Regulation of a voltage-sensitive release mechanism by Ca²⁺-calmodulin-dependent kinase in cardiac myocytes

A role for Ca²⁺-calmodulin-dependent kinase (CamK) in regulation of the voltage-sensitive release mechanism (VSRM) was investigated in guinea pig ventricular myocytes. Voltage clamp was used to separate the VSRM from Ca²⁺-induced Ca²⁺ release (CICR). VSRM contractions and Ca²⁺ transients were absent in cells dialyzed with standard pipette solution but present when 2-5 μM calmodulin was included. Effects of calmodulin were blocked by KN-62 (CamK inhibitor), but not H-89, a protein kinase A (PKA) inhibitor. Ca²⁺ current and caffeine contractures were not affected by calmodulin. Transient-voltage relations were bell-shaped without calmodulin, but they were sigmoidal and typical of the VSRM with calmodulin. Contractions with calmodulin exhibited inactivation typical of the VSRM. These contractions were inhibited by rapid application of 200 μM of tetracaine, but not 100 μM of Cd²⁺, whereas CICR was inhibited by Cd²⁺ but not tetracaine. In undialyzed myocytes (high-resistance microelectrodes), KN-62 or H-89 each reduced amplitudes of VSRM contractions by ~50%, but together they decreased VSRM contractions by 93%. Thus VSRM is facilitated by CamK or PKA, and both pathways regulate the VSRM in undialyzed cells.