Objective: This study investigated the role of calsequestrin (CSQ) in the control of excitation–contraction (E–C) coupling in the heart.

Methods: CSQ over-expression was induced in isolated rabbit ventricular cardiomyocytes using an adenovirus coding for rabbit CSQ (Ad-CSQ). After 24 h of culture, CSQ protein expression was increased by 58 ± 18% (n = 10). An adenovirus coding for β-galactosidase (Ad-LacZ) was used as a control.

Results: In voltage-clamped, Fura-2-loaded cardiomyocytes, L-type Ca²⁺ current (I_Ca,L) and Ca²⁺ transient amplitude were both increased in the Ad-CSQ group by ∼78%. Doubling the external Ca²⁺ concentration in the control group (Ad-LacZ) increased the LTCC amplitude to a similar degree (85 ± 6%), but increased the Ca²⁺ transient amplitude by 149 ± 13%. This suggests that SR Ca²⁺ release may be inhibited upon CSQ over-expression. Alternatively, nifedipine (0.5 μM) was used to reduce I_Ca,L in Ad-CSQ-transfected cells to values comparable to control (Ad-LacZ). Under these conditions, Ca²⁺ transient amplitude was not different from Ad-LacZ, but the SR Ca²⁺ content was ∼60% higher as assessed by both the caffeine-induced Ca²⁺ release and the accompanying Na⁺/Ca²⁺ exchanger current (I_NCX). The cause of the increased I_Ca,L is unknown. No change in the expression level of the α1-subunit of the L-type Ca channel was observed. β-Escin-permeabilized cardiomyocytes were used to study Ca²⁺ sparks imaged with Fluo-3 at 145–155 nmol/L [Ca²⁺]. Spontaneous Ca²⁺ spark frequency, duration, width, and amplitude were unchanged in the Ad-CSQ group, but SR Ca²⁺ content was 48% higher than Ad-LacZ.

Conclusions: CSQ over-expression increased SR Ca²⁺ content but reduced the gain of E–C coupling in rabbit cardiomyocytes.