Calmodulin kinase is functionally targeted to the action potential plateau for regulation of L‐type Ca 2+ current in rabbit cardiomyocytes

L‐type Ca 2+ current ( I Ca−L ) triggers Ca 2+ release from the sarcoplasmic reticulum (SR) and both SR and I Ca−L are potential sources of intracellular Ca 2+ (Ca 2+ i ) for feedback regulation of I Ca−L . Ca 2+ i bound to calmodulin (Ca 2+ –CaM) can inhibit I Ca−L , while Ca 2+ –CaM can also activate Ca 2+ –CaM‐dependent protein kinase II (CaMK) to increase I Ca . However, it is not known whether I Ca−L or the SR is the primary source of Ca 2+ for I Ca−L regulation. The L‐type Ca 2+ channel C terminus is implicated as a critical transduction element for I Ca−L responses to Ca 2+ –CaM and CaMK, and the C terminus undergoes voltage‐dependent steric changes, suggesting that Ca 2+ i control of I Ca−L may also be regulated by cell membrane potential. We developed conditions to separately test the relationship of Ca 2+ –CaM and CaMK to I Ca−L and SR Ca 2+ i release during voltage clamp conditions modelled upon time and voltage domains relevant to the cardiac action potential. Here we show that CaMK increases I Ca−L after brief positive conditioning pulses, whereas Ca 2+ –CaM reduces I Ca−L over a broad range of positive and negative conditioning potentials. SR Ca 2+ release was required for both Ca 2+ –CaM and CaMK I Ca−L responses after strongly positive conditioning pulses (+10 and +40 mV), while Ca 2+ i from I Ca−L was sufficient for Ca 2+ –CaM during weaker depolarizations. These findings show that I Ca−L responses to CaMK are voltage dependent and suggest a new model of L‐type Ca 2+ channel regulation where voltage‐dependent changes control I Ca−L responses to Ca 2+ –CaM and CaMK signalling.