Control of tissue blood flow at very low temperatures

Maximum levels of sustainable swimming activity rely on aerobic metabolism, and hence are limited by the cardiovascular supply capacity which is impaired at low temperatures. It is unclear to what extent species adapted to a continuous cold environment retain control of regional blood flow. Muscle blood flow (MBF) was therefore measured using the radiolabelled microsphere method in the Antarctic teleost Notothenia coriiceps and compared with data for trout at their preferred (optimal) temperatures of 0°C and 11°C, respectively. In resting fish, blood flow distribution to skeletal muscle was similar (around 8 ml/min/100 g), despite a lower cardiac output in Notothenia (7 vs. 27 ml/min). Following maximal exercise there was a significant increase in specific blood flow to slow, but not fast, muscle in both species. However, the relative hyperaemia was greater in trout (6.6- vs. 9.3-fold, respectively), with a similar relative increase in cardiac output (2.3- vs. 3.4-fold, respectively). The increase in blood flow to slow muscle, and decrease in peripheral resistance, during maximal exercise in trout (and hence regional O₂ delivery by convective transport) appears to be determined centrally. In contrast, reduced active redistribution from the viscera in Notothenia is presumably responsible for the attenuated increase in MBF on exercise. However, regional hyper- and hypoperfusion can occur within a single locomotory muscle, suggesting control of vascular tone is maintained.     

Decreased skeletal muscle pump activity in patients with postural tachycardia syndrome and low peripheral blood flow

Standing translocates thoracic blood volume into the dependent body. The skeletal muscle pump participates in preventing orthostatic intolerance by enhancing venous return. We investigated the hypothesis that skeletal muscle pump function is impaired in postural tachycardia (POTS) associated with low calf blood flow (low-flow POTS) and depends in general on muscle blood flow. We compared 12 subjects that have low-flow POTS with 10 controls and 7 patients that have POTS and normal calf blood flow using strain-gauge plethysmography to measure peripheral blood flow, venous capacitance, and calf muscle pump function. Blood volume was estimated by dye dilution. We found that calf circumference was reduced in low-flow POTS (32 +/- 1 vs. 39 +/- 3 and 43 +/- 3 cm) and, compared with controls and POTS patients with normal blood flow, is related to the reduced fraction of calf venous capacity emptied during voluntary muscle contraction (ejection fraction, 0.52 +/- 0.07 vs. 0.76 +/- 0.07 and 0.80 +/- 0.06). We found that blood flow was linearly correlated (r(p) = 0.69) with calf circumference (used as a surrogate for muscle mass). Blood volume measurements were 2.2 +/- 0.3 in low-flow POTS vs. 2.6 +/- 0.5 in controls (P = 0.17) and 2.4 +/- 0.7 in normal-flow POTS patients. Decreased calf blood flow may reduce calf size in POTS and thereby impair the upright ejective ability of the skeletal muscle pump and further contribute to overall reduced blood flow and orthostatic intolerance in these patients.     

Insect-based BioFETs with improved signal characteristics

Insect-based BioFETs (biologically sensitive field-effect transistors) with improved signal characteristics have been developed. These BioFETs require a specifically adapted signal interfacing between a FET as signal transducer and an intact insect antenna as biocomponent. Therefore, different field-effect transistors have been fabricated in order to study the signal transfer at the bioelectronic interface. As relevant features of the BioFET, its current-voltage characteristics, the transconductance and the signal-to-noise ratio have been investigated as affected by the choice of gate insulator materials and gate dimensions (width-to-length ratio, thickness of the dielectric layers). The performance of the improved FET arrangement in the isolated-antenna BioFET was validated by employing dilution series of the plant odour component Z-3-hexen-1-ol.     

Six-minute walk test improved forearm skin blood flow in Tunisian obese women

The purpose of this study was to investigate whether 6-min walk test (6MWT) would improve the forearm skin blood flow (FSBF) response to acetylcholine (ACh), an endothelium-dependent vasodilator, in Tunisian women over a wide range of BMI. The FSBF was measured noninvasively using a laser Doppler flowmeter in response to local infusion of a cumulative dose of ACh, before and after the 6MWT for 102 healthy women; the results were expressed as percentage of baseline. The 6MWT was monitored and recorded. The mean response of FSBF to ACh was significantly greater before as well as after the 6MWT in lean (1,235 ± 123% vs. 1,644 ± 140%) than in overweight (630 ± 62% vs. 1,080 ± 66%) and obese subjects (402 ± 38% vs. 795 ± 40%) (P < 0.0001). Our regression analysis also revealed that the maximal FSBF response to ACh (i.e., its efficacy) was inversely correlated with BMI both before as well as after the 6MWT (r = -0.828, P < 0.0001; r = -0.859, P < 0.0001, respectively), and the efficacies of ACh in the three groups were all significantly elevated following the 6MWT (P < 0.0001). As indicated by ANOVA test, the 6MWT improved the FSBF responses of the lean, overweight, and obese subjects, by 33, 71, and 98%, respectively. We confirm that obesity induced a reduction of skin vasodilatory reserve and altered both endothelial-dependent relaxation and wall compliance. However, our new data clearly demonstrated that the 6MWT not only improved significantly the FSBF responses in the three groups of women, but the obese patients appeared to benefit more from the 6MWT than the overweight and the lean subjects.     

Subarachnoid haemorrhage: older patients have low cerebral blood flow.

Daily estimations of hemispheral cerebral blood flow using the xenon-133 inhalation technique was made in 116 patients during the first three weeks after subarachnoid haemorrhage. The patients'' cerebral perfusion on average remained less than the normal perfusion expected for their age (based on a single estimation of cerebral blood flow in 67 volunteers). On each separate day after subarachnoid haemorrhage cerebral blood flow was inversely related to the patient''s age. Older patients seem especially at risk of developing cerebral ischaemia after subarachnoid haemorrhage. The clinical outcome was more often unfavourable in older patients--that is, in those who tended to have the lowest cerebral blood flow. Present results support the view that episodes of low cerebral blood flow lead to a poor outcome after subarachnoid haemorrhage. Because of the risk of inducing cerebral ischaemia great care should be exercised by physicians administering hypotensive drugs to older patients after subarachnoid haemorrhage.     

A continuum theory of blood cells filtration at low flow rates

Blood cells filtration with decreasing pressure under gravity was studied for evaluation of the cell fluidity or deformability at a low shear state. A continuum approach was made to the flow and pressure in the filter at the low flow state to relate macro- and micro-scopic quantities. The mass conservation law of each species provided a set of differential equations with respect to the pore fraction and filter resistance. The numerical calculation was made for various values of hematocrit and leukocrit. It was shown that the filter resistance might be increased with decreasing pressure, resulting from both red and white cells. The leukocrit, more than 0.05% white cells, may influence the filtration, depending upon the cell deformation. Even in the absence of the white cell, a decrease in pressure increased the filter resistance markedly. The present result indicates that single red cell shows a nonlinear behavior of flow in pores at the low pressure level.     

Frequency response characteristics of cerebral blood flow autoregulation in rats

Transfer function analysis of blood pressure and cerebral blood flow in humans demonstrated that cerebrovascular autoregulation operates most effectively for slow fluctuations in perfusion pressure, not exceeding a frequency of approximately 0.15 Hz. No information on the dynamic properties of cerebrovascular autoregulation is available in rats. Therefore, we tested the hypothesis that cerebrovascular autoregulation in rats is also most effective for slow fluctuations in perfusion pressure below 0.15 Hz. Normotensive Wistar-Kyoto rats (n = 10) were instrumented with catheters in the left common carotid artery and jugular vein and flow probes around the right internal carotid artery. During isoflurane anesthesia, fluctuations in cerebral perfusion pressure were elicited by periodically occluding the abdominal aorta at eight frequencies ranging from 0.008 Hz to 0.5 Hz. The protocol was repeated during inhibition of myogenic vascular function (nifedipine, 0.25 mg/kg body wt iv). Increases in cerebral perfusion pressure elicited initial increases in cerebrovascular conductance and decreases in resistance. At low occlusion frequencies (<0.1 Hz), these initial responses were followed by decreases in conductance and increases in resistance that were abolished by nifedipine. At occlusion frequencies of 0.1 Hz and above, the gains of the transfer functions between pressure and blood flow and between pressure and resistance were equally high in the control and nifedipine trial. At occlusion frequencies below 0.1 Hz, the gains of the transfer functions decreased twice as much under control conditions than during nifedipine application. We conclude that dynamic autoregulation of cerebral blood flow is restricted to very low frequencies (<0.1 Hz) in rats.     

WISE-2005: reduced cerebral blood flow velocity with nitroglycerin--comparison with common carotid artery blood flow

During the WISE-2005 study of 24 women, we observed a reduction (21.6 +/- 0.89%, mean +/- SEM) in cerebral blood flow velocity (CBV) measured by transcranial Doppler ultrasound, following 0.3 mg sublingual nitroglycerin (NG). In parallel, we observed quantitative reductions in leg blood flow (47.3 +/- 7.0%) and corresponding reductions in calculated conductance (Conductance = Femoral Flow / Mean Arterial Pressure; 45.7 +/- 7.2%). To determine if the reduction in CBV was the result of reduced cerebral blood flow or dilation of the middle cerebral artery (MCA), the change in CBV in the MCA was compared with changes in quantitative flow measured in the common carotid artery (CCA). The relationship between CBV and CCA blood flow was tested in five men and four women using hyper- and hypo-ventilation to manipulate arterial PCO2. Changes in CCA blood flow were positively correlated with changes in CBV (p<0.001). We then investigated the CBV and CCA flow responses to sublingual NG in an additional two men and six women. Concurrent with the reduction in CBV there was no change in blood flow through the CCA (p>0.05). These results indicate that the decrease in CBV observed in response to NG was probably the result of dilation of the MCA and that total cerebral blood flow was similar after administration of NG. These results suggest regional differences in the vascular responses to NG during the WISE bed rest. Conduit vessels of both the peripheral and cerebral vasculature dilated; however, the resistance vessels in skeletal muscle constricted causing a reduction in blood flow, while the resistance vessels of the brain appeared to be unaffected by NG so that cerebral blood flow remained constant. These results highlight the need to obtain quantitative measures of cerebral blood flow if there is reason to suspect that the diameter of the MCA might not remain constant.     

The tissue factor requirement in blood coagulation

Formation of thrombin is triggered when membrane-localized tissue factor (TF) is exposed to blood. In closed models of this process, thrombin formation displays an initiation phase (low rates of thrombin production cause platelet activation and fibrinogen clotting), a propagation phase (>95% of thrombin production occurs), and a termination phase (prothrombin activation ceases and free thrombin is inactivated). A current controversy centers on whether the TF stimulus requires supplementation from a circulating pool of blood TF to sustain an adequate procoagulant response. We have evaluated the requirement for TF during the progress of the blood coagulation reaction and have extended these analyses to assess the requirement for TF during resupply ("flow replacement"). Elimination of TF activity at various times during the initiation phase indicated: a period of absolute dependence (<10 s); a transitional period in which the dependence on TF is partial and decreases as the reaction proceeds (10-240 s); and a period in which the progress of the reaction is TF independent (>240 s). Resupply of reactions late during the termination phase with fresh reactants, but no TF, yielded immediate bursts of thrombin formation similar in magnitude to the original propagation phases. Our data show that independence from the initial TF stimulus is achieved by the onset of the propagation phase and that the ensemble of coagulation products and intermediates that yield this TF independence maintain their prothrombin activating potential for considerable time. These observations support the hypothesis that the transient, localized expression of TF is sufficient to sustain a TF-independent procoagulant response as long as flow persists.     

Time-dependent rheological behaviour of blood flow at low shear in narrow horizontal tubes

Magnitude and time-dependence of the effects of red cell aggregation and sedimentation on the rheology of human blood were studied during low shear (tau W 2.5 to 92 mPa) flow through horizontal tubes (ID 25 to 105 microns). Immediately following reduction of perfusion pressure to a low value the red cell concentration near the tube walls decreases as a result of red cell aggregation. This is associated with a transient increase of centerline velocity. Simultaneously, sedimentation begins to occur and eventually leads to the formation of a cell-free supernatant plasma layer. Time-course and extent of this sedimentation process are strongly affected by wall shear stress variation, particularly in the larger tubes. At the lower shear stresses, centerline velocity decreases (flow resistance increases) with time following the initial acceleration period, due to sedimentation of red cells. This is followed by a further increase of resistance caused by the elevation of hematocrit occurring because of the reduction of cell/plasma velocity ratio. The time dependence of blood rheological behaviour under these flow conditions is interpreted to reflect the net effect of the partially counteracting phenomena of sedimentation and red cell aggregation.     

Changes in organ blood flow between high and low voltage electrocortical activity in fetal sheep

The effect of spontaneous changes in high or low voltage electrocortical activity, in the absence of uterine contractions, on the regional distribution of blood flow was studied in normoxic unanaesthetized fetal sheep at 124-134 days gestation in utero, using the isotope-labelled microsphere method. On transition from high to low voltage there was a significant fall in arterial pressure (7%) and an increase in flow (19-38%) to areas of the brain corresponding to the arborization of the reticular formation, i.e. excluding the cerebrum and cerebellum. Blood flow to the gastro-intestinal tract, pancreas and liver (portal vein) also increased.     

Phage display selects for amylases with improved low pH starch-binding

Directed evolution of secreted industrial enzymes is hampered by the lack of powerful selection techniques. We have explored surface display to select for enzyme variants with improved binding performance on complex polymeric substrates. By a combination of saturation mutagenesis and phage display we selected alpha-amylase variants, which have the ability to bind starch substrate at industrially preferred low pH conditions. First we displayed active alpha-amylase on the surface of phage fd. Secondly we developed a selection system that is based on the ability of alpha-amylase displaying phages to bind to cross-linked starch. This system was used to probe the involvement of specific beta-strands in substrate interaction. Finally, a saturated library of alpha-amylase mutants with one or more amino acid residues changed in their Cbeta4 starch-binding domain was subjected to phage display selection. Mutant molecules with good starch-binding and hydrolytic capacity could be isolated from the phage library by repeated binding and elution of phage particles at lowered pH value. Apart from the wild type alpha-amylase a specific subset of variants, with only changes in three out of the seven possible positions, was selected. All selected variants could hydrolyse starch and heptamaltose at low pH. Interestingly, variants were found with a starch hydrolysis ratio at pH 4.5/7.5 that is improved relative to the wild type alpha-amylase. These data demonstrate that useful alpha-amylase mutants can be selected via surface display on the basis of their binding properties to starch at lowered pH values.