Dissociation of human choriogonadotropin from the pseudopregnant rat ovary as a complex to a receptor fragment during perifusion and incubation.

Pseudopregnant rats were injected intravenously with radioactively-labelled human choriogonadotropin (CG). The animals were killed 2 h after the injection and the ovaries, liver and kidney were subjected to perifusion. Radioactivity was released from the ovaries at an increasing rate during perifusion, mainly in a complex form with a molecular size between human CG and the solubilized receptor-human CG complex. The increase in the rate of radioactivity release was inhibited by N-ethylmaleimide and CuCl2, but not by MgCl2, Trasylol, N alpha-tosyl-L phenylalanine chloromethyl ketone or N alpha-p-tosyl-L-lysine and CuCl2 chloromethyl ketone. Intact hormone dissociation from the complex at pH 3. After perifusion the ovarian tissue radioactivity only as receptor-hormone complexes. Only free radioiodine released from the control tissues, liver and kidney during perifusion. The low molecular weight hormone complex was also released from a homogenate of pseudopregnant rat ovaries prelabelled in vivo with radioactivity-labelled human CG during incubation in a hypotonic medium. The release of this complex was likewise inhibited by alkylating agents and heavy metals, and intact hormone dissociated from the complex at pH 3. A similar human CG complex was released also from purified receptor-human CG complex during incubation with ovarian homogenate, and presence of N-ethylmaleimide or use of heat inactivated ovarian homogenate inhibited this process. The present results indicate that the in vivo bound human CG sheds from the luteal tissue in perifusion and incubation as a low molecular weight complex. This may be a facet in the processing and elimination of occupied LH receptors from the ovary.