ABSTRACT
Slices of non-luteinized porcine ovaries have been incubated in the presence or absence of human chorionic gonadotrophin (HCG) and exogenous radioactive substrates. Progesterone, 17α-hydroxyprogesterone and androstenedione were isolated in a radiochemically pure form. The chemical mass and the specific activity were determined by gas liquid chromatography and liquid scintillation spectrometry.
HCG stimulated the rate of formation of androstenedione in the absence of exogenous substrates with a factor of 4–8. In the presence of pregnenolone or progesterone at a concentration of about 2 × 10−6 mol/l the stimulatory effect of HCG was either abolished or markedly reduced. The conversion of exogenous progesterone to androstenedione was reduced in response to HCG indicating that the capacity of the tissue to convert progesterone to androstenedione was limited, and that the limit was reached at this rather low substrate concentration. These findings furthermore suggest that the endogenous rather than the exogenous radioactive substrate will be »preferred« by the tissue. The observations demonstrate the necessity of measuring both the radioactivity and the chemical mass of the products in investigations of this type using radioactive substrates.
The formation of progesterone from endogenous substrates was also stimulated by HCG. [1-14C] acetate and [7α-3H]cholesterol were not utilized by the tissue for steroid formation. Exogenous [4-14C] pregnenolone and [7α-3H] progesterone in similar concentration were both utilized for production of 17α-hydroxyprogesterone and androstenedione. HCG had no effect on the relative utilization of the radioactive substrates.
Body Composition of Dairy Cattle by Potassium-40 Liquid Scintillation Detection