T-type Ca2+ current contribution to Ca2+-induced Ca2+ release in developing myocardium

Abstract In normal adult-ventricular myocardium, Ca 2+ -induced Ca 2+ release (CICR) from the sarcoplasmic reticulum (SR) is activated via Ca 2+ entry through L-type Ca 2+ channels. However, embryonic-ventricular myocytes have a prominent T-type Ca 2+ current ( I Ca,T ). In this study, the contribution of I Ca,T to CICR was determined in chick-ventricular development. Electricallystimulated Ca 2+ transients were examined in myocytes loaded with fura-2 and Ca 2+ currents with perforated patch-clamp. The results show that the magnitudes of the Ca 2+ transient, L-type Ca 2+ current ( I Ca,L ) and I Ca,T , decline with development with the majority of the decline of transients and I Ca,L occurring between embryonic day (ED) 5 and 11. Compared to controls, the magnitude of the Ca 2+ transient in the presence of nifedipine was reduced by 41% at ED5, 77% at ED11, and 78% at ED15. These results demonstrated that the overall contribution of I Ca,T to the transient was greatest at ED5, while I Ca,L was predominate at ED11 and 15. This indicated a decline in the contribution of I Ca,T to the Ca 2+ transient with development. Nifedipine plus caffeine was added to deplete the SR of Ca 2+ and eliminate the occurrence of CICR due to I Ca,T . Under these conditions, the transients were further reduced at all three developmental ages, which indicated that a portion of the Ca 2+ transients present after just nifedipine addition was due to CICR stimulated by I Ca,T . These results indicate that Ca 2+ entry via T-type channels plays a significant role in excitation-contraction coupling in the developing heart that includes stimulation of CICR.