Calmodulin kinase II accelerates L-type Ca2+ current recovery from inactivation and compensates for the direct inhibitory effect of Ca2+i in rat ventricular myocytes

摘要

Some studies report that the positive relationship between L-type Ca²⁺ current (ICa−L) and frequency in cardiac myocytes is mainly due to a direct negative feedback of sarcoplasmic reticulum Ca²⁺ release on ICa−L inactivation while others provide evidence for activation of calmodulin kinase II (CaMKII). To further elucidate the role of endogenous CaMKII activity, the CaMKII inhibitory peptides, autocamtide-2 relating inhibitory peptide (AIP) and myristoylated AIP were applied using conventional and perforated patch-clamp methods. AIP inhibited the normal adaptive increase in ICa−L in response to abrupt increase in pacing frequency from 0.05 to 2 Hz. The positive ICa−L–frequency relationship was reversed by AIP and the inhibitory effect of AIP was significantly exaggerated at fast pacing rates. The onset of inactivation of ICa−L was not altered by AIP. After thapsigargin treatment, AIP slowed recovery from inactivation of ICa−L and this effect was exaggerated during fast pacing. Buffering of [Ca²⁺]i by BAPTA and EGTA accelerated recovery of ICa−L from inactivation, and BAPTA partly eliminated the effect of AIP on the recovery. We conclude that: (1) [Ca²⁺]i directly slows ICa−L recovery from inactivation; and (2) Ca²⁺-dependent endogenous CaMKII activity accelerates the ICa−L recovery. Thus, at fast heart rates, elevated [Ca²⁺]i activates endogenous CaMKII and compensates for its direct inhibitory effect on ICa−L recovery from inactivation. Dynamic activity of endogenous CaMKII enhances the positive ICa−L–frequency relationship.