Sodium and ionic strength sensing by the calcium receptor

Area of Science:

• Biochemistry
• Physiology
• Molecular Biology

Background:

• The calcium-sensing receptor (CaR) regulates serum calcium levels by responding to extracellular calcium.
• CaR is present in various tissues beyond the parathyroid gland, suggesting broader physiological roles.
• CaR activation involves electrostatic interactions with polycationic agonists.

Purpose of the Study:

• To investigate the effect of ionic strength on CaR activation.
• To determine if ionic strength modulates CaR sensitivity to agonists like extracellular calcium and spermine.
• To explore the potential role of CaR in sensing ionic strength in different tissues.

Main Methods:

• Experiments measuring CaR activation by varying extracellular calcium and NaCl concentrations.
• Assessment of CaR sensitivity to spermine under different ionic strengths.
• Analysis of parathyroid hormone secretion in response to calcium and ionic strength changes.

Main Results:

• Changes in NaCl concentration significantly altered CaR activation by extracellular calcium and spermine.
• Ionic strength inversely affected CaR sensitivity: lower ionic strength increased sensitivity, higher ionic strength decreased it.
• Parathyroid cells showed potent modulation by ionic strength, with altered EC50 for calcium inhibition of PTH release.

Conclusions:

• Ionic strength is a critical modulator of CaR activity, independent of osmolality.
• The CaR may function as a sensor for ionic strength and NaCl in various tissues, including brain regions.
• These findings have implications for understanding CaR function in calcium homeostasis and other physiological processes.