Estradiol inhibition of pituitary luteinizing hormone release is antagonized by serum proteins

We tested the influence of protein binding upon the rapid 17 beta-estradiol (E2) inhibitory effect on luteinizing hormone (LH) responsiveness to LH releasing hormone (LHRH) in perifused rat anterior pituitaries. LHRH pulses were given before 1 h after changing the perifusion medium to contain various combinations of protein and E2. In the absence of albumin an E2 concentration of about 27 pg/ml inhibited LH secretion in response to LHRH by 50% (judged from the change in secretion ratio, i.e LH response to LHRH pulse no. 2 divided by that to pulse no. 1). Albumin (1 g/dl) and human plasma enhanced the self-priming effect of LHRH (P less than 0.1) and blunted the slope of the dose-response relationship between E2 and LH secretion. These effects suggest a stimulatory effect of proteins on LH secretion which is independent of E2. However, proteins appear to antagonize the E2 effect in part by binding E2. A total E2 dose of 270 pg/ml or more was required to achieve about 50% inhibition of the LH secretion ratio in the presence of albumin. Since 20% of E2 was unbound in the presence of albumin, the 50% effective dose of free E2 was about 54 pg/ml. This closely approximates the comparably effective dose of E2 in the absence of protein. Although a total E2 concentration of 216 pg/ml was required for a 50%-inhibitory E2 effect in the presence of heat-inactivated plasma, this was equivalent to between 28 and 37 pg/ml of free E2 in these experiments. The effect of E2 in a total dose of 216 pg/ml was attenuated more by the use of TEBG-positive plasma than of TEBG-negative (heat-inactivated) plasma (P less than 0.01). Our data indicate that the E2 inhibitory effect upon LH responsiveness to LHRH is attenuated by albumin and TEBG to approximately the extent expected by binding of E2 to these proteins. Our data are certainly contrary to those expected if either albumin or TEBG augmented this E2 action. Our data support the concept that the bioavailable fraction of plasma sex steroids is that which is free from binding to plasma proteins.