INFLUENCE OF OXYTOCIN REMOVAL FROM THE CORPUS-LUTEUM ON SECRETORY FUNCTION AND DURATION OF THE ESTROUS-CYCLE IN CATTLE

Mature heifers (n = 3 1) were infused with either saline or noradrenaline (0.3 mug kg-1 min-1) via a cannula inserted into the aorta abdominalis through the coccygeal artery (cranial to the origin of the ovarian artery). Noradrenaline was infused for three periods of 30 min on days 11 and 12 and on days 15 and 16 of the oestrous cycle. After the noradrenaline treatment, saline was given for 1 h in the same way. During each infusion, peripheral blood samples were collected for progesterone and oxytocin determination every 5-10 min and then once a day until oestrus. As a control, four heifers were infused with saline in latin square design on days 11, 12 and on days 15, 16; they were bled once a day until oestrus. Other heifers were infused on the same days, but 1 h after the last infusion of noradrenaline, 500 mug of prostaglandin F2alpha (PGF2alpha) analogue was injected, to measure any remaining luteal oxytocin. For comparison four heifers were injected with PGF2alpha analogue alone on day 12 and four others on day 16. Blood samples were taken as described previously. Each infusion of noradrenaline stimulated (P < 0.01) progesterone secretion. There was a significant (P < 0.05) response of oxytocin to each noradrenaline stimulation on days 11 and 12, although on each day the response to the second infusion was reduced and further reduced after the third infusion. On days 15 and 16, only the first noradrenaline infusion caused a clear surge of oxytocin with much smaller increases in oxytocin secretion after subsequent infusions. Noradrenaline stimulation reduced the total content of oxytocin in the corpus luteum on day 12 by 68-82% (mean 73.3%) and by 74% on day 16. The duration of the oestrous cycle was 20.8 +/- 0.6 and 21.0 +/- 0.4 days, respectively, in heifers infused with noradrenaline compared with 21.0 +/- 0.5 days in controls. Noradrenaline infusions markedly reduced the total amount of oxytocin in mid- and late-cycle bovine corpora lutea but this did not affect either spontaneous luteolysis or the duration of the oestrous cycle. It is therefore assumed that ovarian oxytocin may have a permissive role rather than a direct action on luteolysis in cattle.