Plasma progesterone and prolactin concentrations in overtly pseudopregnant bitches: A clinical study

Plasma concentrations of progesterone (P4) and prolactin (PRL) were measured in 35 bitches presented at veterinary clinics for symptoms of overt pseudopregnancy (PSP) between 50 and 95 days after the onset of proestrus. Results were compared to those from samples collected from 35 control bitches at comparable stages of the ovarian cycle (expressed as days after the onset of observed signs of proestrus). In the PSP bitches at 71.4 ± 1.6 (mean ± S.E.M.) days of the cycle, P4 (1.5 ± 0.2 ng/mL) was lower (P < 0.01) and PRL (16.0 ± 1.9 ng/mL) was higher (P < 0.01), compared to P4 (2.7 ± 0.4 ng/mL) and PRL (2.9 ± 0.6 ng/mL) in control bitches at 70.6 ± 1.5 days of the cycle. Low P4 was not a prerequisite for elevated PRL. Although elevated (≥10 ng/mL) PRL (20.9 ± 2.0 ng/mL) occurred more often with low (<2 ng/mL) P4 (20 of 24 cases) it also occurred with P4 above 3 ng/mL in two affected bitches and in two control bitches. Whether the occurrence of relatively low PRL concentrations (<4 ng/mL) in samples obtained from 4 of the 35 pseudopregnant bitches reflected variable and often elevated PRL secretion or increased sensitivity to PRL in the absence of elevated prolactin in those animals was not determined. We inferred that elevated plasma PRL was often involved in the etiology of overt PSP; furthermore, a premature decline in circulating P4 concentrations may be a factor in some instances.