PTH and secondary hyperparathyroidism: To evaluate the calcium-responding capacity of parathyroid cells in secondary hyperparathyroidism (SHPT), we isolated and cultured parathyroid cells from patients with SHPT, then used electrophysiological recordings, calcium fluorescence and a PTH assay to assess the calcium-induced response, including membrane potential, intracellular Ca2+ and PTH levels, respectively (Fig. 1a).