Exogenous Ca2+-ATPase isoform effects on Ca2+ transients of embryonic chicken and neonatal rat cardiac myocytes

1. Sarco-endoplasmic reticulum Ca2+-ATPase from fast skeletal (SERCA1) or c ardiac muscle (SERCA2a) was expressed in embryonic chicken and neonatal rat cardiac myocytes by adenovirus vectors, with c-1. myc tags on both constru cts to compare expression and distinguish exogenous from endogenous SERCA2a in myocytes. 2. Expression of the two isoforms was similar (approximately 3-fold higher than endogenous SERCA). However, SERCA1 activity wax 2-fold greater than SE RCA2a activity, due to intrinsic differences in turnover rates. Activation of both exogenous SERCA isoforms by Ca2+ was displaced to slightly lon er [ Ca2+], suggesting that the overexpressed isoforms were independent of phosp holamban. In fact, phospholamban and calsequestrin expression were unchange d. 3. Decay time constants of cytosolic Ca2+ transients from cells overexpress ing SERCA1 were reduced by 30-40% and half-widths by 10-15% compared to con trols. SERCA2a overexpression produced much less acceleration of transients in chick than in rat, and less acceleration than SERCA1 overexpression in either species. There was no significant change in resting [Ca2+], peak amp litudes, or in the amount of Ca2+ releasable by: caffeine from overexpressi on of either SERCA isoform. However, the amplitudes of the transients incre ased with SERCA1 overexpression when pacing frequency limited refilling of the sarcoplasmic reticulum. 4. It is concluded that total SERCA transport velocity has a primary effect on the decay phase of transients. Transport velocity is affected by SERCA isoform turnover rate, temperature, and/or SERCA copy number.