Regulation of L-type calcium channel by phospholemman in cardiac myocytes

We evaluated whether phospholemman (PLM) regulates L-type Ca2 + current (ICa) in mouse ventricular myocytes. Expression of α1-subunit of L-type Ca2 + channels between wild-type (WT) and PLM knockout (KO) hearts was similar. Compared to WT myocytes, peak ICa (at − 10 mV) from KO myocytes was ~ 41% larger, the inactivation time constant (τinact) of ICa was ~ 39% longer, but deactivation time constant (τdeact) was similar. In the presence of isoproterenol (1 μM), peak ICa was ~ 48% larger and τinact was ~ 144% higher in KO myocytes. With Ba2 + as the permeant ion, PLM enhanced voltage-dependent inactivation but had no effect on τdeact. To dissect the molecular determinants by which PLM regulated ICa, we expressed PLM mutants by adenovirus-mediated gene transfer in cultured KO myocytes. After 24 h in culture, KO myocytes expressing green fluorescent protein (GFP) had significantly larger peak ICa and longer τinact than KO myocytes expressing WT PLM; thereby independently confirming the observations in freshly isolated myocytes. Compared to KO myocytes expressing GFP, KO myocytes expressing the cytoplasmic domain truncation mutant (TM43), the non-phosphorylatable S68A mutant, the phosphomimetic S68E mutant, and the signature PFXYD to alanine (ALL5) mutant all resulted in lower peak ICa. Expressing PLM mutants did not alter expression of α1-subunit of L-type Ca2 + channels in cultured KO myocytes. Our results suggested that both the extracellular PFXYD motif and the transmembrane domain of PLM but not the cytoplasmic tail were necessary for regulation of peak ICa amplitude. We conclude that PLM limits Ca2 + influx in cardiac myocytes by reducing maximal ICa and accelerating voltage-dependent inactivation.