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 (Kd = 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.
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