| In normal adult ventricular myocardium, Ca2+-induced Ca 2+ release (CICR) from the sarcoplasmic reticulum (SR) is activated via Ca2+ entry though L-type Ca2+ channels. However, embryonic ventricular myocytes have a prominent T-type Ca 2+ current (ICa,T). In this study, the contribution of ICa,T in CICR was determined in chick ventricular development. Electrically stimulated Ca2+ transients were examined in myocytes loaded with fura-2. The results show that the magnitude of the Ca2+ transient declines with development with the majority of the decline occurring between embryonic day (ED) 5 and ED11 (972 ± 19nM vs. 675 ± 30nM). By ED15 the calcium transient had declined to 630 ± 17nM. The contribution of ICa,T to the transient was determined after blockade of L-type channels with nifedipine. The magnitude of the Ca 2+ transient was reduced to 575 ± 15nM at ED5, 153 ± 10nM at ED11, and 138 ± 10nM at ED15. The greatest effect of nifedipine was at ED11 and ED15, which indicated a decline in the contribution of I Ca,T to the Ca2+ transient with development. In the presence of nifedipine, caffeine was added to deplete the SR of Ca2+ and eliminate CICR due to ICa,T. Under these conditions, the transient was reduced to 392 ± 10nM at ED5 and reduced further to 120 ± 5nM and 30 ± 5 nM at ED11 and ED15. The α1H isoform of the T-type calcium channel is the only isoform expressed in developing chick myocardium. The relative amounts of mRNA for the α 1H isoform coincide with the decrease in current. These results indicate that CICR due to the presence of ICa,T occurs in ventricular myocardium throughout development even when ICa,T is expected to be small and the SR relatively mature. |
