CONTROL OF THE MARE'S OESTROUS CYCLE BY PROSTAGLANDINS

Several authors have recently demonstrated the luteolytic action of exogenous prostaglandin F2\g=a\(PGF2\g=a\)in various laboratory and domestic animals and there is now ample evidence that PGF2\g=a\is the uterine luteolytic hormone which terminates the life of the CL in many species (McCracken, 1972; McCracken, Carlson, Glew, Goding, Baird, Gr\l=e'\en & Samuelsson, 1972). In cattle, Rowson, Tervit & Brand (1972) showed that an intrauterine (i/u) in- fusion of PGF2\g=a\on 2 successive days at any time between the 5th and 16th day of the cycle, induces oestrus 3 days later; they found that for 0\m=.\5mg PGF2\g=a\to be strongly luteolytic in the cow, the substance must be placed in the uterine horn which is ipsilateral to the ovary containing the active CL. This observation is in accordance with the experiments of Moor & Rowson (1966), Harrison, Heap & Linzell (1968) and others, which have indicated that in the sheep, cow and pig, uterine luteolysin passes directly from the uterine horn to the adjacent ovary by way of a local pathway. In the mare, however, unilateral hysterectomy prolongs the life-span of the CL in some animals but not in others, and the proportion in which luteal maintenance occurs seems to be similar irrespective of whether the CL is situated contra- lateral or ipsilateral to the remaining uterine horn (Ginther & First, 1971). This finding suggests that uterine luteolysin may reach the ovaries through the general circulation rather than by 'a local pathway' in the mare. The average length of the oestrous cycle in mares is 22 days. Oestrus is highly variable and may last 3 to 10 days, ovulation occurring 24 to 48 hr before the end of oestrus (Day, 1939). Douglas & Ginther (1972) have recently demonstrated that a single subcutaneous (s.c.) injection of 1 -25 to 10 mg PGF2(Z given to a mare on the 6th day after ovulation significantly reduces the length of dioestrus. In the present study, we describe the high luteolytic potency of a new synthetic analogue of PGF2a (ICI-79939) in the mare. Fourteen Welsh Pony mares were given the compound ICI-79939 at various stages of dioestrus during a total of forty oestrous cycles. In twenty-nine cycles involving twelve mares, treatment consisted of the infusion of 2 ml of an aqueous solution containing 100, 150 or 200 µg of the synthetic compound directly into the body of the uterus on 2 successive days. In eleven cycles