Calcium, parathyroid hormone, oxytocin and pH profiles in the whelping bitch

Despite the high prevalence of primary uterine inertia in whelping bitches, the underlying pathogenesis remains unclear. The objectives were to i) determine serum concentrations of total calcium, ionized calcium (iCa), parathyroid hormone (PTH), and blood pH in normally whelping bitches throughout the peri-parturient period; and ii) investigate relationships among iCa, PTH, and acid-base status, and the role that they and oxytocin may have in the underlying pathogenesis of canine uterine inertia. Bitches were randomly selected from a population of German Shepherd Dog bitches with a history of uncomplicated parturition (Group 1; n = 10), and from a population of Labrador bitches with a clinical history of an increased incidence of uterine inertia and stillbirths (Group 2; n = 20). Jugular blood samples were collected daily from -4 d to the onset of whelping (t = 0 h), and then every 4 h until the last pup was born. Overall, bitches from Group 2 had higher mean ± SEM serum concentrations of PTH (4.72 ± 2.45 pmol/L, P < 0.001), lower iCa (1.31 ± 0.08 pmol/L, P < 0.05), and higher venous pH (7.41 ± 0.03, P < 0.005) than bitches from Group 1 (2.9 ± 1.44 pmol/L, 1.38 ± 0.06 mmol/L, and 7.33 ± 0.02, respectively) during the periparturient period. However, there was no significant difference between Groups 1 and 2 for serum oxytocin concentrations during the periparturient period (45.5 ± 40 and 65.5 ± 82 pg/mL). We inferred that low iCa resulting from a rising pH and decreasing PTH during the periparturient period may have contributed to decreased uterine contractility and increased risk of stillbirths. Therefore, manipulating the cationic/anionic difference in diets of pregnant bitches, similar to the bovine model for hypocalcamia, may reduce the incidence of stillbirths in the bitch.