Uptake of the neutral amino acids glutamine, leucine, and serine by Pneumocystis carinii.

Experiments to elucidate the mechanism by which Pneumocystis carinii transports glutamine, leucine, and serine were performed in this study. Uptake of all three radiolabeled amino acids exhibited first-order, saturation kinetics as extracellular substrate concentrations were increased, thus ruling out simple diffusion and indicating carrier-mediated transport. Kinetic analyses of amino acid uptake and the results of competitive inhibition experiments suggested that leucine, serine, and glutamine were taken up via a common transporter system. The uptake of serine was examined in greater detail to characterize the nature of the carrier. Serine uptake was not affected by N, N'-dicyclohexylcarbodiimide, carbonyl cyanide m-chlorophenyl hydrazone, ouabain, gramicidin, valinomycin, sodium azide, salicylhydroxamine acid (SHAM), iodoacetate, iodoacetate plus SHAM, KCN, and azide. Thus serine uptake did not require sodium or energy from ATP, an electrochemical proton gradient or a membrane potential across the cell surface (i.e., proton-motive force). Serine uptake was dependent on glucose in the extracellular compartment. In the presence of glucose, serine uptake was inhibited by chloramphenicol but not cycloheximide. The results from these experiments are most consistent with facilitated diffusion as the mechanism. After 30 min of incubation, most of the radioactivity was in the cellular soluble fraction. In most cases, incorporation into the extractable total lipids and the remaining particulate cellular components were detectable after this incubation period.

Transport of aspartic acid, arginine, and tyrosine by the opportunistic protist Pneumocystis carinii.

In order to improve culture media and to discover potential drug targets, uptake of an acidic, a basic, and an aromatic amino acid were investigated. Current culture systems, axenic or co-cultivation with mammalian cells, do not provide either the quantity or quality of cells needed for biochemical studies of this organism. Insight into nutrient acquisition can be expected to lead to improved culture media and improved culture growth. Aspartic acid uptake was directly related to substrate concentration, Q(10) was 1.10 at pH 7.4. Hence the organism acquired this acidic amino acid by simple diffusion. Uptake of the basic amino acid arginine and the aromatic amino acid tyrosine exhibited saturation kinetics consistent with carrier-mediated mechanisms. Kinetic parameters indicated two carriers (K(m)=22.8+/-2.5 microM and K(m)=3.6+/-0.3 mM) for arginine and a single carrier for tyrosine (K(m)=284+/-23 microM). The effects of other L-amino acids showed that the tyrosine carrier was distinct from the arginine carriers. Tyrosine and arginine transport were independent of sodium and potassium ions, and did not appear to require energy from ATP or a proton motive force. Thus facilitated diffusion was identified as the mechanism of uptake. After 30 min of incubation, these amino acids were incorporated into total lipids and the sedimentable material following lipid extraction; more than 90% was in the cellular soluble fraction.

Slow pathway modification for atrioventricular node re-entrant tachycardia: fast junctional tachycardia predicts adverse prognosis.

OBJECTIVE: To examine the cycle length of the junctional tachycardia often seen during successful slow pathway ablation for atrioventricular (AV) node re-entrant tachycardia, to determine whether shorter cycle lengths predict imminent atrioventricular block. DESIGN: Retrospective analysis of consecutive patients undergoing slow pathway modification. Intracardiac recordings were analysed after digital storage to determine the development of junctional tachycardia, its duration and maximum, minimum, and mean cycle length, occurrence of heart block, persistent slow pathway conduction, or later confirmed recurrence of AV node re-entrant tachycardia. SETTING: Regional cardiac centre. PATIENTS: 136 consecutive patients undergoing electrophysiological study found to have typical "slow-fast" AV node re-entrant tachycardia and subject to 137 slow pathway modification procedures. RESULTS: During successful temperature feedback controlled radiofrequency energy application, junctional tachycardia developed in 133 of 137 procedures. During ablation, 10 patients had evidence of AV block (first degree in seven patients and third degree in three), and 17 others had retrograde junctional atrial (JA) block. In these 27 patients, the junctional tachycardia was rapid, with a minimum (SD) cycle length 291 (47) ms. Conduction recovered quickly in all but two patients, one of whom required permanent pacing. Junctional tachycardia with normal AV and JA conduction in the other 111 patients was of a significantly slower minimum cycle length (537 (123) ms; p < 0.0001). CONCLUSIONS: Fast junctional tachycardia with cycle lengths under 350 ms seen during slow pathway modification is a predictor of conduction block, suggesting proximity to the compact node. Radiofrequency energy application should be terminated immediately to prevent development of AV block. An "auto cut off" facility for cycle lengths shorter than 350 ms could be built into radiofrequency ablation systems to increase safety.

Subpectoral implantation of a cardioverter defibrillator under local anaesthesia.

OBJECTIVE: To evaluate patient acceptability of submuscular implantation of a cardioverter defibrillator (ICD) under local anaesthesia with conscious sedation. DESIGN: Retrospective review. Patient acceptability in the second half of the study was routinely assessed within 24 hours. SETTING: Regional cardiac centre. PATIENTS: 45 consecutive patients with either aborted sudden death or haemodynamically unstable ventricular tachycardia were referred for ICD implantation. INTERVENTIONS: A subpectoral implantation technique was employed. Twelve procedures were performed under general anaesthesia. Thirty three patients were sedated with midazolam and diamorphine, and local anaesthesia was achieved with bupivicaine. Ventricular fibrillation for defibrillation threshold testing was induced by alternating current, T wave shock, or ultrarapid burst pacing. Patients were contacted after the procedure to assess acceptability. RESULTS: 32 patients having implantation under local anaesthesia did not recall the surgical procedure. One patient described an awareness of "pushing" as the generator was positioned in the pocket. Seven patients said that the procedure was painless but recalled a test shock, four describing it as mildly uncomfortable. All 33 patients stated that they would be willing to have a second implant under local anaesthesia. Twelve patients who had the implant performed under general anaesthesia had no recollection of the procedure. Mean (SD) total procedure duration was significantly longer in those who had general anaesthesia (93 (16) v 67 (17) minutes; p = 0.0009). CONCLUSIONS: Subpectoral implantation of ICDs may be performed safely with patient acceptability under local anaesthesia with conscious sedation.

Outcome of pregnancy in women with Marfan's syndrome.

OBJECTIVE: To improve life expectancy and prevent premature mortality in women with Marfan's syndrome. METHODS: During the development of a regional genetic register for Marfan's Syndrome the outcome of 91 pregnancies in 36 women with this condition was established retrospectively and the cardiovascular and obstetric complications documented. RESULTS: No patient had a significant cardiovascular abnormality limiting function before her pregnancy. Of 36 women, four had an aortic dissection relating to pregnancy and two others required aortic surgery following delivery. Thirty women had uncomplicated gestational histories. The incidence of obstetric complications did not exceed expectation. CONCLUSIONS: Women with Marfan's syndrome are at significant risk of aortic dissection in pregnancy even in the absence of preconceptional cardiovascular abnormality. Aortic root dilatation may be a predictor of risk but dissection may occur without significant dilatation. Guidelines for obstetric care are suggested and preconceptional assessment recommended.

Evolving phenotype of Marfan's syndrome.

AIM: To examine evolution of the physical characteristics of Marfan's syndrome throughout childhood. METHODS: 40 children were ascertained during the development of a regional register for Marfan's syndrome. Evolution of the clinical characteristics was determined by repeat evaluation of 10 patients with sporadic Marfan's syndrome and 30 with a family history of the condition. DNA marker studies were used to facilitate diagnosis in those with the familial condition. RESULTS: Musculoskeletal features predominated and evolved throughout childhood. Gene tracking enabled early diagnosis in children with familial Marfan's syndrome. CONCLUSIONS: These observations may aid the clinical diagnosis of Marfan's syndrome in childhood, especially in those with the sporadic condition. Gene tracking has a role in the early diagnosis of familial Marfan's syndrome, allowing appropriate follow up and preventive care.