Reduction of the Ca2+ transport systems in mouse young brown fat cells

Principal differences in the kinetics and amplitude of Ca2+ response to norepinephrine were found between freshly isolated young differentiated brown fat cells. An increase in the Ca2+ concentration in the cytoplasm ([Ca2+]i) in the young cells was unusually slow (A[Ca2+]i = 0.03 nM/s) in comparison with that in the differentiated cells, and the Ca2+ influx from the outside was not induced by Ca2+ mobilization agents, such as thapsigargin and ionomycin. Ionomycin increased [Ca2+]i up to 150 nM in a Ca2+-free medium and up to 270 nM in the normal medium. This results in that the intracellular Ca2+ stores in freshly isolated young cells are rather poor, and the mechanism of capacitive calcium entry does not virtually function. The data on chemical modification of Ca2+ channels in the plasma membrane by thimerosal suggest that the conductance of these channels is low and/or their number in young brown fat cells is insignificant.