Iain F. Gow

The sensitivity of human blood platelets to the aggregating agent ADP during different dietary sodium intakes in healthy men

SummaryWe have investigated the effect of varying sodium intake on the renin-angiotensin system, ADP-induced patelet aggregationin vitro, and blood 5-HT concentrations in 9 male volunteers.Systolic blood pressure was slightly reduced during a low sodium diet, whereas the diastolic pressure remained unchanged. Plasma renin activity and aldosterone concentration both fell significantly when sodium intake was increased; plasma angiotensin lI concentraion also fell, but not significantly.There was a significant fall in haematocrit after an increased sodium intake, but there was no change in the whole-blood platelet count after correcting for this. There were no significant changes in either total (i. e. PRP) or platelet 5-HT concentrations.The extent of platelet aggregation induced by 5 and 20 μmol · 1−1 of ADP increased significantly when dietary sodium intake was increased. When compared with low or normal sodium intakes, lower concentrations of ADP were required to produce 50% of maximum aggregation after a high sodium intake. The 5-HT2, receptor antagonist ketanserin (1 μmol · 1−1in vitro) reduced the extent of aggregation induced by 5 μmol · 1−1 ADP after the volunteers had taken a high sodium diet, whereas the angiotensin 11 receptor anatgonist saralasin (1 nmol-1−1) increased the rate of aggregation after the low sodium diet.Thus, during a high sodium intake, human platelets become more sensitive to the aggregating agent ADP It is possible that this effect is mediatedvia platelet 5-HT2 receptors, since ketanserin abolished the increase in salt-induced aggregation seen with 5 μmol · 1−1 ADP.

Development and validation of an improved radioimmunoassay for serotonin in platelet-rich plasma

A radioimmunoassay (RIA) using a 125I-tracer is described for measurement of serotonin (5-hydroxytryptamine, 5-HT) in human platelet-rich plasma (PRP). Antisera were raised against 5-HT-succinamate conjugated to bovine albumin and, to improve assay sensitivity, the analyte was made chemically similar to the immunogen by conversion to N-acetylserotonin prior to assay, using the specific amino reagent N-acetoxysuccinimide. The assay shows good correlation with a high-pressure liquid chromatography (HPLC) reference method (5-HT RIA = 1.007 X 5-HT HPLC + 29.3, r = 0.936, p less than 0.001, n = 40), indicating that no significant cross-reactions were detected. Samples of PRP are diluted 1/20 to fall within the working range (80-15% B/B0) of the assay, which is 4.75-325 nmol/l, (0.95-65.0 pmol/tube), corresponding to 95-6500 nmol/l in PRP. Intra- and interassay coefficients of variation were 5.0-10.5% and 12.0-21.2% respectively for serotonin concentrations of 250-2,500 nmol/l added to platelet-poor plasma. With this improved assay, it is possible to analyse up to 100 samples/day, compared with 10-20 samples/day by HPLC.

Properties of Volume-Activated Taurine Efflux from Human Breast Cancer Cells

Cell membranes are permeable to water, which means that cell volume will be determined by the osmolality of the extracellular fluid and by the cellular content of osmotically active solutes. Although some cells can be exposed to anisosmotic conditions, most cells experience a change in their volume due to alterations in the rate of solute transport (Lang et al., 1998). For example, an increase in solute uptake or an increase in solute efflux could lead respectively to cell swelling and shrinking. However, it is well established that most cells are able to regulate their volume (Hoffmann and Simonsen, 1989). Thus, if cells are placed in a hyposmotic solution they initially behave like perfect osmometers and swell, but subsequently reduce their volume. This process is known as a regulatory volume decrease (RVD) and is dependent upon a net efflux of solutes from the cells. In particular, the efflux of K, Cl and amino acids, especially taurine, underlies a RVD (Kirk, 1997). Conversely, if cells are bathed in a hypertonic solution they shrink but increase their volume towards normal. This response relies upon an increase in the cellular uptake of solutes (Hoffmann and Simonsen, 1989). Although cells have to regulate their volumes within relatively narrow limits, it is now established that cell volume per se acts as a signalling system to control key metabolic processes such as protein synthesis and lipogenesis (Lang et al., 1998). For example, mammary protein synthesis is stimulated by cell swelling and inhibited by cell shrinking (Millar et al., 1997; Grant et al., 2000). Therefore, it follows that volumeactivated transport processes may have a key role to play in regulating cell growth and proliferation as a consequence of altering cell metabolism. It is now becoming apparent that membrane transport processes, which are normally activated by an increase in cell volume, may be involved in the early stages of apoptosis. For example, volume-sensitive K efflux appears to play a role in initiating programmed cell death in mouse one-cell embryos (Trimarchi et al., 2002). Activation of volume-