Functional Properties of SR Cl- and K+ Channels during Postnatal Development of Cardiac Muscle

Abstract

In adult (AD) heart, the sarcoplasmic reticulum (SR) contains Cl- & K+ channels presumably involved in controlling RyR-mediated SR Ca2+ release. These channels provide a countercurrent mechanism that attenuates the drop in Ca2+ driving force across the SR membrane, thereby preventing early termination of Ca2+ release. We showed that in newborn (NB), Ca2+ sparks occur with similar frequency than in AD but have shorter duration and smaller amplitude, implying an early termination of Ca2+ release. Although the functional properties of SR Cl- & K+ channels have been thoroughly described in AD, little is known about their presence and their role in NB. Consequently, we first tested the hypothesis that the early termination of Ca2+ release in NB coincides with absence/low density of SR Cl- & K+ channels at this stage. To this end, the heavy microsomal fraction was obtained from 5-days-old NB and AD rat hearts and SR Cl- & K+ channels were reconstituted into artificial planar lipid bilayers. Our results indicate that Cl- & K+ channels can be reconstituted from NB heavy SR microsomes with a similar success rate (number of SR channel incorporations / total number of bilayers) than in AD (∼0.2 for Cl- channels & ∼0.1 for K+ channels). Thus, an alternative mechanism would imply that in NB, smaller counterion fluxes result from different functional properties of SR Cl- & K+ channels. This assumption was tested by measuring their unitary conductance, open probability, and voltage dependence. The results in NB channels revealed no significant differences in any of these parameters in comparison to AD. Thus, we concluded that SR Cl- & K+ channels do not contribute to the developmental changes of Ca2+ release in NB cardiomyocytes. Supported by AHA-0655656Z to RMA.