Chick embryo heart cells with high and low intracellular calcium concentrations respond differently to ouabain

In cell cultures of 10-day-old chick embryo hearts, we found two cell populations, one with high intracellular calcium concentration ([Ca2+]i) of 116 +/- 34 nM (S.E., high [Ca2+]i cells, n = 154) and another one with low [Ca2+]i of 46 +/- 14 nM [Ca2+]i (S.E., low [Ca2+]i cells, n = 171), as revealed by fura-2 digital imaging fluorescence microscopy. The proportion of the high [Ca2+]i cells varied as a function of the cell density from 10-60% of all cells. Histochemical staining of the cells showed that cells with high and low [Ca2+]i did not represent differences between muscle and non-muscle cells. When the cells were exposed to different concentrations of ouabain, the high [Ca2+]i cells showed a half maximal effect at 2.10(-9) M ouabain, but only a small increase in [Ca2+]i of 30%. The low [Ca2+]i cells reached their half maximal increase in [Ca2+]i at 4.10(-8) M ouabain. A second increase in [Ca2+]i in this cell type was observed between 10(-6) and 10(-5) M ouabain. Toxic concentrations of ouabain produced an excessive increase in [Ca2+]i in low [Ca2+]i cells, whereas high [Ca2+]i cells showed morphological degeneration due to their higher sensitivity to ouabain. In conclusion, we demonstrate that chick embryo heart contains cells with high and low [Ca2+]i which show differences in the sensitivity of their sodium pumps to cardiac glycosides.