Properties of β-adrenergic receptors on porcine corpora lutea and granulosa cells

The nature of β-adrenergic binding by swine corpora lutea and granulosa cells was examined with the specific β-adrenergic radioligand, (±)3-[¹²⁵I]iodocyanopindolol (ICYP). Saturation analyses revealed the presence of high affinity (K_d = 15.2 ± 2.1 pM; n = 8 experiments) and low capacity (6.7 ± 0.8 fmol/mg protein) β-adrenergic receptors on porcine corpora lutea membranes. The properties of β-adrenergic binding were determined by computer modeling of competition studies with a variety of compounds selective for β-adrenergic subtypes. These studies disclosed predominantly β₁-adrenergic receptors on pig luteal membranes. This inference from radioligand binding studies was corroborated functionally by the approximately equipotent biological effects of L-norepinephrine and L-epinephrine on cAMP production by luteal tissue (respective EC₅₀s of 282 ± 31 and 187 ± 66 nM; n = 3 experiments). Physiological regulation of specific β-adrenergic receptor content in the swine ovary was indicated by prominent (up to 9-fold) variations in receptor concentrations among corpora lutea and granulosa cells at various stages of maturity. In addition, there was differential expression of β-adrenergic receptor subtype. Whereas the β-adrenergic receptor subtype was predominantly β₁ in corpora hemorrhagica and corpora lutea, granulosa cells and corpora albicantia contained principally β₂ receptors. This difference could not be accounted for by blood cell contamination of corpora lutea, since swine blood cells contained predominantly (>98%) β₂-receptors, which were present at less, than 8.6% the concentration of total β-receptors in luteal tissue. In summary, swine corpora lutea and granulosa cells contain specific high affinity, low capacity β-adrenergic receptors that are functionally coupled to biological responses. Moreover, total receptor content as well as β-adrenergic subtype exhibit significant physiological variation in relation to maturational status of ovarian follicular and luteal tissue.