[Calcium, modulator of the expression of gonadoliberin at the intracellular level]

GnRH has been entrapped in liposomes. Chromatographic studies and enzymatic peptidase treatments, show the efficiency of the encapsulation. A purification method on G75 Sephadex of the entrapped GnRH is described. This method prevents any dilution of the liposome fraction. A free GnRH contamination, lower than 0.4 per cent, has been observed. Superfused hypophyses respond to the message of the internalized GnRH only when calcium is present in the extracellular medium. The intensity of the answer depends on the duration of the entrapped GnRH infusion. The decrease observed in the response intensity after a long stay of the GnRH in the cytoplasm allows us to say that GnRH controls its own expression: The binding of GnRH to the membrane receptor during the early phase induces a calcium uptake necessary to the expression of the internalized GnRH, this being the late phase in LH release. A too low calcium concentration does not allow GnRH expression. As a consequence, GnRH is enzymatically degradated by the cytoplasmic peptidases. The LH release during the late phase is the result of a combined action of calcium and cytoplasmic peptidases. To support this idea we show: 1- that an extracellular calcium concentration around 0.5 or 0.6 mM is the best condition for the expression of the internalized GnRH. 2- that a GnRH agonist (D-Ala6-GnRH) known to be peptidase resistent induces a higher LH release in our experimental conditions.