Differentiation of BC₃H1 smooth muscle cells changes the bivalent cation selectivity of the capacitative Ca²⁺ entry pathway

Differentiation of BC₃H1 cells leads to expression of a variety of proteins characteristic of smooth muscle and to changes in the behaviour of intracellular Ca²⁺ stores. Treatment of both differentiated and undifferentiated cells with thapsigargin (2 μM) emptied their intracellular Ca²⁺ stores, and in the presence of extracellular Ca²⁺ caused an increase in cytosolic [Ca²⁺] that rapidly reversed after its removal. The amplitudes of these capacitative Ca²⁺ entry signals were 101±8 nM (n = 42) in differentiated cells and 188±16 nM (n = 35) in undifferentiated cells. Mn²⁺ entry in thapsigargin-treated cells, measured by recording the quenching of cytosolic fura 2 fluorescence, was 374±26% (n = 34) and 154±7% (n = 41) of control rates in differentiated and undifferentiated cells, respectively. Empty stores caused Ba²⁺ entry to increase to 282±20% (n = 8) of its basal rate in differentiated cells and to 187±20% (n = 8) in undifferentiated cells. Rates of Ca²⁺ extrusion, measured after rapid removal of extracellular Ca²⁺ from cells in which capacitative Ca²⁺ entry had been activated, were similar in differentiated (t_½ = 23±2 s, n = 7) and undifferentiated (23±1 s, n = 6) cells. The different relationships between capacitative Ca²⁺ and Mn²⁺ signals are not, therefore, a consequence of more active Ca²⁺ extrusion mechanisms in differentiated cells, nor are they a consequence of different fura 2 loadings in the two cell types. We conclude that during differentiation of BC₃H1 cells, the cation selectivity of the capacitative pathway changes, becoming relatively more permeable to Mn²⁺ and Ba²⁺. The change may result either from expression of a different capacitative pathway or from modification of the permeation properties of a single pathway.