Calmodulin kinase and a calmodulin-binding 'IQ' domain facilitate L-type Ca²⁺ current in rabbit ventricular myocytes by a common mechanism

1. Ca²⁺-calmodulin-dependent protein kinase II (CaMK) and a calmodulin (CaM)-binding 'IQ' domain (IQ) are both implicated in Ca²⁺-dependent regulation of L-type Ca²⁺ current (I_Ca). We used an IQ-mimetic peptide (IQmp), under conditions in which CaMK activity was controlled, to test the relationship between these CaM-activated signalling elements in the regulation of L-type Ca²⁺ channels (LTCCs) and I_Ca in rabbit ventricular myocytes. 2. A specific CaMK inhibitory peptide nearly abolished I_Ca facilitation, but the facilitation was 'rescued' by cell dialysis with IQmp. 3. IQmp significantly enhanced I_Ca facilitation and slowed the fast component of I_Ca inactivation, compared with an inactive control peptide. Neither effect could be elicited by a more avid CaM-binding peptide, suggesting that generalized CaM buffering did not account for the effects of IQmp. 4. I_Ca facilitation was abolished and the fast component of inactivation eliminated by ryanodine, caffeine or thapsigargin, suggesting that the sarcoplasmic reticulum (SR) is an important source of Ca²⁺ for I_Ca facilitation and inactivation. IQmp did not restore I_Ca facilitation under these conditions. 5. Engineered Ca²⁺-independent CaMK and IQmp each markedly increased LTCC open probability (P_o) in excised cell membrane patches. The LTCC P_o increases with CaMK and IQmp were non-additive, suggesting that CaMK and IQmp are components of a shared signalling pathway. 6. Both CaMK and IQmp induced a modal gating shift in LTCCs that favoured prolonged openings, indicating that CaMK and IQmp affect LTCCs through a common biophysical mechanism. 7. These findings support the hypothesis that CaMK is required for physiological I_Ca facilitation in cardiac myocytes. Both CaMK and IQmp were able to induce a modal gating shift in LTCCs, suggesting that each of these signalling elements is important for Ca²⁺-CaM-dependent LTCC facilitation in cardiac myocytes.