Dihydropyridine-sensitive calcium entry activated by physiological increase in extracellular calcium concentration in human hyperparathyroid cells

Abstract

In the companion abstract, we showed that cardiac-like L-type Ca²⁺ channel is expressed in hyperparathyroid cells. We then posed the question whether this channel could be activated under physiological conditions. Cytoplasmic Ca²⁺ concentration was measured with a Ca²⁺-sensitive fluorescent indicator probe, fluo-3. The cells were immersed in Hepes-buffered solution containing 1-2 mM Ca²⁺. Local application of the same solution via a small-tipped pipette resulted in no change in the fluo-3 fluorescence, suggesting that stretch-activated Ca²⁺ permeable channels were not activated under our experimental conditions . When Ca²⁺ concentration of the pipette solution was raised to 2.5-3 mM, however, application of the high Ca²⁺ solution for 10 s evoked a transient increase in fluo-3 fluorescence (a fluo-3 transient). In the presence of 10 μM nitrendipine, the fluo-3 transient was markedly reduced. These results could be explained by the hypothesis that increase in extracellular Ca²⁺ concentration induces Ca²⁺ entry, at least in part, through the dihydropyridine-sensitive L-type Ca²⁺ channels. In addition, neither skeletal myotubes nor cardiac myocytes showed fluo-3 transients upon exposure to 10 mM extraccelular Ca²⁺. [Jpn J Physiol 55 Suppl:S209 (2005)]