The concentration of the common α-subunit of the glycoprotein hormones and of the β-subunit of luteinizing hormone (LHβ) in peripheral blood and follicular fluid was measured throughout the menstrual cycle, and the effects of these subunits, either alone or in combination, on the production of progesterone by human granulosa cells in tissue culture were investigated.
Changes in the serum concentration of α-subunit and immunoreactive `LHβ-like' material throughout the menstrual cycle were similar to those of LH. The concentrations of the subunits before the mid-cycle gonadotrophin peak were not significantly different from those during the luteal phase of the menstrual cycle. Gel filtration of a pooled serum sample obtained at mid-cycle confirmed the presence of immunoreactive α-subunit together with intact LH; however, because of the cross-reactivity of LH in the LHβ assay a distinct peak of LHβ-subunit could not be demonstrated.
In follicular fluid, α-subunit was detectable in large follicles ( ≥ 8 mm) throughout the menstrual cycle at concentrations similar to those found in serum. By contrast α-subunit in small follicles ( < 8 mm) and `LHβ-like' material in all follicles were only detectable during or just after peak concentrations in peripheral plasma.
The LH subunits did not increase the rate of progesterone secretion by human granulosa cells when each was added alone, even at concentrations five times higher than those in plasma. However, when both subunits were added simultaneously there was an increased rate of progesterone secretion comparable to that achieved with intact LH.
It is concluded that the common α-subunit circulates in blood independently of the intact hormones, and that it is present in a proportion of developing Graafian follicles without affecting either the viability or biosynthetic potential of their granulosa cells. During the late follicular phase however, when both the α- and LHβ-like subunits are present in follicular fluid, they may recombine and enhance steroid production by granulosa cells which are undergoing luteinization at this time.