Specific cytoplasmic binding of 17beta-[3H]estradiol ([3H]E2beta) by unoccupied receptors in bovine uterine endometrium was determined by saturation analysis and correlated with endometrium levels of E2beta in 21 cows. The concentrations of these estrogen receptors (ER) during the estrous cycle were significantly greater during proestrus, estrus, and postestrus (days 18-20 and 0-4 of the cycle) than during the midluteal period (days 10-12; P less than 0.05). These increases in ER concentration paralleled increases in endometrial and plasma E2beta concentrations. In a preliminary experiment involving six heifers killed during the estrous cycle, a comparison of the ER concentrations of the horns ipsilateral and contralateral to the corpus luteum showed no significant differences, as did a study of the dissociation constant of the steroid-receptor interaction during the estrous cycle and early pregnancy. The order of inhibition of cytoplasmic binding of [3H]E2beta by estrogens was as follows: E2beta greater than E1 greater than E2alpha greater than E3 at 4 C. The concentration of ER in six pregnant animals was higher on days 2-3 than on days 13-14 after insemination, which was similar to that found in cycling animals; however, the difference between days was not significant. Ovariectomy of two heifers resulted in slightly higher ER concentrations than in intact heifers, whereas immunization of two heifers with an E2beta conjugate resulted in levels 2-fold higher. In these altered animals plasma progesterone (P4) was nearly nondetectable. ER concentration was inversely related to the level of plasma P4 in cycling heifers (r = -0.58, P less than 0.01). Endometrial estrogen and plasma estrogen levels were also inversely correlated with plasma P4 levels (r = -0.65, P less than 0.01 and r = -0.48, P less than 0.05, respectively). Thus, if P4 level influences ER levels, the hormone may make the tissue less responsive to ovarian estrogens at such time as the luteal phase of the cycle and during pregnancy.

The effect of methionine on the uptake, distribution, and binding of the convulsant methionine sulfoximine (MSO) in 7 rat brain regions, the spinal cord, the liver, and the kidney was investigated. The administration of methionine decreased the uptake of MSO in all brain regions. The uptake of MSO by and its distribution in the nervous tissue was uniform and failed to result in any preferential accumulation of the drug. Methionine decreased the amount of MSO bound to cerebral structures and to the spinal cord. MSO bound to the spinal cord was less susceptible to release by Triton X-100 than was brain-bound MSO.