Circulating blood progesterone is pulsatile throughout the rat oestrous cycle

In the present experiment, we determined circulating progesterone (P₄) levels of intact cycling female rats. A cannula constructed from polyethylene-50 tubing containing heparinized saline was inserted into the jugular vein of the rat under light ether anaesthesia and after a 3-6 h recovery period sequential blood samples were withdrawn from unanaesthetized, freely-moving rats. Blood samples (80-100 μl) were collected at 10-min intervals for 6 h and the volume was replaced with saline. Plasma P₄ was determined using a specific P₄ radioimmunoassay. Four rats in each of three phases of the rat oestrous cycle (E, D1, D2) were examined. In addition, rats in proestrus were diveded into an early (EP, N = 4) and late (LP, N = 4) proestrous conditions. Individual data were analyzed by PULSAR as adapted for an IBM-PC. P₄ was found to fluctuate in a pulsatile mode throughout the rat oestrous cycle. Mean levels of circulating P₄ were lowest (38.8 ± 1.7 nmol/l plasma) in EP and highest (122.5 ± 1.3) in D1. The frequency of P₄ pulses was dramatically decreased in D1 (2.50 ± 0.86 pulses/6 h), whereas no difference was found among the other days of the cycle (7.25 ± 0.47 in E, 7.25 ± 0.85 in D2, 5.75 ± 1.31 in EP, and 6.50 ± 1.04 in LP). Interestingly, the amplitude of P₄ pulses was significantly lower in EP (14.0 ± 2.91 pmol), whereas no difference was found among other groups. Another group of experimental animals were ovariectomized (N = 4; two with and two without an implant of a silastic capsule of P₄) and similarly bled. These animals demonstrated a fairly stable circulating pattern of P₄. The present data demonstrate for the first time that P₄ in rat blood fluctuates in a pulsatile manner.