Characterisation of Ca2+ membrane permeability of renal A6 cells upon different osmotic conditions

The nature of the calcium-transporting mechanisms involved in A6 cell calcium homeostasis under iso- and hypo-osmotic conditions was investigated using fura-2 (AM) as a cell calcium indicator. Under steady-state conditions, intracellular calcium (Ca2+i) was increased by Bay K8644 or by gramicidin, an ionophore which depolarises A6 cell membranes. The Ca2+i increase following calcium addition (to calcium-depleted cells) or membrane depolarisation was blocked by nifedipine but not by verapamil or omega-conotoxin, indicating that the membrane calcium permeability may be mediated by voltage-dependent and dihydropyridine-sensitive calcium channels. Ca2+i could also be increased by a hypo-osmotic shock having a linear relationship with the osmolarity change. This osmotically induced Ca2+i increase had an extracellular origin since it was absent when cells were suspended in a calcium-free medium and it was not affected by thapsigargin or TMB-8 application. In addition, it was inhibited by the calcium channel inhibitor, nifedipine. Furthermore, under hypo-osmotic conditions, an additional Ca2+i increase, sensitive to nifedipine, was measured when cells were depolarised by gramicidin or K-gluconate addition. It is proposed that the hypo-osmotically induced cell calcium increase implies the activation of voltage-dependent and nifedipine-sensitive calcium channels, presenting the same pharmacological characteristics as those involved in cell calcium homeostasis under iso-osmotic conditions. The initial Ca2+i increase was transient and stabilised to a value nevertheless higher than the iso-osmotic level; this secondary and incomplete regulatory phase did not occur in the presence of thapsigargin or TMB-8, thus providing evidence of intracellular calcium storage.