The continuous measurement of tubular volume changes in response to step changes in contraluminal osmolality

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Abstract
A new method to measure time dependent (t) volume (V) changes in proximal straight tubules (PST) is described.V is calculated from diameter (d) measurements for which a video camera and an integrating circuit are used. A tubular image of high optical contrast is recorded with the TV camera such that the scan lines run crosswise to the tubule. The video signal is analyzed by a special processor which adds 225 tubular diameters of each TV frame and feeds this analog signal to a pen recorder. The fractional error ind measurements is 10−3. Diameter changes of less than 0.05 μm can be detected, as compared to the usual error of a single measurement of about 0.4 μm.P os cb , the osmotic water permeability of the contraluminal cell membrane was measured by setting up osmotic steps across it in less than 0.1 s and following the ensuing Δd/Δt. the time delay between solution change and the linear part of the osmotic response was 0.51±0.05 s.P os cb was found to be 50.4 (±8.7)×10−4 cm3·cm−2 of basement membrane area ·s−1·osmolar−1.