A Comparison of the Potassium Content and Osmolality of Plaque Fluid and Saliva, and the Effects of Plaque Storage

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Abstract
Previous determinations of osmolality and potassium concentrations in plaque fluid, much higher than those in saliva, suggest a restricted exchange between the two, which must be reconciled with recent findings of quite rapid diffusion in plaque. Possible reasons for the high values were considered, and of these the effect of solute leakage from bacteria to the plaque fluid during typical periods of storage was investigated. It was also shown that the osmotic pressure of plaque fluid could be measured quite accurately by vapor pressure osmometry on whole plaque samples without the need for centrifugation. Samples of plaque, or plaque fluid prepared by centrifugation at 12,000 g, were compared for osmolality or potassium content with matched samples prepared from plaque stored chilled or in liquid nitrogen. Saliva samples obtained just prior to plaque collection were also analyzed. Freshly collected plaque from overnight-fasted subjects had a plaque fluid osmolality of 156 ± 35 as compared with 98 ± 23 mOs/kg for saliva. Potassium in plaque fluid from freshly collected "mature" plaque was 40.6 ± 5.1 as compared with 20.3 ± 5.3 mmol/L for saliva, but for 1-2-day-old plaque from fasted subjects it was significantly lower (30.4 ± 5.6 mmol/L). These values for plaque fluid are all much less than those previously found, and storage was found to cause a marked increase (range, 35-100%). Centrifugation at 12,000 g caused little change in plaque fluid osmolality but seemed to accelerate the rate of increase during subsequent storage.