Sarcoplasmic reticulum Ca2+ release in neonatal rat cardiac myocytes

In the neonatal mammalian heart, the role of ryanodine receptor (= Ca²⁺ release channel)-mediated sarcoplasmic reticulum (SR) Ca²⁺ release for excitation–contraction coupling is still a matter of debate. Using an adenoviral system, we overexpressed separately the junctional SR proteins triadin, junctin, and calsequestrin, which are probably involved in regulation of ryanodine receptor function. Infection of neonatal rat cardiac myocytes with triadin, junctin, or calsequestrin viruses, controlled by green fluorescent protein expression, resulted in an increased protein level of the corresponding transgenes. Measurement of Ca²⁺ transients of infected cardiac myocytes revealed unchanged peak amplitudes under basal conditions but with overexpression of calsequestrin and triadin caffeine-releasable SR Ca²⁺ content was increased. Our results demonstrate that an increased expression of triadin or calsequestrin is associated with an increased SR Ca²⁺ storage but unchanged Ca²⁺ signaling in neonatal rat cardiac myocytes. This is consistent with an ancillary role of the sarcoplasmic reticulum in excitation–contraction coupling in the developing mammalian heart.