L‐type Ca²⁺ channels play an important role in vital cell functions such as muscle contraction and hormone secretion. Both a voltage‐dependent and a Ca²⁺‐dependent process inactivate these channels. Here we present evidence that inhibition of the mitochondrial Ca²⁺ import mechanism in rat (Sprague‐Dawley) ventricular myocytes by ruthenium red (RR), by Ru360 or by carbonyl cyanide m‐chlorophenylhydrazone (CCCP) decreases the magnitude of electrically evoked transient elevations of cytosolic Ca²⁺ concentration ([Ca²⁺]_c). These agents were most effective at stimulus rates greater than 1 Hz.

RR and CCCP also caused a significant delay in the recovery from inactivation of L‐type Ca²⁺ currents (I_Ca). This suggests that sequestration of cytosolic Ca²⁺, probably near the mouth of L‐type Ca²⁺ channels, into mitochondria during cardiac contractile cycles, helps to remove the Ca²⁺‐dependent inactivation of L‐type Ca²⁺ channels.

We conclude that impairment of mitochondrial Ca²⁺ transport has no impact on either L‐type Ca²⁺ currents or SR Ca²⁺ release at low stimulation frequencies (e.g. 0.1 Hz); however, it causes a depression of cytosolic Ca²⁺ transients attributable to an impaired recovery of L‐type Ca²⁺ currents from inactivation at high stimulation frequencies (e.g. 3 Hz). The impairment of mitochondrial Ca²⁺ uptake and subsequent effects on Ca²⁺ transients at high frequencies at room temperature could be physiologically relevant since the normal heart rate of rat is around 5 Hz at body temperature. The role of mitochondria in clearing Ca²⁺ in the micro‐domain near L‐type Ca²⁺ channels could be impaired during high frequencies of heart beats such as in ventricular tachycardia, explaining, at least in part, the reduction of muscle contractility.