BLOOD LEVELS OF PROGESTERONE AND 15-KETO-13,14-DIHYDRO-PROSTAGLANDIN F2 DURING THE NORMAL OESTROUS CYCLE AND EARLY PREGNANCY IN HEIFERS

Peripheral plasma levels of progesterone1) and 15-keto-13,14-dihydroprostaglandin F2α were measured during the normal oestrous cycle in three heifers. Before the experiment the animals received an infusion each of different amounts of prostaglandin F2α to estimate the conversion of this compound into the main plasma metabolite, 15-keto-13,14-dihydroPGF2a. The experimental period was initiated by heat synchronization with 25 mg prostaglandin F2a given intramuscularly. Daily blood samples were collected from the jugular vein until day 14 of the oestrous cycle and subsequently every third hour until heat occurred. During this heat the three heifers were mated, and blood samples were collected in the same way during the following cycle and over the next expected heat. Pregnancy occurred in one of the animals. High levels of 15-keto-13,14-dihydro-prostaglandin F2a were recorded as pulses during the last days of the normal oestrous cycle. These pulses coincided with decrease in progesterone levels. In the pregnant heifer neither decrease in progesterone nor increase in the prostaglandin metabolite level was seen.