Showing papers by "Mark D. Johnson published in 1991"

Analysis of anomalous multilayer magnetoresistance within the thermomagnetoelectric system.

Abstract: The formalism of thermodynamics is used to calculate a spin-coupled resistance at the interface between a ferromagnet and a nonferromagnet. The resulting model gives a good qualitative and quantitative explanation of recent measurements of magnetoresistance in ferromagnet-nonferromagnet superlattices

Direct real time measurement of quasiparticle lifetimes in a superconductor.

Abstract: A short laser pulse breaks Cooper pairs and photoexcites electrons in a superconducting film of niobium. As the quasiparticles recombine and rejoin the condensate, changes in the electrodynamic response of the sample are detected by a fast oscilloscope. The observed biexponential decay is consistent with a previous measurement. The fast recovery time is used to obtain the pair recombination time, which is in good agreement with theory.

Reduced G protein function in desensitized rat aorta.

Abstract: These studies investigated the role of G proteins in vascular desensitization. Rats were infused with norepinephrine (NE) subcutaneously (0.1 mg/kg/hr) for 6 days using osmotic minipumps and aortas were obtained. NE-stimulated contraction was blunted in aortas obtained from NE infused rats compared to aortas from vehicle infused controls. Contractile responses to KCl and serotonin (5-HT) were not affected by NE infusion indicating desensitization was specific to certain receptors. Dose response curves for NaF-stimulated contraction were right-shifted in aortas from NE infused rats suggesting reduced G protein function in these vessels. G protein function was studied further by assessing G protein activation in response to receptor stimulation. This was done by measuring agonist-stimulated increases in 35S-GTP gamma S binding to G protein alpha subunits in aortic membranes. The alpha 1 adrenergic agonist phenylephrine stimulated GTP tau S binding to Gs and Gi in aortic membranes and the degree of stimulation was reduced in desensitized vessels. 5-HT stimulated binding to Gi and Go in aortic membranes and the response was not altered in desensitized vessels. Thus, vascular desensitization produced by NE infusion involves selective reductions in the ability of alpha 1 receptors to activate Gs and Gi.

Continuous Microcatheter Spinal Anesthesia With Subarachnoid Meperidine for Labor and Delivery

Abstract: The patient was a 28-yr-old, white, gravida 1 para 0, 176.3-cm, 73.8-kg woman who was seen in consultation at 38 wk gestation because of a history of ”allergy to local anesthetics.” She had a history of mild asthma since childhood for which she used a metaproterenol inhaler as necessary, but had never been admitted to a hospital for treatment of asthma. She reported allergies to povidone-iodine solution, tetracycline, erythromycin, lidocaine, and “Novocaine” manifested by rash and urticaria. In addition, she reported an extrapyramidal reaction to proclorperazine. Six months earlier she had suffered an apparent allergic reaction in a dentist’s office when given a maxillary injection of local anesthetic. She developed a rash and hives on her neck, face, arms, and trunk that subsided after several hours. The

Maternal and fetal plasma atrial natriuretic peptide concentrations during elective caesarean section.

Abstract: Atrial natriuretic peptide (ANP) is stored in the atrial cardiocyte and is capable of exerting potent, selective, and transient effects on fluid and electrolyte balance and on blood pressure. Because fluid shifts and hemodynamic adjustments occur during parturition, ANP might play a homeostatic role in the parturient and fetoplacental unit. We measured maternal and fetal plasma ANP concentrations in 19 parturients during elective caesarean section. Plasma ANP levels were also measured in seven nonpregnant women of the same age group. The baseline ANP concentration in parturients was significantly higher (29.77 +/- 6.06 pg/ml vs 7.37 +/- 2.1 pg/ml; mean +/- s.e.mean) than in their nonpregnant counterparts. The umbilical artery (UA) ANP concentration was significantly higher than the umbilical vein concentration (91.91 +/- 14.91 pg/ml vs. 40.04 +/- 9.71 pg/ml). Factors under the anaesthesiologist's control may influence maternal and fetal plasma ANP levels. There was a significant correlation between the volume of maternal Ringer's lactate infusion received and maternal ANP concentration. A significant correlation was seen between the total dose of ephedrine administered acutely prior to delivery and the UA ANP concentration. These data suggest that: 1) increased blood volume during pregnancy is associated with increased maternal plasma ANP levels, and 2) the fetus can produce its own ANP, and is thereby capable of responding to ANP stimulating factors.

Anaesthetic Management of Myocardial Infarction in a Parturient

Abstract: Myocardial infarction is encountered rarely during pregnancy, but when it occurs the event is life-threatening to both mother and fetus. Data on maternal and fetal outcome are limited, but overall maternal mortality approaches 35%, and 40% of deaths occur during the third trimester. We present a case of myocardial infarction occurring at 38 weeks gestation, and discuss the anaesthetic management of the problems encountered during labour and delivery.

Alterations in cardiovascular responsiveness and adrenoceptor binding during catecholamine infusion hypertension in rats.

Abstract: Chronic continuous infusion of norepinephrine in rats causes alterations in biochemical and physiologic responses of the cardiovascular system and in cardiovascular adrenoceptor number. The response of cardiac and aortic ornithine decarboxylase (ODC) activity to stimulation by norepinephrine was decreased in rats receiving norepinephrine infusion. These responses are due to stimulation of beta- and alpha-adrenergic receptors, respectively. Additionally, there was reduced stimulation of aortic ODC activity by angiotensin II and vasopressin. The cardiac ODC response to angiotensin II was decreased, but the response to vasopressin was not affected. The decreased ODC response is accompanied by decreased pressor responses to the alpha-adrenergic agonist phenylephrine. Decreased numbers of alpha- and beta-adrenoceptor binding sites (as measured by the binding of [3H]prazosin and [125I]pindolol) might mediate, in part, the altered responses to adrenergic agonists. The decreased cardiovascular responsiveness measured in these animals after several days of norepinephrine infusion hypertension contrasts with the increased responses found in most other forms of hypertension. This provides a useful model in which to examine the consequences of prolonged adrenergic receptor stimulation.