Two types of external Cl(-)-dependent Cl(-) channels and one type of stretch receptor cation channel contribute to the formation of isotonic blastocoel fluid in early medaka fish embryo.

Ionic channels in blastoderm cells dissociated from medaka fish embryos at the late blastula stage were studied by the patch-clamp technique in whole-cell, inside-out and cell-attached patch configurations. These cells were mechanically dissociated without using proteo-lytic enzymes. I have reported previously an external anion-dependent type I Cl(-) channel, which was observed at the early blastula stage, and its voltage dependency shifted toward the positive direction with a reduction in [Cl(-)](o). The type I Cl(-) channel was observed in about half of the blastoderm cells at the late blastula stage. I also observed another external anion-dependent Cl(-) channel (type II) whose voltage dependency shifted toward the negative direction with a reduction in [Cl(-)](o) and a cation selective stretchactivated channel. The$coexistence of the type II Cl(-) channel and stretch-activated cation channel would produce the efflux of both cations and anions in response to low ionic strength fluid. Further, the initial rise of blastocoel [Cl(-)] by this mechanism would trigger positive feedback between the type I Cl(-) channel conductance and blastocoel [Cl(-)]. At the early blastula stage, the blastocoel cavity fluid has a low ionic strength similar to that of pond water. I proposed that the cooperation of three types of ion channels at the late blastula stage generates isotonic blastocoel cavity fluid.