Measurements of macula densa cell volume changes in isolated and perfused rabbit cortical thick ascending limb. I. Isosmotic and anisosmotic cell volume changes

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Abstract
A method to measure time‐dependent volume changes in macula densa (MD) cells is described. Cell volume is calculated from cell height measurements for which an image‐splitting eyepiece is used. This paper presents the results of experiments designed to investigate the behaviour of the macula densa cells in anisosmotic media, to evaluate the cell volume response to sudden decreases in luminal or peritubular osmolarity and to examine the effect of different luminal NaCl concentrations on the steady‐state isosmotic cell volume and on the regulatory volume response to anisosmotic media. The result showed that induced alteration in macula densa cell volume did not change macula densa surface area, but only cell height. The mean control cell height was 13.3 μm ± 0.4. When MD cells were exposed to a luminal osmolarity of 180 mosM (control 300 mosM) they swelled only to 1.19 ± 0.02 of the control value and with furosemide present to 1.13 ± 0.02 or with low NaCl to 1.13 ± 0.01. While after 5 min of exposure values were 1.15 ± 0.03, 0.99 ± 0.02 and 1.02 ± 0.02, respectively. Addition of furosemide (10‐1 M) to the luminal perfusate (300 mosM) resulted in a rapid decrease in cell height to 0.8 ± 0.02 in relation to control. When furosemide was removed cell volume was restituted (0.98 ± 0.03). When luminal perfusate was replaced by mannitol and (12 mM Na+, 7 mM CI‐) cell volume decreased to 0.83 ± 0.02 of the control value.The results of this study are in accordance with the view that the macula densa cells have a furosemide‐sensitive NaCl absorption system (cation and chloride co‐transport) across the apical membrane. The basolateral cell membrane have very rapid regulatory mechanisms which maintain the cell volume during reductions in basolateral osmolarity. The transport properties of the macula densa cells are not affected by maximal and supramaximal doses of vasopressin. These findings offer more direct support to the hypothesis that the initiation of the tubuloglomerular feedback response involve furosemide‐inhibitable uptake of sodium and chloride ions into the macula densa cells ‐ where the ions would, subsequently, be absorbed and accumulated inside the juxtaglomerular interstitium.