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Biomedical subjects

A R Gear

Publications and source records attributed to A R Gear.

12 recordsLinked to original sources
PubMed1992

High-speed platelet adhesion under conditions of rapid flow.

The recognition of exposed collagen by circulating platelets is an initial step in the formation of the hemostatic plug or a thrombus after vascular injury. Theoretical calculations of the speed of platelet function required for effective hemostasis have suggested very short reaction times. However, it is not known how fast platelets can adhere to collagen under arterial flow conditions or which membrane proteins are involved. We have used a continuous-flow, microaffinity column linked to a resistive-particle counter to detect platelet adhesion. Adhesion of human platelets to native type I collagen was extremely rapid, with exponential half-times as short as 240 ms, and was nearly complete by 2 s. This RGD-independent process was not associated with platelet aggregation or secretion. The monoclonal antibody 6F1 directed against the glycoprotein Ia/IIa complex inhibited adhesion, suggesting that this complex plays an important role in the initial phases of platelet-collagen interaction under flow conditions. In addition, divalent cations were required for adhesion, as indicated by inhibition with EDTA in plasma and the dependence on Mg2+ for washed platelets.

Amino Acid Sequence↗
PubMed1992

Adhesion efficiency, platelet density and size.

We have previously shown that adhesion of human platelets to immobilized collagen is extremely rapid, with initial rates approaching 3% of single particles adhering per 10 ms. Here, we have investigated adhesion efficiency to collagen as a function of platelet density. Platelet subpopulations: low-density (1.040 < d < 1.065 g/ml), intermediate-density (1.065 < d < 1.070 g/ml) and high-density (1.070 < d < 1.080 g/ml) were separated by Percoll density gradient centrifugation. They constituted 24%, 47% and 29% of the total platelet population and had mean volumes of 6.01, 7.37 and 8.21 fl, respectively. Using a continuous-flow, micro-affinity column, we found that the most dense (large) platelets exhibited initial rate of adhesion 4 times greater than the least dense (small) platelets. They were also less sensitive to inhibition by prostacyclin (PGI2). In contrast, there was no significant difference in aggregation induced by high doses of ADP and collagen, indicating that the most dense platelets were not preferentially involved in aggregation induced by high doses of agonists. These results suggest that normal circulating platelets can be distinctly heterogeneous in their ability to adhere to collagen under arterial-flow conditions. The greater efficiency of high-density platelets may be related to increased content of the glycoprotein Ia/IIa (GPIa/IIa) complex.

Antigens, CD↗
PubMed1991

Radiographic contrast agents and platelet function: a quenched-flow study.

Radiographic contrast media (RCM) may alter platelet behavior at concentrations achieved during cardiac angiography. We used quenched-flow aggregometry coupled to single-particle counting to study the influence of RCM on the kinetics of platelet aggregation (less than 5.0 sec) induced by adenosine diphosphate (ADP, 2.86 microM). At a concentration in platelet-rich plasma (PRP) of 5 per cent RCM by volume, platelet aggregation was inhibited by diatrizoate, iopamidol and ioxaglate either directly or following incubation of each contrast agent with PRP for 20 minutes. Diatrizoate inhibited more than did iopamidol or ioxaglate (56 +/- 6, versus 39 +/- 3 and 40 +/- 9 per cent respectively; P less than 0.003, p less than 0.009, n = 20 normal subjects). A small reduction (about 16 per cent) in aggregation velocity occurred within 5 seconds of exposure of PRP to all 3 RCM and the onset time (t) or lag period before aggregation begins was significantly prolonged by diatrizoate (p less than 0.03). The RCM vehicles alone (iodinated moiety removed, osmolality readjusted) had no effect on the ADP-induced aggregation. Platelet counts fell significantly after incubation with diatrizoate (12%; p = 0.04). Our data therefore show that early platelet aggregation was inhibited by 3 commonly-used ionic and nonionic contrast agents. Inhibition was apparently caused by the iodinated contrast molecule, began within seconds of platelet-RCM contact and was independent of vehicle composition. Since diatrizoate inhibited aggregation more than iopamidol or ioxaglate, its use may be of additional value during angiographic procedures in clinical situations involving enhanced platelet activation.

Adult↗
PubMed1989

Endothelium-dependent potentiation of human platelet aggregation.

The endothelium produces many substances which have the potential to alter hemostasis. We examined the influence of a substance(s) released by cultured bovine aortic endothelial cells on very early events (less than 5 sec) in platelet function, using a new quenched-flow approach. Contributions of the endothelial cell surface to alterations in platelet function were avoided by separating the endothelium and platelets. Superfusates from endothelial cells were allowed to interact with platelets for several seconds, followed by challenge with ADP to induce aggregation. Single-particle counting provided a sensitive index of aggregation kinetics. In this system, unstimulated endothelial-cell superfusate had no effect on aggregation induced by ADP. Stimulation of the endothelial cells with bradykinin produced an unstable factor(s) that potentiated ADP-induced platelet aggregation, was not affected by hemoglobin or cyclooxygenase inhibitions, and had no platelet aggregatory activity on its own. Nitric oxide inhibited aggregation and its effect was blocked by hemoglobin. The identity of the potentiating factor is unknown, but it does not appear to be platelet activating factor, thrombin, ADP, thromboxane, nitric oxide, or endothelium-derived relaxing factor.

Adenosine Diphosphate↗
PubMed1989

Atrial natriuretic factor in essential hypertension.

We measured circulating levels of immunoreactive atrial natriuretic factor (ANF) in 10 patients with untreated, uncomplicated mild to moderate essential hypertension and in 15 normotensive controls. ANF concentrations were significantly higher in the hypertensive group than in the control group (38.4 +/- 6.9 pg/ml versus 18.3 +/- 1.8 pg/ml, p less than 0.02). A positive correlation between ANF levels and systolic, diastolic and mean blood pressure was noted in the total study population (p less than 0.008, r = 0.52; p less than 0.005, r = 0.55; p less than 0.02, r = 0.46, respectively). Thus, plasma ANF concentrations are elevated in essential hypertension and may result from increased intraarterial pressure.

Adult↗
PubMed1989

Platelet calcium and quenched-flow aggregation kinetics in essential hypertension.

Abnormal platelet function may contribute to the complications of essential hypertension. We have studied the kinetics of platelet aggregation induced by adenosine diphosphate (ADP) or epinephrine, plasma beta-thromboglobulin, and basal, cytosolic, and free calcium, as correlates of platelet function. Fifteen untreated patients with essential hypertension and without detectable atherosclerosis, 18-40 years old, were compared with 30 matched normotensive control subjects. Maximal rates of platelet aggregation (Vmax) with ADP and epinephrine were significantly higher in patients than in control subjects (p less than 0.03), as assessed by quenched-flow aggregometry. However, significance was lost when Vmax was corrected for the platelet count. Paradoxically, the activation constants (Ka) for ADP were higher in patients than in control subjects (p less than 0.03). With ADP as the inducing agent, onset time (t) or lag period before aggregation begins was longer in patients than in control subjects (p less than 0.02). beta-thromboglobulin levels, an index of in vivo platelet activation, were not significantly different between the two groups (p = 0.13). The mean platelet cytosolic free calcium concentration was higher in patients (213 +/- 19 nM) than in control subjects (172 +/- 14 nM), but this difference was not statistically significant (p = 0.07). However, there was a close correlation between the free calcium level and systolic, diastolic, and mean blood pressure (p less than 0.003, p less than 0.04, p less than 0.004, respectively). No difference in platelet volume between the two groups was found. Our data suggest that platelets in the early stages of essential hypertension display an overall increased aggregation potential but a diminished sensitivity to ADP.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Diphosphate↗
PubMed1977

Age-dependent separation of erythrocytes by preparative electrophoresis.

Human and rat erythrocytes have been fractionated according to charge by density-gradient electrophoresis. Some 1 to 2 ml, or about 10(10) of packed cells, can be electrophoresed up a stabilizing sucrose gradient, and require 3 to 4 hr for effective fractionation. The fastest migrating erythrocytes were the least fragile, were larger, incorporated the most 59Fe, and contained more glucose 6-phosphate dehydrogenase activity. These findings are consistent with the most negatively charged erythrocytes being the youngest cells, agreeing with other separative procedures based on density, phase partitioning, or serial osmotic hemolysis.

Animals↗
PubMed1977

Erythrocyte osmotic fragility: micromethod based on resistive-particle counting.

A new procedure has been developed to measure the osmotic fragility of human erythrocytes. It is based on the resistive-particle counter and the finding that lysed erythrocytes have a lower electrical resistance than intact cells after the current in the sensing orifice reaches a critical value. The most suitable test conditions involve a 19 mu orifice, a high current setting, and the simple use of size discriminators to exclude lysed cells from being counted. Since only 1 to 5 microliter of blood are needed per salt concentration, a complete fragility test can be carried out with less than 50 mu of blood. Results with the new procedure were comparable to the classic spectrophotometric one for normal blood, in clinical conditions of increased or decreased erythrocyt osmotic fragility, and for different anticoagulants. It has a specific advantage in that ratio counting makes accurate pipetting of blood unnecessary.

Computers↗
PubMed1975

Control of platelet glycogenolysis; Activation of phosphorylase kinase by calcium.

An apparent enigma during platelet aggregation is that increased glycogenolysis occurs despite a fall in cyclic AMP levels; Activation by a classical cascade is therefore unlikely, and an alternative stimulus for phosphorylase a formation was sought. It was found that low levels of Ca-2+ markedly activate phosphorylase b kinase from human platelets, with a Ka of 0i muM Ca-2+, which is similar to that for the skeletal muscle enzyme; The kinase activity is unstable, and on enzyme ageing is a 50% loss in activity with the Ka decreasing to 0.33 muM Ca-2+. In unstilulated platelets, phosphorylase a was 13.3% of toal measured activity, and glycogen synthetase I was 32.3%. Aggregation induced by ADP did not change the percentage of I synthetase, while increasing that for phosphorylase a. Dibutyryl cyclic AMP did, as expected, increase the percentage of both phosphorylated enzymes; These findings suggest that the natural activator of platelet glycogenolysis during aggregation is Ca-2+, which directly stimulates phosphorylase b kinase without altering glycogen synthetase activity. The cyclic AMP-dependent protein kinase does not appear to be involved;

Adenosine Diphosphate↗