Calcium transient was triggered by Na+ influx in phase 0 and regulated by Ca2+ influx in phase 2 of the action potential in adult rat left ventricular cardiomyocyte

Abstract

BackgroundCa2+-induced Ca2+ release (CICR) has been the prevailing model in cardiomyocytes for over 50 years. However, whether Na+ influx plays a direct role in triggering Ca2+ release has remained unclear. We previously discovered that action potential (AP) phase 0 triggered Ca2+ release while phase 2 regulated the decay phase during the Ca2+ transient (CT). We hypothesized that CICR might only exist under experimental conditions, not during physiological AP cycles. MethodsSimultaneous recordings of ion currents and CT from the same rat left ventricular cardiomyocyte were employed to investigate the temporal relationship between cation influxes and CT. ResultsThe reactivation time for CT was approximately 250 ms. AP phase 0 triggered the CT, while phase 2 only regulated CT width. Pulses from -40 to +80 mV with 6 ms duration were applied to validate whether the reverse Na+/Ca2+ exchanger (NCX) could trigger CT. Pulses of -40 and -30 mV with 6 ms duration induced giant Na+ influx and tail current, but only Na+ influx triggered CT, not tail current. Pulses from -20 mV to +80 mV with 6 ms duration triggered CT via tail current, not via reduced Na+ influx. In Ca2+ free solution, Na+ influx was still able to trigger CT until sarcoplasmic reticulum (SR) Ca2+ content was remarkably reduced. Moreover, increased reverse NCX current induced by ramp-like depolarizing pulses did not trigger CT but widened its shape. Additionally, when SR function was destroyed by caffeine (20 mM/L), field stimulations caused progressively elevated cytosolic Ca2+ levels, indicating that SR has a buffering function for maintaining cytosolic Ca2 concentration. ConclusionsOnce Na+ influx triggers CT, Ca2+ influx cannot trigger a second CT during an AP cycle due to the limitation imposed by the reactivation time. Na+ influx might enter SR via an unknown channel or transporter at the terminal cisternae during the AP and induce Ca2+ release by acting on the SR luminal-facing site of the ryanodine receptor 2 (RyR2) channel and/or by increasing the positive potential gradient across the SR membrane.