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Pi transport regulation by chicken growth plate chondrocytes
C Montessuit1, J P Bonjour, J Caverzasio
1Department of Medicine, University Hospital, Geneva, Switzerland.
Insights
Inorganic phosphate transport in chicken growth plate chondrocytes is sodium-dependent and regulated by growth factors. This carrier-mediated process is crucial for cartilage growth and mineralization.
Area of Science:
- Cell Biology
- Biochemistry
- Skeletal Biology
Background:
- Inorganic phosphate (Pi) is essential for epiphyseal cartilage growth and mineralization.
- Understanding Pi transport mechanisms in chondrocytes is key to studying skeletal development.
Purpose of the Study:
- To characterize the inorganic phosphate (Pi) transport in primary cultures of chicken growth plate chondrocytes.
- To investigate the regulation of Pi transport by insulin-like growth factor I (IGF-I) and parathyroid hormone (PTH).
Main Methods:
- Primary cultures of chicken growth plate chondrocytes were used.
- Sodium-dependent Pi uptake was measured kinetically.
- Inhibition studies with Pi analogues were performed.
- The effects of IGF-I and PTH on Pi transport were assessed.
Main Results:
- Pi uptake was significantly enhanced by extracellular sodium, indicating sodium-dependent Pi transport (NaPiT).
- Kinetic analysis revealed a Michaelis constant for Pi of 0.443 ± 0.095 mM and a sodium concentration for half-maximal transport of 48.0 ± 8.7 mM.
- Stoichiometric analysis suggested cotransport of multiple sodium ions with each Pi molecule.
- NaPiT was inhibited by phosphonoformic acid and arsenate.
- IGF-I stimulated NaPiT in a dose-dependent manner, with maximal effect at >5 nM.
- PTH exhibited a biphasic effect, with maximal transient stimulation observed at 8 hours.
Conclusions:
- Pi uptake by growth plate chondrocytes is mediated by a saturable, sodium-dependent carrier system.
- This NaPiT system is regulated by key growth factors like IGF-I and PTH, influencing cartilage homeostasis.
- These findings provide insights into the molecular mechanisms underlying skeletal growth and mineralization.
Abstract:
Inorganic phosphate (Pi) is a key element for the growth and mineralization of the epiphyseal cartilage. In this study, the characteristics of the transport of Pi in growth plate chondrocytes have been determined using primary cultures of chicken growth plate cartilage cells. The uptake of Pi was significantly increased in the presence of extracellular sodium. The kinetic parameters of the saturable sodium-dependent Pi transport (NaPiT) were determined. The Michaelis constant for Pi was 0.443 +/- 0.095 mM, and the concentration of sodium with which half-maximal Pi transport was observed was 48.0 +/- 8.7 mM. Stoichiometric analysis suggested that more than one sodium ion was cotransported with each Pi molecule. NaPiT was sensitive to inhibition by Pi analogues such as phosphonoformic acid and arsenate. These data strongly suggest that Pi uptake by chicken growth plate chondrocytes is a carrier-mediated process driven by the transmembrane electrochemical gradient of sodium. Two important regulators of biosynthetic activities of growth plate chondrocytes, insulin-like growth factor I (IGF-I) and parathyroid hormone (PTH), selectively regulated Pi transport. With IGF-I, maximal stimulation (117 +/- 7% above control) was observed at doses > 5 nM, with an half-maximal effective concentration of 0.46 +/- 0.18 nM. A significant effect was observed after 1 h of exposure and was maintained for up to 24 h. PTH increased Pi transport with a biphasic dose-response curve. The change in NaPiT was transient, being maximally observed after 8 h (58 +/- 8%) and unexpressed after 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)