Cytoplasmic Ca2+ concentration of single normal human and bovine parathyroid cells measured by dual wavelength microfluorometry

Dual wavelength microfluorometry was utilized to measure the cytoplasmic calcium concentration (Ca_i ²⁺) of single parathyroid cells loaded with the indicator fura-2. The method enabled the first registrations of Ca_i ²⁺ of normal human parathyroid cells, available only in minute numbers. At 0.5 mM extracellular Ca²⁺, the Ca_i ²⁺ levels were similar in normal human and bovine cells. Both cell types responded with an initial Ca_i ²⁺ transient followed by a sustained increase when raising extracellular Ca²⁺ to 3.0 mM. The sustained effect exhibited a sigmoidal relation to extracellular Ca²⁺ in the 0.5–3.0 mM range. Although the increase was somewhat greater in the human cells, the half maximal responses were obtained at almost identical extracellular Ca²⁺ concentrations. Whereas K⁺ depolarization decreased Ca_i ²⁺, the Ca_i ²⁺ channel blocker D-600 had dual actions, raising Ca_i ²⁺at 0.5 mM Ca²⁺ and decreasing it at 3.0 mM Ca_i ²⁺, and the effects were similar in the bovine and human cells. The present experimental approach verified the validity of utilizing bovine cells as controls in studies of human parathyroid tissue and it appears suitable for analysis of the role of different subpopulations of parathyroid cells in the abnormal parathyroid tissue of patients with hyperparathyroidism.