Glucose-induced Oscillations in Cytoplasmic Free Ca²⁺ Concentration Precede Oscillations in Mitochondrial Membrane Potential in the Pancreatic β-Cell

Using dual excitation and fixed emission fluorescence microscopy, we were able to measure changes in cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_i) and mitochondrial membrane potential simultaneously in the pancreatic β-cell. The β-cells were exposed to a combination of the Ca²⁺ indicator fura-2/AM and the indicator of mitochondrial membrane potential, rhodamine 123 (Rh123). Using simultaneous measurements of mitochondrial membrane potential and [Ca²⁺] _i during glucose stimulation, it was possible to measure the time lag between the onset of mitochondrial hyperpolarization and changes in [Ca ²⁺]_i. Glucose-induced oscillations in [Ca ²⁺]_i were followed by transient depolarizations of mitochondrial membrane potential. These results are compatible with a model in which nadirs in [Ca²⁺]_i oscillations are generated by a transient, Ca²⁺-induced inhibition of mitochondrial metabolism resulting in a temporary fall in the cytoplasmic ATP/ADP ratio, opening of plasma membrane KATP channels, repolarization of the plasma membrane, and thus transient closure of voltage-gated L-type Ca²⁺ channels.