Variation in size of the 'readily releasable pool' of luteinizing hormone during the oestrous cycle of the rat.

The size of the 'readily releasable pool' of luteinizing hormone at various times of the oestrous cycle has been determined by injecting a supramaximal dose of luteinizing hormone releasing factor (LH-RF) i.v. into rats anaesthetized with sodium pentobarbitone. In an attempt to block replenishment of the 'pool' during release, cycloheximide was administered 30 min before LH-RF. A 20-fold increase in pool size occurred between the morning of dioestrus and the evening of pro-oestrus in the absence of any significant change in total pituitary content of LH. This suggests that increased responsiveness may be brought about by a change in the receptor-release apparatus and/or a transfer of LH from a 'storage pool' which leads to an apparent increase in the proportion of LH available for release.