Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002)

The definition of a "primary method of measurement" [1] has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harned cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Syst\ue8me International d'Unit\ue9s). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [2]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 \ub0C. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances

H+ permeation through the cardiac gap junction

Although H+ ions modulate junctional conductance, their junctional permeabilità has not been measured. We investigated this in end-to-end pairs of guinea pig ventricular myocytes superfused with 20 mM HEPES-buffered Tyrode (pH 7.4 37°C). HCl(pH 3 in isotonic KCl) was diffused into cell-1 from patch pipette while confocal imaging pHi (AM-loaded SNARF). Acidification above an arbitrary threshold of 20 nM Delta[H+]I was used to record longitudinal acid movement between 2 downstream regions of interest (ROIs, 60 um apart) in cell-1 and two similarly spaced ROIs in cell-2. Intracellular delay between ROIs was 18s and 21s in cell-1 and 2 respectively (n=7). In contrast, delay over an intercellular distance of 10-20 um was about 75s. 40uM Beta-glycyrrhetinic acid, a junctional uncoupler, blocked junctional H+ permeation (n=2) and greatly increased resting intercellular resistance (from 6 MOhms to 1.7 GOhms; n=2, double patch experiments). In contrast, 30 uM Hoe 642 (a Na-H exchange inhibitor) had no effect on junctional H+ delay. Measurements of junctional SNARF flux (n=3) indicated it accounted for <0.7% of junctional acid flux. We conclude that acid permeates gap junctions, but that the rate of movement along a cell pair is limited by the junctional transfer. This will help to restrict the spread of an intracellular acid insult within the heart, such as may occur during myocardial ischaemia

Intrinsic mobility of H+ ions (DHapp) in guinea pig ventricular myocytes

This was estimated by diffusing HCl (pH 3.0 in isotonic KCl) into a myocyte through a patch pipette while confocally imaging pHi (AM-loaded SNARF). Cells were superfused with 20 mM HEPES-(am-loaded SNARF). Cells were superfused with 20 mM HEPES-buffered Tyrode, pH 7.4 at 37°C. The time-course of fall of pHi was averaged in two regions of interest (ROIs) positioned 10 um and 105 um downstream from the pipette. Inter-ROI time delay, estimated at a 20 nM threshold rise of [H+]I, was 28s (n=14), and was unaffected (n=3) by 30 uM Hoe 642, a potent Na-H exchange inhibitor. Acid movement was associated with a longitudinal pHi gradient of about 0.25 units. Separate measurements of SNARFi mobility (n=4) indicated that protonated fluorophore movement accounted for <0.2% of the longitudinal acid flux. Modelling the spatiotemporal behaviour of pHi using equations for two-dimensional H+ diffusion indicated a DHapp of about 1.0 x 10-6 cm2/s, 2-3 fold faster than apparent in rabbit ventricular myocytes (Vaughan-Jones et al. Biophys J [2000] 78, 223A) but still 100 fold slower than for H+ in unbaffered solution. We conclude that effective intracellular H+ mobility is slow, most likely limited by the mobility of high capacity intrinsic buffers

Possibilities and limitations of current stereo-endoscopy

Background Stereo-endoscopy has become a commonly used technology. In many comparative studies striking advantages of stereo-endoscopy over two-dimensional presentation could not be proven. To show the potential and fields for further improvement of this technology is the aim of this article. Method The physiological basis of three-dimensional vision limitations of current stereo-endoscopes is discussed and fields for further research are indicated. New developments in spatial picture acquisition and spatial picture presentation are discussed. Results Current limitations of stereo-endoscopy that prevent a better ranking in comparative studies with two-dimensional presentation are mainly based on insufficient picture acquisition. Conclusion Devices for three-dimensional picture presentation are at a more advanced developmental stage than devices for three-dimensional picture acquisition. Further research should emphasize the development of new devices for three-dimensional picture acquisition