Study of the change of prolactin and progesterone during dopaminergic agonist treatments in pseudopregnant bitches.

In order to clarify the role of prolactin (PRL) and progesterone (P(4)) in the pathophysiology of canine pseudopregnancy (PSP) we designed an experiment, where we induced an abrupt pharmacological blockade of PRL secretion with dopaminergic agonists (DA) or placebo (PL). Thirty overtly pseudopregnant (PSPT) bitches were randomly allocated to three groups of 10 animals each: PL, bromocriptine (BR), and cabergoline (CA), which were treated with PL, 7.5microg/kg BR and 5microg/kg CA, respectively. On days 1, 7 and 14 (day 0: beginning of the treatment) all the animals were classified into grades of intensity of PSP clinical signs, considering serum or milk secretion and enlargement of the mammary glands. Presence or absence of treatment side effects were recorded and blood samples for PRL and P(4) determinations collected. Serum PRL and P(4) concentrations (ng/ml) of all the animals on day 1 were (least squares means [LSM]+/-S.E.M.) 17.70+/-2.05 and 1.13+/-0.13, respectively. During the experiment, serum PRL and P(4) concentrations decreased (day effect, P<0.05). During the experiment, serum PRL concentrations were lower in the DA treated group (BR and CA) compared with PL group (P<0.05). After a week of treatment, the percentage change of PRL was -62.52 versus 102.16+/-46.20 (P<0.01) for the treated (BR and CA) and PL groups, respectively. Conversely, no significant differences were found in the percent change in PRL between the BR and the CA groups nor in P(4) percentage change among all groups for the same week. Significant differences in the achievement of complete remission between treated and PL groups were found on days 7 (40 versus 0%, P<0.05) and 14 (90 versus 0%, P<0.01). No significant correlation between PRL and P(4) was found on day 1 in any of the animals. However, a significant correlation for the same hormones was found on days 7 and 14 for the DA treated groups (r=0.46, P<0.01). While in the PL group, PRL concentrations and intensity of clinical signs were not significantly correlated on days 1, 7 and 14; in the DA treated groups they were significantly correlated on days 7 and 14 (r=0.34, P<0.05). The presence of a positive correlation between PRL concentrations and the grades of intensity of clinical signs in the treated animals indicates the major role of PRL in PSP physiopathology. However, the lack of correlation during spontaneous involution of PSP in the PL group demonstrates that PRL concentrations do not completely explain the problem. In summary, abrupt changes in serum PRL seemed to be more important in ceasing PSP signs than total PRL concentrations in these groups of animals.