Secretion of prolactin and growth hormone in relation to ovarian activity in the dog

In pregnant bitches an apparent increase in plasma prolactin concentrations is observed during the second half of pregnancy, mean plasma prolactin con centrations peak on the day of parturition, fall for the next 24-48 h and t hen rise again. During lactation, high plasma prolactin concentrations are observed. Plasma prolactin levels in non-pregnant bitches appear to be lowe r than in pregnant animals, particularly in the last part of the luteal pha se. Pulsatile secretion of prolactin has been observed during the luteal ph ase and mid-anoestrus. Progression of the luteal phase is found to be assoc iated with an increase in prolactin release. The association of a strong in crease of prolactin release and a decrease of plasma progesterone concentra tions has also been demonstrated in overtly pseudopregnant bitches. Elevate d prolactin secretion during progression of the luteal phase in the bitch m ay play a role in mammogenesis and is important because of the luteotrophic action of prolactin. Acromegaly is a syndrome of tissue overgrowth and ins ulin resistance due to excessive growth hormone (GH) production. In the bit ch, acromegaly can be induced either by endogenous progesterone or by exoge nous progestagens. Progestagen-induced GH production in this species origin ates from foci of hyperplastic ductular epithelium of the mammary gland. Pu lsatile secretion of GH has been observed in normal cyclic bitches. In cont rast with the pulsatile GH secretion seen in healthy dogs, the progestagen- induced plasma GH levels in bitches with acromegaly do not have a pulsatile secretion pattern. Just as with prolactin, the plasma progesterone levels influence the secretion pattern of GH in the bitch. The pulsatile secretion pattern of GH changes during the progression of the luteal phase in health y cyclic bitches, with higher basal GH secretion and less GH being secreted in pulses during the first part of the luteal phase. The progesterone-indu ced GH production may promote the proliferation and differentiation of mamm ary gland tissue during the luteal phase of the bitch by local autocrine/pa racrine effects and may exert endocrine effects.