Figure 9. Hypothesis of the regulation of parathyroid cell proliferation by extracellular [Ca2+]. 1) Inhibitory pathway via the calcium-sensing receptor (CaR). 2) Stimulatory pathway via an unknown transmembrane transduction mechanism. Physiologically, pathway 1 predominates over pathway 2. In presence of parathyroid hyperplasia with calcium-sensing receptor down-regulation pathway 2 could become dominant and favor parathyroid cell proliferation over suppression. After Roussanne et al (75).