg deficiency and AS for 12 wk exhibited the greatest deficits in plasma and vascular muscle Mg and the greatest elevations in systolic and diastolic ABP; vascular tissue Ca contents also showed the greatest increases. In situ measurements of mesenteric arterioles, venules, and precapillary sphincters in the various subgroups revealed that the lower the plasma [Mg], the more constricted the microvessels, and the higher the ABP, the lower the plasma [Mg]. Capillary blood flow velocities were decreased in relation to the degree of plasma Mg deficit.(ABSTRACT TRUNCATED AT 250 WORDS)
A novel ion selective electrode (ISE) for ionized magnesium (IMg2+) in whole blood (WB), plasma (PL) and serum (S) has now been designed and characterized in normal human subjects, diseased and pregnant subjects. Using this ISE on various levels of aqueous Mg2+ solutions (0.1-3.0mM), mean Mg2+ values are within 94.6 and 99.2% of their targets. The linearity of the ISE (0.1-3.0mM) in aqueous solution and human PL and S ranges between 92.0 and 99.3%. The ISE is highly selective for IMg2+, yielding measurements in less than 2 min, and exhibiting no or negligible effects from physiologic concentrations of Ca2+,Na+,K+ or H+. Ligand binding studies indicate that pathophysiologic concentrations of most smaller molecular weight anions fail to interfere with IMg2+ measurements in aqueous solution, PL or S. Likewise, pathophysiologic concentrations of heavy metals or lipids do not interfere with measurements for IMg2+. Comparison of healthy, normal WB, PL and S IMg2+, using the ISE, with measurements of ultrafilterable Mg by atomic absorption spectroscopy (AAS) are excellent. The mean value for IMg2+ in normal WB, PL and S is 0.58-0.60mM (range = 0.53-0.67mM). Compared to total Mg (TMg) (0.75-0.96mM) and ICa2+ (1.10-1.30mM), IMg2+ is held in a narrow, tight range, representing 71% of TMg. Preliminary studies on plasma and serum IMg2+ during open heart surgery and term pregnancy demonstrate significant abnormalities in IMg2+ and % IMg2+. The new ISE for IMg2+ should prove invaluable and revolutionary in studies of Mg metabolism in both healthy and diseased states.
Replacement of extracellular chloride ions ([Cl-]o) by other anions, on contractility and the effects of extracellular magnesium ions ([Mg2+]o) on spontaneous mechanical activity, as well as on agonist-induced responses of rat aorta and portal vein, were studied. Replacement of [Cl-]o with acetate (Ac-) or isethionate (Ise-) ions resulted in an increase and decrease, respectively, of the spontaneous mechanical activity frequency in portal vein; the amplitudes of the spontaneous mechanical activity were attenuated by Ac- and Ise- substitution. Withdrawal of [Mg2+]o in Cl(-)-containing media resulted in elevation of tension development in rat aortas, whereas a similar maneuver in media with Ac- or Ise-, substituted for [Cl-]o, resulted in abrogation of this tension development. Use of disulfonic stilbene anion-channel blockers, DIDS (4,4'-diisothiocyano-2,2'-stilbene disulfonate, 400-600 microM) and SITS (4,4'-acetamido-4'-isothiocyano-2,2'-stilbene disulfonic acid, 400-600 microM), failed to influence either spontaneous mechanical activity or basal tone of rat portal portal vein or aortas. Incubation of DIDS or SITS in Mg(2+)-free media also failed to influence mechanical responses to withdrawal of [Mg2+]o. Use of the Cl- cation transport inhibitor bumetanide (30-80 microM) also failed to alter spontaneous mechanical activity or basal tone in either the presence or absence of [Mg2+]o. Ac- and Ise- substitution attenuated norepinephrine- and K(+)-induced contractile responses in portal vein and aorta, Caffeine-induced contractions of aortas were potentiated by withdrawal of [Mg2+]o in Cl(-)-containing media but inhibited in Ac(-)- or Ise(-)-substituted solutions. In the presence of [Mg2+]o, substitution of foreign anions resulted in alterations in the agonist contractile dose-response curves; EC50s were increased whereas maximum tensions were depressed. Withdrawal of [Mg2+]o amplified these effects. Substitution of Ac- or Ise- for [Cl-]o in the presence or absence of [Mg2+]o depressed contractions induced by Ca acetate in aortas and portal vein. These results suggest that: (1) Cl- plays an important role in regulating spontaneous mechanical activity, basal tone, and contractility in rat aorta and portal vein; and (2) Cl- probably physiologically mediates some of the effects and actions of [Mg2+]o on intracellular release of and influx of Ca2+ in these smooth muscles.
Replacement of extracellular chloride ions by thiocyanate anions (SCN-) followed by washout in normal chloride-containing solution produced contractions in isolated rat aortas and portal veins of female rats followed by slow relaxation; these contractions consisted of fast and slow phases. These SCN(-)-induced biphasic contractions were also noted in rat aortas precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic strips versus intact ring preparations. The SCN(-)-induced contractions in both the aorta and portal vein were inhibited markedly by denervation with 6-hydroxydopamine. Use of prazosin, rauwolscine, propranolol, atropine, methysergide, diphenydramine, indomethacin or procaine (10(-3) M) failed to alter the SCN(-)-induced responses. However, use of phentolamine at 10(-5) M, but not at lower concentrations of the drug, resulted in complete inhibition of SCN(-)-induced contractions. Treatment of the vascular tissues with EGTA (5 mM) or incubation in Ca(2+)-free media abolished the SCN(-)-induced contractile responses. Treatment with verapamil (10(-6) M) or washing in Ca(2+)-free Krebs Ringer solution after incubation with SCN(-)-Krebs Ringer selectively inhibited the slow phases of the aortic contractions. Replacement of SCN- anions with other foreign monovalent anions or with sucrose modified the amplitude of the SCN(-)-induced contractions. These foreign anions seemed to follow a relative order of potency similar to that for a lyotropic series of anions, where acetate greater than isethionate greater than chloride greater than bromide greater than nitrate greater than iodide ions.(ABSTRACT TRUNCATED AT 250 WORDS)
Alteration of the extracellular anion environment by replacement of chloride ions (Cl-) with thiocyanate ions (SCN-) in normal Krebs-Ringer bicarbonate solution (NKRB) induced sustained development of basal tension in isolated aortas from male adult Sprague-Dawley and Long-Evans rats, but not from females of these strains. However, aortic smooth muscle isolated from sexually immature male and female Wistar rats underwent more marked tension development under such treatment, exhibiting no gender differences. Such SCN(-)-induced contractile responses are not tachyphylactic, completely dependent on extracellular calcium ([Ca2+]0), and could be aborted (relaxed to basal tone) by readmission of Cl-. Castration and replacement of male sex hormones with estradiol inhibited SCN(-)-induced contractions in isolated aortas from adult Wistar male rats, while castration and testosterone supplementation failed to induce contraction in aortic muscle of adult female Wistar rats exposed to SCN-. We believe these data are compatible with the notion that gender differences in vascular activity may be modulated by actions of estradiol on the metabolism of Cl- and other anions in vascular smooth muscle, which may be linked to transport of Ca2+ across the vascular muscle cell membrane.
Phencyclidine (PCP) abuse is reaching alarming proportions. PCP has recently been shown to induce hypertensive encephalopathies, microvascular cerebrovasospasm and acute intracerebral hemorrhage. Since we have shown in vitro that cerebral vasospasms induced by PCP could be completely reversed, or prevented, by use of organic calcium antagonists, we utilized a television microscope recording system to determine whether magnesium ions (Mg2+) could inhibit the ability of PCP to induce contraction of pial arterioles and its sequelae of microvascular damage. Administration of either MgCl2 or Mg aspartate HCl, i.a. or i.v. (1, 10, and 20 mumol/min), before or after administration of PCP produced dose-dependent inhibition (30-80%) of PCP-induced arteriolar spasms and the subsequent vascular damage. A variety of pharmacologic receptor antagonists and cyclooxygenase inhibitors failed to influence PCP-induced cerebrovasospasms. These data suggest that a naturally-occurring Ca2+ antagonist, viz. Mg2+, may be useful in the treatment of PCP intoxication and its cerebral vascular consequences.
It has been suggested that anions may play important roles in the regulation of membrane potentials, intracellular calcium and pH of vascular smooth muscle. It is, thus, possible that anions may be important in the sex-linked differences observed in contractile functions of vascular smooth muscle. With this in mind, we investigated the influence of alteration of the anionic environment, i.e., Cl- chloride ions, on the contractile activity of isolated aortic strips and rings from sexually mature male and female rats. After a few minutes of changing the incubation medium from normal chloride-containing Krebs-Ringer bicarbonate (NKRB) to a SCN- (thiocyanate) modified Krebs-Ringer bicarbonate solution (SCN-KRB; 118 mM SCN- substituted for 118 mM Cl-), resting tension increased dramatically, e.g., to about 85% of KCl maximum in males, but not in females. This increased resting tension could not be inhibited or attenuated by specific adrenergic, cholinergic, histaminergic, serotoninergic or cyclo-oxygenase inhibitors. Likewise use of a local anesthetic in high concentration (i.e., procaine HCl, 10(-3) M) also failed to inhibit or attenuate the SCN(-)-induced contractions. However, use of 5 mM EGTA, incubation in Ca2(+)-free SCN-KRB media or addition of [Mg2+]o (extracellular Mg2+) (2.4-14.4 mM) all resulted in inhibition of the enhancement of vascular tone in aortas from male animals, whereas addition of 5 mM CaNa2EDTA or removal of [Mg2+]o from SCN- KRB potentiated it. Use of intact aortic rings or aortic strips produced identical results in response to SCN-KRB and the experimental maneuvers. Our results suggest that the gender differences in the contractile responsiveness of vascular smooth muscle are probably related to anionic metabolism and distribution of Cl- ions across the vascular smooth muscle cell membranes. Chloride and other anions may mediate the diverse effects and actions of sex hormones on vascular tone, reactivity and calcium movement in vascular smooth muscle.
Cocaine HCl (10 micrograms/ml) delivered perivascularly to the surface of the rat brain resulted in rapid contraction of pial arterioles, which reduced the diameters by 26% compared to controls. This was followed by venular vasospasm and rupture of postcapillary venules and micro-hemorrhages at postcapillary sites. Administration of Mg aspartate HCl, by intraarterial or intravenous infusion (1, 10 and 20 mumol/min), before or after the cocaine, produced dose-dependent inhibition (20-85%) of the cocaine-induced arteriolar spasms and prevention and attenuation of the venular vasculotoxicity and hemorrhaging. These data suggest that magnesium salts might be useful agents in the treatment of cocaine-induced intoxication and prevention of brain damage.
In this study, we have examined the effects of variation in dietary Mg on the atherogenic process. Oral supplementation of rabbits fed a high cholesterol diet (1% or 2%) with the Mg salt magnesium aspartate hydrochloride (Magnesiocard) (i) lowers the level of serum cholesterol and triglycerides in normal (25-35%) as well as atherosclerotic (20-40%) animals and (ii) attenuates the atherosclerotic process markedly. In addition, we found that dietary deficiency of Mg augments atherogenesis markedly and stimulates (or activates) macrophages of the reticuloendothelial system. Evidence is presented to indicate that the hypercholesterolemic state may cause the loss of Mg from soft tissues to the serum, thereby masking an underlying Mg deficiency.
The possible importance of facilitation of sodium-calcium (Na(+)-Ca2+) exchange by removal of extracellular magnesium ions ([Mg2+]o) in expression of endothelium-dependent relaxation was investigated in aortic rings isolated from female rats. Simultaneous [Mg2+]o withdrawal (0 mM Mg2+) and reduction in extracellular Na+ (Total [Na+]o = 84 mM), by replacement of NaCl with isosmolar amounts of sucrose in normal Krebs-Ringer bicarbonate (NKRB), induced significant increases of basal tone of denuded rat aortic rings, but not in tissues with intact endothelium. These vascular effects were not affected by indomethacin, phentolamine or atropine in any of the tissues tested. Reintroduction of 1.2 mM Mg2+ or removal of extracellular Ca2+ ([Ca2+]o) from the Mg2+ and Na(+)-deficient incubation media induced complete relaxation of the denuded tissues. Methylene blue (10(-5) M), an inhibitor of endothelium-derived relaxant factor (EDRF), potentiated tension development in intact tissues. These results suggest that: (1) as in vascular muscle, Mg2+ plays an important role in Ca2+ homeostasis in endothelial cells (EC), probably via Na(+)-Ca2+ exchange; and (2) such Mg(+)-regulated internal Na(+)-dependent Ca2+ entry participates in the expression of endothelium-dependent relaxation.
Possible gender differences in contractile responses to reduction in extracellular Mg2+ ([Mg2+]0) and [Na+]0 in isolated aortic rings and strips from adult rats were investigated. In consonance with our previous studies, [Mg2+]0 withdrawal (0 mM Mg2+) and reduction in [Na2+]0 (total = 84 mM Na+) by replacement of NaCl with isosmolar amounts of sucrose in normal Krebs-Ringer bicarbonate (NKRB) induced significant increases of basal tone of aortic rings and strips in male, but not in female, rats. However, such tension development was observed in endothelium-denuded aortic preparations isolated from both sexes. No effects of indomethacin (10(-5) M) were found in any of the tissues tested. Adding 1.2 mM MgSO4 to the Mg2+ and Na(+)-deficient incubation media relaxed the tension increase to a normal basal level. The endothelium-dependent vasodilator responses to acetylcholine (10(-10) - 10(-6) M) of aortic rings from male or female rats in NKRB were not different. These results suggest that: (1) as in vascular smooth muscle cells, Mg2+ plays an important role in Ca2+ homeostasis in endothelial cells, probably via Na(+)-Ca2+ exchange and (2) sex steroid hormones may influence contractile responses of vascular smooth muscle by modifying endothelium function through such Mg(2+)-regulated internal Na(2+)-dependent Ca2+ entry.