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Biomedical subjects

P Hendry

Publications and source records attributed to P Hendry.

33 records · Page 2Linked to original sources
PubMed1995

The role of magnesium in myocardial preservation.

The purpose of this review is to look at the role of magnesium in the formation of preservation and reperfusion solutions for the ischaemic heart. Preservation of the heart during cardiac surgery procedures, including cardiac transplantation, can be divided into distinct phases: arrest, cold storage in the case of transplantation, global ischaemia during implantation or cardiac surgery procedures, followed by reperfusion when the heart is rewarmed and restarted. Although the magnesium ion can play a significant role in myocardial protection, it is important to recognize the different types of protection required during these different phases of surgical procedures. The rationale for the inclusion of magnesium in cardioplegic solutions is threefold: (i) for its negative inotropic effect; (ii) to prevent ischaemia-induced magnesium loss; (iii) to influence cellular ionic movements. Preservation temperature as well as the concentration of other ionic constituents present in the preservation solution alter the effects of magnesium. Results obtained from animal models suggest that elevated magnesium (16 mM) is beneficial to the hypothermic preservation of hearts with extracellular type solutions, especially when calcium is elevated in the solution formulation. Research has shown that the amplitude of the inotropic effect of magnesium varies from one species to another so that the beneficial effect of magnesium is inferior in the less sensitive species. Using the human atrial trabecular preparation as a model for myocardial preservation, we have assessed the effects of elevated magnesium on the recovery of developed force, both for long-term preservation (24 h) during hypothermic arrest (4 degrees C) and for reperfusion during rewarming of the trabeculae. No clear pattern emerged when the ratio of calcium to magnesium was altered in St Thomas' I and II solutions used for the storage. However, when the atrial trabecular preparation was rewarmed in a Krebs Henseleit buffer containing an elevated level of magnesium (16 mM), a greater number of trabeculae reached a greater developed force and had higher levels of energetic metabolites than when the magnesium in the Krebs Henseleit buffer was 1.2 mM. Several studies have suggested that an elevated magnesium prevents calcium overload by competing with this ion at the membrane, and reduces the workload, while ATP reserves and ion homoeostasis are re-established. The role of the magnesium ion in hypothermic preservation of the human myocardium is still not clear after many clinical and experimental studies and requires further investigation.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

The mechanism of the hammerhead ribozyme.

The rate of cleavage of a 13-mer substrate by an all-RNA ribozyme with 2 10-mer hybridising arms (TAT RA), appears to be limited by a conformational change involving the longer than necessary arms. The same limitation does not apply to a similar ribozyme with DNA arms (TAT RB) or to an all-RNA ribozyme with only 6 hybridising bases on each arm (TAT RA 6x2). The limitation does not arise in the hybridisation step nor in the dissociation of products. An extra step involving a conformational change of the pre-formed ribozyme-substrate prior to cleavage is introduced to explain the observed kinetics.

Base Sequence↗
PubMed1994

Using linkers to investigate the spatial separation of the conserved nucleotides A9 and G12 in the hammerhead ribozyme.

Two series of hammerhead-derived ribozymes, or 'minizymes', in which helix II has been replaced by linkers of non-nucleotidic moieties, have been synthesised by solid-phase methods. In the first series, the minizymes had linkers containing one, two, three, four or five repeated units of phosphopropanediol, so that the number of atoms in the chain connecting the 3'O of the conserved A9 to the 5'O of the conserved G12 varied from 7 to 31. In the second, more-limited series, the minizymes contained linkers of either tetra- or hexa-ethyleneglycol. The rates at which these minizymes cleaved their cognate 13-nucleotide substrate were determined at 30 degrees C, and compared with the rates of cleavage by an analogous series of minizymes containing from two to six repeated units of thymine deoxyribonucleotide in place of helix II. In all three series, the cleavage rates increased with increasing linker length, with a plateau being reached at the longer lengths tested. Relative cleavage rates within the phosphopropanediol and the thymidine series depended strongly on linker length, but maximal activity was achieved in both series with 25 atoms in the chain joining A9 and G12. The lengths of linkers required to achieve maximal activity of the minizymes are considerably greater than the linkers of 13 atoms which are sufficient to stabilise the ends of double-helices of DNA or RNA.

Base Sequence↗
PubMed1992

A ribozyme with DNA in the hybridising arms displays enhanced cleavage ability.

Hammerhead ribozymes cleave RNA substrates containing the UX sequence, where X = U, C or A, embedded within sequences which are complementary to the hybridising 'arms' of the ribozyme. In this study we have replaced the RNA in the hybridising arms of the ribozyme with DNA, and the resulting ribozyme is many times more active than its precursor. In turnover-kinetics experiments with a 13-mer RNA substrate, the kcat/Km ratios are 10 and 150 microM-1min-1 for the RNA- and DNA-armed ribozymes, respectively. The effect is due mainly to differences in kcat. In independent experiments where the cleavage step is rate-limiting, the DNA-armed ribozyme cleaves the substrate with a rate constant more than 3 times greater than the all-RNA ribozyme. DNA substrates containing a ribocytidine at the cleavage site have been shown to be cleaved less efficiently than their all-RNA analogues; again however, the DNA-armed ribozyme is more effective than the all-RNA ribozyme against such DNA substrates. These results demonstrate that there are no 2'-hydroxyl groups in the arms of the ribozyme that are required for cleavage; and that the structure of the complex formed by the DNA-armed ribozyme with its substrate is more favourable for cleavage than that formed by the all-RNA ribozyme and its substrate.

Animals↗
PubMed1992

Minimal sequence requirements for ribozyme activity.

The hammerhead ribozyme, as engineered by J. Haseloff and W. L. Gerlach [(1988) Nature (London) 334, 585-591], is an RNA molecule containing two regions of conserved nucleotides, a double helix, called helix II, which connects the two conserved regions, and flanking arms of variable sequence, which hybridize the ribozyme to its specific target. Here we show that this ribozyme may be reduced in size and still retain cleavage activity by replacing helix II with just a few nucleotides that cannot form Watson-Crick base pairs between themselves. Furthermore, the nucleotides replacing helix II and the nucleotides in the flanking arms may be substituted with DNA, and this small, DNA-containing ribozyme is fully as active as the original, full-size ribozyme. Cleavage activity of the minimized ribozyme depends on the number and sequence of the few nucleotides that replace helix II; optimal activity, thus far, is achieved by four or five deoxyribopyrimidines. The minimized ribozyme, or "minizyme," is active as a monomer, as shown by its nearly constant activity over a concentration range varying 25,000-fold, by the mobility of the minizyme-substrate complex in nondenaturing polyacrylamide gels as compared with other nucleic acid molecules of known size, and by other observations. These minizymes provide an excellent model system for studying the structure and mechanism of catalytic RNA; they might also be useful in a variety of biological applications.

Base Sequence↗
PubMed1990

Modified bacteriophage lambda promoter vectors for overproduction of proteins in Escherichia coli.

A new series of expression vectors that direct high-level overproduction of gene products in Escherichia coli is described. All contain strong bacteriophage lambda promoters, PR and PL, arranged in tandem so that both promote transcription into genes inserted into or between unique restriction sites. The vectors also direct expression of the lambda cI857 gene (from its natural promoter, PM), which enables their use in any E. coli host strain to effect controlled expression by shifting the temperature of cultures from 30 to 42 degrees C. The vectors pCE30, pND201, pPT150 and pMA200U are derivatives of the high-copy-number plasmid pUC9. Vector pCE33 is an analogous derivative of the heat-inducible runaway-replication plasmid, pMOB45, and directs overproduction of proteins by virtue of increase in both gene dosage and transcription following treatment at 42 degrees C. The vectors pND201 and pPT150 bear a ribosome-binding site (RBS) perfectly complementary to the 3' end of E. coli 16-S rRNA a few bp upstream from a unique HpaI site. Ways in which they may be used to improve the efficiency of translation of mRNA by substitution of a natural RBS with selection for optimal spacing from an ATG (or GTG) start codon are described. The phagemid vector pMA200U is a direct analog of pCE30 designed to facilitate preparation of single-stranded DNA templates for use in oligodeoxyribonucleotide-directed mutagenesis of overexpressed genes.

Bacteriophage lambda↗
PubMed1988

Coronary endarterectomy. An adjunct to coronary artery bypass grafting.

There is a wide variation in the reported results of endarterectomy in conjunction with coronary artery bypass grafting. Operative mortality ranges from 0 to 10 per cent, perioperative infarction ranges from 5 to 30 per cent, and patency rates range from 38 to 100 per cent, with 74 to 95 per cent being asymptomatic or improved. This wide discrepancy in clinical outcome occurs for a number of reasons. First, there is nonhomogeneity of patient populations resulting from a lack of standardized patient selection criteria. Second, the operative experience of most published reports spans a decade throughout which major advances in cardiac surgery have occurred. Finally, endarterectomy is a technically challenging procedure with its own learning curve. Differences in technique and expertise almost certainly contribute to the variability of results. Patients with diffuse coronary artery disease pose a challenge to the cardiac surgeon. Endarterectomy entails the risks of increased morbidity and mortality and therefore should be done only if conventional bypass grafting is precluded. However, the risks of these complications must always be weighed against the possible benefits. At the present time carefully selected patients can benefit from this procedure. Unfortunately, until such time as controlled randomized studies are carried out on a prospective basis, statistical support for this procedure will not be available. Given that such a study is questionable from an ethical point of view, clinical decisions for this select group of patients must be done on a case-by-case basis. The best that can currently be done is to carefully follow these patients and scrutinize existing data to ensure optimal clinical management.

Coronary Artery Bypass↗
PubMed1985

Should obese patients not undergo coronary artery surgery?

To assess the effect of obesity on the patient's recovery from coronary artery bypass surgery, a prospective study was performed on 200 patients. Group 1 comprised 101 nonobese patients and group 2, 99 obese patients. The mean percentage over the ideal weight was 8.2% in group 1 and 33.4% in group 2 (p less than 0.001). Preoperative assessment revealed no difference in age, sex, height, incidence of diabetes, family history or smoking habits between the two groups. Group 2 had a higher incidence of hypertension (p less than 0.01) and hyperlipidemia (p less than 0.02). The average number of grafts placed per patient was 3.4 +/- 0.8 in group 1 and 3.5 +/- 0.08 in group 2. Operative mortality was 2.9% and 2.0% in groups 1 and 2 respectively. Obesity was a predictive factor in postoperative hypertension (p less than 0.025), in the development of wound infection (p less than 0.01) and in an increased requirement of bronchodilators (p less than 0.01). The postoperative progression of physical activity and length of hospital stay was similar, being 10.9 +/- 5.8 days and 9.7 +/- 9.5 days in groups 1 and 2 respectively. The authors conclude that bypass grafting is an acceptable therapeutic option for obese patients with coronary artery disease.

Adult↗
PubMed1985

Benign bronchoesophageal fistulas.

Two cases of benign bronchoesophageal fistula are presented which are representative of both congenital and acquired forms. The presentation of this relatively rare condition is characterized by recurrent cough especially after drinking liquids and is easily diagnosed by esophagogram. A high index of suspicion is required in all cases of recurrent cough and lung suppuration in order for this condition to be recognized. Benign bronchoesophageal fistulas are rare but the symptoms are often classic and the diagnosis is made easily once proper investigation is undertaken. Bronchoesophageal fistulas may be either congenital or acquired, with the latter being more common. The treatment is usually straightforward and prognosis is excellent for long-term survival. We present two cases of benign bronchoesophageal fistula.

Aged↗
PubMed1982

Early deleterious hemorheologic changes following acute experimental coronary occlusion and salutary antihyperviscosity effect of hemodilution with stroma-free hemoglobin.

Viscometric measurements were made on the blood of anesthetized dogs before and following acute occlusion of the left anterior descending coronary artery. Within an hour of the occlusion, the low-shear viscosity of whole blood was significantly increased and continued to rise thereafter. This was accompanied by significant increments in the yield-shear stress of whole blood, suggesting accelerated tendencies in aggregation and plasma viscosity. In another group of dogs, 1 hour after coronary occlusion, rapid isovolemic hemodilution was performed with an 8% solution of stroma-free hemoglobin, reducing the hematocrit to 24%. This was followed by marked reduction of whole blood viscosity at all shear rates, abolition of the yield-shear stress, and reduction of plasma viscosity. These findings suggest that a hyperviscosity state develops early in acute myocardial ischemia which is detrimental to collateral perfusion of the marginal zone of ischemic myocardium. Such hyperviscosity syndrome can be blocked by hemodilution with stroma-free hemoglobin.

Acute Disease↗
PubMed1981

Intravenous solutions in parenteral nutrition.

A review of the theoretical basis on which amino acid intravenous solutions are formulated leads to the conclusion that the first class protein model is not ideal for the very small premature infant. Comparison of the serum and urine amino acid levels in a controlled trial between intravenous feeding with "Vamin" and intragastric milk feeding, and further comparison of these values with the cord blood values of premature infants supports the conclusion that the first class protein model is not ideal. The balance studies also support the view that the urinary output of amino acids, as well as the serum levels, should be taken into account in determining the ideal amino acid input.

Amino Acids↗
PubMedDate not supplied

In vivo evaluation of an intrathoracic ventricular assist device.

In this series of experiments, the Unified System components of the HeartSaver Ventricular Assist Device (VAD) version 5.0 were isolated from the controller and power supply for independent assessment. Five systems with external controller/power supply via a percutaneous lead configuration were tested in 13 male calves (101.8+/-4.3 kg). Two studies were ended acutely because of improper filling and air embolism, respectively. Duration of support was from 2.2 hours to 30 days (mean, 99+/-62 hours). The 30 day survivor was euthanized electively. Study termination was related to postoperative complications in five calves: two with bleeding/tamponade, one with thromboembolism caused by inadequate anticoagulation, and two with respiratory insufficiency. Other causes of termination were: one caused by main building power failure, two from errors in communication between the device and controller, and two caused by hydraulic fluid loss related to housing defects. From these experiments, an intrathoracic position for the calf has been defined, the procedure for implantation without cardiopulmonary bypass has been developed, refinements to the controller have been made, and inflow and outflow cannulae have been reinforced. Hydraulic fluid losses will be solved by proceeding with use of a titanium housing instead of polyurethane. In conclusion, the development of the HeartSaver VAD is progressing, in part because of these experimental and informative animal studies. Further in vivo evaluation of the final version will be conducted before clinical trials.

Animals↗