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Biomedical subjects

J M Robertson

Publications and source records attributed to J M Robertson.

At least 145 records · Page 8Linked to original sources
PubMed1984

Safety of prolonged aortic clamping with blood cardioplegia. II. Glutamate enrichment in energy-depleted hearts.

Cold blood cardioplegic techniques are limited in their ability to protect energy-depleted hearts during aortic clamping. This study extends our previous observations on the benefits of amino acid (L-glutamate) enrichment of blood cardioplegic solutions as well as the added metabolic benefits of normothermic induction of cardioplegic arrest to increase the rate of repair of energy-depleted hearts. The results demonstrate that L-glutamate enrichment of blood cardioplegic solutions significantly improves metabolic recovery (greater oxygen consumption, better anaerobic metabolism) and ventricular performance. Normothermic induction of glutamate-enriched cardioplegia allowed complete recovery of myocardial metabolism and function compared to cold blood cardioplegic technique and may be used as a form of "active resuscitation" of energy-depleted hearts.

Adenosine Triphosphate↗
PubMed1983

Topological arrangement of two transfer RNAs on the ribosome. Fluorescence energy transfer measurements between A and P site-bound tRNAphe.

The relative arrangement of two tRNAPhe molecules bound to the A and P sites of poly(U)-programmed Escherichia coli ribosomes was determined from the spatial separation of various parts of the two molecules. Intermolecular distances were calculated from the fluorescence energy transfer between fluorophores in the anticodon and D loops of yeast tRNAPhe. The energy donors were the natural fluorescent base wybutine in the anticodon loop or proflavine in both anticodon (position 37) and D loops (positions 16 and 17). The corresponding energy acceptors were proflavine or ethidium, respectively, at the same positions. Four distances were measured: anticodon loop-anticodon loop, 24(+/- 4) A; anticodon loop (A site)-D loop (P site), 46(+/- 12) A: anticodon loop (P site)-D loop (A site), 38(+/- 10) A: D loop-D loop, 35(+/- 9) A. Assuming that both tRNAs adopt the conformation present in the crystal and that the CCA ends are close to each other, the results are consistent with the two anticodons being bound to contiguous codons and suggest an asymmetric arrangement in which the planes of the two L-shaped molecules enclose an angle of 60 degrees +/- 30 degrees.

Anticodon↗
PubMed1983

Stenosis severity effects for unbalanced simple-pulsatile bifurcation flow.

A numerical finite-difference analysis is made of a plane simple-pulsatile flow past a symmetrical bifurcation which contains an asymmetrical smooth-contoured stenosis in the trunk. In essence, such a situation could represent a stenosed common carotid artery immediately upstream from the carotid junction. The flow is unbalanced; two-thirds of it exits or enters through the lower branch. The effect on various flow parameters of the stenosis itself and on changes in its severity is investigated by comparing the results for a severe stenosis, a mild stenosis, and no stenosis. The simple-pulsatile forcing function is specified in terms of an oscillatory and a steady Karman number. To obtain a significant amount of backflow, the oscillatory trunk Karman number is taken as 1000 compared to the steady value of 250. The frequency of oscillation is stipulated by a trunk Stokes number of 10 pi. The numerical procedures utilize the vorticity-transport version of the Navier-Stokes governing equations. A non-orthogonal coordinate transform allows the calculations to be made in a rectangular grid where the central difference expressions are easily applied. The results are presented in terms of both kinematic and kinetic parameters. The variation in the basic kinematic variables of stream function and vorticity is shown by temporal sequences of contour plots at times of peak flow and during the flow reversal stages as well as by several velocity vector plots. Kinetic results are given in terms of the temporal variation in shear stresses along boundaries. The peak shears are found to occur at the zenith of the stenosis at times of peak flow: the value for the severe stenosis is twice as large as that for the mild stenosis. The midline pressure distribution in the trunk and the centerline pressure distributions in the branches are also included.

Carotid Arteries↗
PubMed1983

Comparison of distribution beyond coronary stenoses of blood and asanguineous cardioplegic solutions.

In seven dogs on cardiopulmonary bypass, a critical stenosis (75% to 90%) of the left anterior descending coronary artery (LAD) was produced. Alternate 250 ml/min infusions of asanguineous and blood cardioplegic (4 degrees C) solutions were made for 3 to 5 minutes. Poststenotic flow (flowmeter), intramyocardial temperature, and aortic pressure were measured. During cardioplegic infusions of 250 ml/min, aortic pressure was 34 +/- 4 mm Hg higher with blood cardioplegia than with asanguineous cardioplegia (82 +/- 7 versus 48 +/- 8 mm Hg). Poststenotic cardioplegic flow was 39% +/- 9% higher (29 +/- 5 versus 18 +/- 5 ml/min) with blood cardioplegia. Consequently, blood cardioplegia resulted in more rapid arrest (20 +/- 2 versus 45 +/- 5 seconds) and lower myocardial temperature (6 degrees +/- 1 degree C) in the region of LAD blood supply; posterior ventricular myocardial cooling was similar (unobstructed vessels) with both solutions. These data show that the reduced viscosity of asanguineous cardioplegia compared to blood cardioplegia results in lower aortic pressure. Consequently, the higher aortic pressure with blood cardioplegia results in superior cardioplegic delivery beyond obstructed coronaries and better myocardial cooling. We conclude that the decreased viscosity of 4 degrees C asanguineous cardioplegia causes diversion of cardioplegic solution from the obstructed to the normal coronary bed.

Animals↗
PubMed1982

Effect of ribosome binding and translocation on the anticodon of tRNAPhe as studied by wybutine fluorescence.

The complexes of N-AcPhe-tRNAPhe (or non-aminoacylated tRNAPhe) from yeast with 70S ribosomes from E. coli have been studied fluorimetrically utilizing wybutine, the fluorophore naturally occurring next to the 3' side of the anticodon, as a probe for conformational changes of the anticodon loop. The fluorescence parameters are very similar for tRNA bound to both ribosomal sites, thus excluding an appreciable conformational change of the anticodon loop upon translocation. The spectral change observed upon binding of tRNAPhe to the P site even in the absence of poly(U) is similar to the one brought about by binding of poly(U) alone to the tRNA. This effect may be due to a hydrophobic binding site of the anticodon loop or to a conformational change of the loop induced by binding interactions of various tRNA sites including the anticodon.

Anticodon↗
PubMed1982

Left ventricular O2 requirements of pressure and volume loading in the normal canine heart and inaccuracy of pressure-derived indices of O2 demand.

The purpose of this study was to: 1) re-evaluate the left ventricular O2 requirements (MVO2) of pressure and volume-loading; and b) assess the accuracy of pressure-derived indices of left ventricular (LV) O2 demand under pressure-loading and volume-loading conditions. Using a right heart bypass preparation (heart rate 150 beats . min-1), mean arterial pressure (7.3 to 22.9 kPa) and cardiac output (0.8 to 6.0 litre . min-1) were varied independently. Adequate left ventricular O2 supply was demonstrated by normal transmural distribution of blood flow, normal myocardial lactate metabolism, and intact reactive hyperaemic responses. For equivalent increases in cardiac external work (from 2 to 4 joules . min-1) pressure-loading and volume-loading resulted in similar increases in O2 uptake (59% and 49%, respectively, P = 0.21). MVO2 was consistently greater for volume-loading than for pressure-loading at any SPTI, pressure-rate product, and triple product. The prediction of O2 demands by these indices under these loading conditions collectively was unreliable (r = 30 to 0.42). O2 requirements of the left ventricle with both pressure-loading and volume-loading were highly correlated (r = 0.82 to 0.99) with meridional wall stress. We conclude that: 1) volume-loading conditions have a greater O2 requirement than appreciated previously; 2) pressure loads and volume loads require similar O2 uptake in the normal canine heart for similar external work; 3) currently used pressure-derived indices are unreliable predictors of LV O2 demand when loading conditions are varied; and 4) O2 requirements are more uniformly related to meridional wall stress.

Animals↗
PubMed1982

A study of the effects of a transversely moving boundary on plane Poiseuille flow.

Numerical (finite-difference) solutions in vorticity-stream function variables using a nonorthogonal geometric transform are found for viscous flow through a plane channel in which a portion of the boundary oscillates to change the flow. Calculations were made for three rates of inflow and for three frequencies of oscillation. The boundary pumpage relative to inflow decreased with inflow Karman number and with the oscillatory period of the boundary. The maximum shear stress, as indicated by the maximum vorticity, increased with Karman number and occurred when the boundary was in the maximum stenotic position. It did not change with boundary period except for the case when the period was the smallest. The channel pressure drop was significantly affected by the pumpage as well as the boundary nonuniformity.

Hemodynamics↗
PubMed1981

Critical importance of ensuring cardioplegic delivery with coronary stenoses.

This study examines the relative importance of the duration of ischemia versus the adequacy of cardioplegic distribution and protection in hearts with coronary stenoses. Of 18 dogs on cardiopulmonary bypass, 12 underwent critical narrowing (greater than 90%) of the left circumflex artery (LCA) and total occlusion of the anterior descending coronary artery (LAD). In six dogs (control) the coronary arteries were patent. A 16 degrees C blood cardioplegic solution was given at 20 minute intervals of aortic clamping. In control dogs and in six dogs with stenoses, the aorta was clamped for 60 minutes. In the latter group, the stenoses were removed after 20 and 40 minutes to simulate sequential completion of grafts and better cardioplegic distribution. In the remaining dogs with stenoses, the aorta was clamped for only 30 minutes, with stenoses removed after the heart had been returned to the beating empty state for 30 minutes to simulate doing distal grafts with cardioplegic protection and proximal grafts during reperfusion (traditional technique). With sequential grafting, myocardial temperature was lower (16 degrees C versus 22 degrees C) and incidence of reperfusion fibrillation less than with the traditional technique. Despite a greater ischemic interval, sequential grafting with adequate cardioplegic distribution resulted in less lactate washout (5 +/- 15 versus 35 +/- 6 cc/100 gm/min), greater recovery of compliance, and higher stroke work indices (1.32 +/- 0.12 versus 0.75 +/- 0.15 kg-m/min). We conclude that the success of myocardial protection with potassium cardioplegia in hearts with coronary stenoses is related more to ensuring its distribution than to limiting the duration of ischemic arrest with the false assumption that the heart is reperfused adequately while proximal grafts are completed in the beating empty state.

Animals↗
PubMed1980

Fluorescent tRNA derivatives and ribosome recognition.

The use of fluorescent derivatives of tRNAPhe (yeast) in studies on tRNA conformation and on tRNA-ribosome recognition is described. Evidence is presented which indicates that under physiological conditions with respect to ionic strength and Mg2+ concentration, tRNAPhe exists in at least two conformations. The functional significance of this behavior is discussed on the basis of aminoacylation experiments. The investigation of the ribosome complexes of tRNAPhe labeled in the anticodon and D-loops has provided evidence suggesting that the presence of the codon, although not appreciably altering the apparent association constant, leads to qualitatively different complexes in which the tRNA appears to be rigidly bound to the codon even in the P-tRNA to the ribosome occurs in several steps, which take place only in the presence of the proper codon. One or more of these steps may represent codon-induced conformational changes of the tRNA molecule, which constitute the molecular basis of the highly specific binding of the tRNA to the ribosome.

Amino Acyl-tRNA Synthetases↗
PubMed1980

Intramyocardial pH during elective arrest of the heart: relative effects of hypothermia versus potassium cardioplegia on anaerobic metabolism.

Using an intramyocardial pH needle probe (21 gauge) to monitor myocardial metabolism during ischemia, we determined the effect of potassium cardioplegia at both moderate and deep hypothermia. Five groups of 5 dogs each were placed on cardiopulmonary bypass and the pH probe was inserted approximately 10 mm into the left ventricular free wall. Cardiac ischemia was achieved by cross-clamping the ascending aorta at 37 degrees C (Group 1), 27 degrees C (Group 2), or 17 degrees C (Group 3). In the remaining two groups, aortic cross-clamping was followed by the infusion of 600 to 800 ml of potassium cardioplegic solution adjusted to cardiac temperatures of 27 degrees C (Group 4) or 17 degrees C (Group 5). In each group, myocardial temperature was maintained constant, electrical and mechanical activity observed, and pH recorded until a plateau was reached or for 3 hours. Our results show a progressive and significant decrease in the metabolic rate with reduction in temperature over the 37 degrees to 17 degrees C range. By abolishing contractile activity, potassium cardioplegia markedly reduces the rate of hydrogen ion accumulation at 27 degrees C, but at 17 degrees C the additive effect of cardioplegia is much less pronounced. These observations support the principle of reducing contractile activity to a minimum during elective arrest of the heart but indicate that potassium cardioplegia does little to further reduce the rate of anaerobic metabolism, as shown by the measurement of intramyocardial pH, under conditions of deep hypothermia.

Animals↗