Abstract
The influence of membrane potential (Δψ) on Ca 2+ transport through the Ca 2+ uniporter (UP) was investigated in fura-2-loaded rat heart mitochondria at physiologically relevant-submicromolar-external [Ca 2+]. In the absence of Δψ the UP could not mediate Ca 2+ uptake even when an 8-fold external (∼500 nM) to internal (∼60 nM) [Ca 2+] gradient was present and charge compensation was provided by acetate and the protonophore, CCCP. A small (∼−120 mV) and transient Δψ (generated by valinomycin) resulted in a rise in matrix [Ca 2+] only when external [Ca 2+] exceeded 150 nM. At physiologically high (∼−180 mV) and stable Δψ this threshold value for Ca 2+ uptake dropped to 15 nM. The results indicate that (1) at physiological [Ca 2+] m Δψ in addition to being a component of Δμ Ca 2+ seems to be necessary for providing a transport-competent conformation for the UP; and (2) below a threshold [Ca 2+] m the UP cannot operate even in the presence of a high electric driving force.
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