D

FIGURE 7-7 (see Color Plate)

Effects of a diet low in phosphate on renal tubular phosphate reabsorption in rats. A, Chronic high Pi diet. B, Acute low Pi diet. C, Colchicine and high Pi diet. D, Colchicine and low Pi diet. In response to a low phosphate diet, a rapid adaptive increase occurs in the sodium-phosphate (Na-Pj) cotransport activity of the proximal tubular apical membrane (A, B). The increase in Na-Pi cotransport activity is mediated by rapid upregulation of the type II Na-Pi cotransport protein, in the absence of changes in Na-Pi messenger RNA (mRNA) levels. This rapid upregulation is dependent on an intact microtubular network because pretreatment with colchicine prevents the upregulation of Na-Pi cotrans-port activity and Na-Pi protein expression (C, D). In this immunofluorescence micrograph, the Na-Pi protein is stained green (fluorescein) and the actin cytoskeleton is stained red (rhodamine). Colocalization of green and red at the level of the apical membrane results in yellow color [14].

FIGURE 7-8 (see Color Plate)

Effects of parathyroid hormone (PTH) on renal tubular phosphate reabsorption in rats. In response to PTH administration to parathyroidectomized rats, a rapid decrease occurs in the sodium-phosphate (Na-Pi) cotransport activity of the proximal tubular apical membrane. The decrease in Na-Pi cotransport activity is mediated by rapid downregulation of the type II Na-P( cotransport protein. In this immunofluores-cence micrograph, the Na-Pi protein is stained green (fluorescein) and the actin cytoskeleton is stained red (rhodamine). Colocalization of green and red at the level of the apical membrane results in yellow color [13]. A, parathyroidectomized (PTX) effects. B, effects of PTX and PTH.

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