Role of dopamine in the secretory function of corpus luteum in cattle

To investigate the role of dopamine (DA) in the secretion of progesterone and ovarian oxytocin (OT) in cattle in vivo and in vitro studies were performed. Luteal slices (middle stage of the luteal phase) were incubated-with five different amounts of DA (10(-3)-10(-7) M) and after 1, 2 and 4 h medium was collected for the progesterone and OT determination. The only 10(-5) M of DA enhanced OT medium concentrations (P < 0.05-0.001) and this response increased within duration of the incubation. The same dose of DA affected (P < 0.05-0.001) progesterone release during the incubation period. Next we wanted to study whether this mechanism will operate in conscious cattle. Mature heifers had cannulae inserted into the aorta abdominalis through the coccygeal artery for local infusion of saline or drugs, and into the jugular vein and the cava vena caudalis for the blood sample collection. In Experiment 2 six heifers on days 10-12 of the cycle received in Latin square design DA (8.4 micrograms/kg/min) for 2h, and they were pre-treated for 2 h with saline or for 1 h with droperidol (DROP; 1 microgram/kg/min) or with 4 micrograms/kg/min of beta-blocker, propranolol (PROP). DA increased (P < 0.05) OT plasma concentrations in all groups but secretion of progesterone was not changed (P > 0.05). DA decreased prolactin concentrations if pre-infused with saline and PROP and DROP diminished this effect of DA. Data suggest that DA affected OT release from both pituitary and from the ovary. Hence in Experiment 3 we wanted to elucidate whether DA caused those effects itself of after its conversion to the noradrenaline (NA). DA was given in six heifers as in Experiment 2 and blood samples were taken simultaneously from the vena cava and from the jugular vein. Within a few minutes from the start of DA infusion, large amount of DOPAC was measured by HPLC in blood. Next NA increase (P < 0.01) was observed in both veins but it was always higher (P < 0.05) in the vena cava. However, OT concentration in blood samples from both veins was not different (P > 0.05). Data indicate that DA can stimulate the secretory function of CL in cyclic heifers supposedly after its conversion to NA.