Measurement of intracellular free calcium in monkey kidney cells with aequorin

A method has been developed for the measurement of intracellular free calcium in mammalian cells. The calcium-sensitive photoprotein aequorin can be incorporated into isolated cells by hypo-osmotic treatment without altering the cell viability, permeability, or metabolism. Intracellular calcium activity (Cai2+) was monitored in a perfusion system. In monkey kidney cells (LLC-MK2), Cai2+ is approximately 57 nanomoles per liter. Changes in Cai2+ with time can also be followed: exposure of the cells to anaerobiosis or the calcium ionophore A23187 reversibly increases Cai2+. The method has also been successfully tested in rat hepatocytes.