Ca2+ signaling in cardiac myocytes overexpressing the α1 subunit of L-type Ca2+ channel

Voltage-gated L-type Ca2+ channels (LCCs) provide Ca2+ ingress into cardiac myocytes and play a key role in intracellular Ca2+ homeostasis and excitation-contraction coupling. We investigated the effects of a constitutive increase of LCC density on Ca2+ signaling in ventricular myocytes from 4-month-old transgenic (Tg) mice overexpressing the a, subunit of LCC in the heart. At this age, cells were somewhat hypertrophic as reflected by a 20% increase in cell capacitance relative to those from nontransgenic (Ntg) littermates. Whole cell I-Ca density in Tg myocytes was elevated by 48% at 0 mV compared with the Ntg group. Single-channel analysis detected an increase in LCC density with similar conductance and gating properties. Although the overexpressed LCCs triggered an augmented SR Ca2+ release, the "gain" function of EC coupling was uncompromised, and SR Ca2+ content, diastolic cytosolic Ca2+, and unitary properties of Ca2+ sparks were unchanged. Importantly, the enhanced I-Ca entry and SR Ca2+ release were associated with an upregulation of the Na+-Ca2+ exchange activity (indexed by the half decay time of caffeine-elicited Ca2+ transient) by 27% and SR Ca2+ recycling by approximate to35%. Western analysis detected a 53% increase in the Na+-Ca2+ exchanger expression but no change in the abundance of ryanodine receptor (RyR), SERCA2, and phospholamban. Analysis of I-Ca kinetics suggested that SR Ca2+ release-dependent inactivation of LCCs remains intact in Tg cells. Thus, in spite of the modest cardiac hypertrophy, the overexpressed LCCs form functional coupling with RyRs, preserving both orthograde and retrograde Ca2+ signaling between LCCs and RyRs. These results also suggest that a modest but sustained increase in Ca2+ influx triggers a coordinated remodeling of Ca2+ handling to maintain Ca2+ homeostasis.