Abstract
The role of microtubules and actin microfilaments in adaptive changes of the apical Na-dependent transport of phosphate (Pi) was investigated in opossum kidney (OK) cells. Up-regulation of Na/Pi cotransport was achieved by incubating OK cells in a medium containing 0.1 mM Pi; down-regulation of Na/Pi cotransport was provoked by refeeding adapted cells with 2 mM Pi. Up-regulation of Na/Pi cotransport was found to be inhibited by approximately 50% after a pretreatment of the cells with the microtubule disrupting agents nocodozole and colchicine; indirect immunofluorescence indicated complete depolymerization of the microtubular network. No inhibition of the adaptive response was observed after treatment of the cells with cytochalasin B to depolymerize actin microfilaments. In adapted cells, depolymerization of microtubules by nocodozole led to a reversibility of Na/Pi cotransport similar to that observed after refeeding adapted cells with 2 mM Pi. No effects of the microtubule disrupting drugs were observed on Na/L-glutamic acid transport. Depolymerization of microtubules did not prevent parathyroid-hormone-mediated inhibition of Na/Pi cotransport. It is concluded that microtubules are (at least in part) involved in the correct insertion of newly synthesized apical Na/Pi cotransport systems and that microtubules are not involved in the internalization of Na/Pi cotransport systems.
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