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Development of mechanisms regulating intracellular Ca2+ concentration in cardiac muscle cells of early chick embryos.

The development of mechanisms for the regulation of intracellular-free calcium ion concentration ([Ca2+]i) was investigated in precardiac mesodermal cells (PMC) and cardiac muscle cells (CMC) from early chick embryos by microfluorometry using a Ca2+-sensitive fluorescent probe, fura-2, and transmission electron microscopy. Microfluorometry indicated that two types of regulatory mechanisms, involving the dihydropyridine receptor (DHPR) and the ryanodine receptor (RYR), are present in CMC when the heartbeat begins at the 8-9 somite stages. Nifedipine completely suppressed the beating of hearts isolated from embryos on Days 1.5 and 2. Ryanodine had no effect on the beating of hearts isolated from embryos on Day 1.5, though it completely suppressed beating in hearts from Embryonic Day 2. Microfluorometry revealed that a change occurred in the Ca2+-regulating mechanisms of CMC on Day 2. Transmission electron microscopy showed the appearance in CMC, also on Day 2, of peripheral couplings with feet structures, and SR adjacent to the Z-line of myofibrils. These findings suggest that the calcium-induced calcium-release (CICR) mechanism appears in the CMC of the chick on the second day of embryonic development.