Vasopressin contamination as a cause of some apparent renal actions of prolactin

Secretion and Function of Pituitary Prolactin in Evolutionary Perspective

The hypothalamo-pituitary system developed in early vertebrates. Prolactin is an ancient vertebrate hormone released from the pituitary that exerts particularly diverse functions. The purpose of the review is to take a comparative approach in the description of prolactin, its secretion from pituitary lactotrophs, and hormonal functions. Since the reproductive and osmoregulatory roles of prolactin are best established in a variety of species, these functions are the primary subjects of discussion. Different types of prolactin and prolactin receptors developed during vertebrate evolution, which will be described in this review. The signal transduction of prolactin receptors is well conserved among vertebrates enabling us to describe the whole subphylum. Then, the review focuses on the regulation of prolactin release in mammals as we have the most knowledge on this class of vertebrates. Prolactin secretion in response to different reproductive stimuli, such as estrogen-induced release, mating, pregnancy and suckling is detailed. Reproduction in birds is different from that in mammals in several aspects. Prolactin is released during incubation in avian species whose regulation and functional significance are discussed. Little information is available on prolactin in reptiles and amphibians; therefore, they are mentioned only in specific cases to explain certain evolutionary aspects. In turn, the osmoregulatory function of prolactin is well established in fish. The different types of pituitary prolactin in fish play particularly important roles in the adaptation of eutherian species to fresh water environments. To achieve this function, prolactin is released from lactotrophs in hyposmolarity, as they are directly osmosensitive in fish. In turn, the released prolactin acts on branchial epithelia, especially ionocytes of the gill to retain salt and excrete water. This review will highlight the points where comparative data give new ideas or suggest new approaches for investigation in other taxa.

Perennial Ryegrass Alkaloids Increase Respiration Rate and Decrease Plasma Prolactin in Merino Sheep under Both Thermoneutral and Mild Heat Conditions

A study was undertaken to determine the effects of feeding two levels of perennial ryegrass alkaloids (nil vs. moderate) under two climatic conditions. Alkaloids were fed via endophyte-infected perennial ryegrass seed and hay. Twenty-four Merino ewe weaners (six months, initial BW 32 ± 1.7 kg) were used in a study that lasted for 21 days after 14 days of adaptation. Sheep were fed either a control or alkaloid (Alk, 110 μg/kg LW ergovaline and 75 μg/kg LW lolitrem B) supplemented diet. Sheep were exposed to either constant thermoneutral (TN, 21–22 °C, 49% RH) or mildly heated (HS, 33 °C 1000–1500 h, 28% relative humidity) conditions. Dietary Alk and HS reduced dry matter intake (DMI) (p < 0.001, p = 0.02, respectively) with the combination of both reducing DMI by 42%. Reductions in DMI resulted in a lower daily gain in the Alk treatment (p < 0.001). Feed digestibility was reduced in the combined treatment (p = 0.03). Rectal temperature, respiration rate, and skin temperature increased in the Alk treatment. Plasma prolactin concentrations were decreased by Alk and increased by mild HS. The data indicate that production is compromised in the presence of Alk and mild HS, with this effect being exacerbated by a combination of both.

Hydration as a limiting factor in lactation

A relatively untapped area of research concerning lactation and hydration exists for bioanthropologists interested in the ecology of breastfeeding. This review details current limited knowledge on the topic with recommendations about relevant research questions for human biologists. On average, lactating mothers produce >750 ml/day milk for their infants. Breastfeeding thus promotes a powerful thirst stimulus in the lactating mother, resulting in a 12-16% increase in fluid intake among Western women. Thirst during lactation may be mediated by oxytocin release, since this hormone is structurally similar to the antidiuretic hormone, vasopressin. Prolactin also may be involved. A few studies among Western women concluded that moderate dehydration does not affect milk production, but it is not known how lactating women in hot and/or dry climates respond to chronically limited water supplies. Lactating women in such environments may use both physiological and behavioral adaptations to conserve fluid requirements, such as reducing activities during breastfeeding and carrying extra fluid supplies while travelling. Given the limited state of knowledge on lactation and hydration, future field studies should incorporate research to determine the importance of adequate fluids in maintaining milk production and the health of both mothers and infants. Am. J. Hum. Biol. 10:151-161, 1998. © 1998 Wiley-Liss, Inc.

Neuroendocrine mechanisms mediating fluid intake during the estrous cycle

Gonadal steroids appear to influence fluid-electrolyte homeostasis through behavioral as well as renal mechanisms. The marked fluctuations in drinking behavior observed during the estrous cycle of the female rat may be due to an interaction between estrogen and the dipsogenic peptide hormone, angiotensin II, at the level of basal forebrain receptors. The preoptic region in particular may play an important integrative role in the maintenance of extracellular fluid balance in synchrony with the estrous cycle, since it contains receptors for angiotensin and estrogen. Prolactin may also directly participate in mechanisms of extracellular thirst, while an exact role for vasopressin has yet to be established. Recent studies also suggest that estrogens may influence body fluid regulation by interacting with several neurotransmitters, including serotonin, dopamine and noradrenaline.

Plasma prolactin levels and body fluid deficits in the rat: causal interactions and control of water intake

A study was made of the responses of chronically hyperprolactinaemic rats to selected dipsogenic stimuli. Measurements were also made of the correlation between the state of hydration of the animal and the plasma prolactin levels. After 24 h water deprivation. S.C. isoprenaline (10 micrograms/kg body wt.) or I.P. injection (5 ml/kg body wt.) of a hypertonic solution (50% w/w) of polyethylene glycol (mol. wt 20000) there was no difference between the hyperprolactinaemic and control rats with respect to the total water intake, the time course of drinking or the urine output. After I.V. injection of 2 M-NaCl (5 ml/kg body wt.) there was no difference between the hyperprolactinaemic and control rats with respect to the total water intake or urine output. However, the hyperprolactinaemic rats drank more slowly than the controls. When angiotensin II was infused I.V. at a rate of 0.2 micrograms/min, the water intake was greater and the threshold to drinking lower in the hyperprolactinaemic than control rats. After 24 h water deprivation, plasma prolactin levels rose significantly in both the control and hyperprolactinaemic rats. When the rats were injected I.V. with hypertonic saline (5 ml, 2 M-NaCl/kg body wt.) and denied access to water, plasma prolactin levels had not changed 1 h later in either the control or hyperprolactinaemic animals. It is concluded that there is no interaction either between the plasma osmolality and prolactin secretion or between the plasma prolactin levels and the amount of water drunk in response to intracellular fluid deficits. However, prolactin secretion is stimulated by the combined intra- and extracellular deficits resulting from water deprivation and there is a clearly demonstrated interaction between prolactin and the extracellularly mediated stimulus of angiotensin II.

Effect of prolactin on urinary excretion and renal haemodynamics in conscious rats

1. Conscious rats received a saline infusion (5.8 ml . hr-1) via a tail vein, for a 6 hr period. For the final 2 hr of this infusion period, prolactin (7.1 microgram (100 g body wt.)-1 . hr-1) was incorporated in the infusate. A control group of animals continued to receive saline alone. 2. In the first hour of prolactin administration (in comparison with the control group), urine flow, sodium output and osmolal output were all significantly reduced (P less than 0.02); there was also a reduction (P less than 0.05) in renal plasma flow (p-amino-hippurate clearance), but the filtration fraction did not alter. The changes in urinary excretion, and in renal haemodynamics, did not persist for the second hour of prolactin administration, but there was a sustained increase in body fluid volume. 3. It is possible that altered renal haemodynamics are responsible for the reduced urinary water and solute excretion which occurs during acute prolactin administration.

The dipsogenic activity of prolactin in male and female rats

1. Ovine prolactin injected intravenously in doses of 10.0, 5.0, 1.0, 0.1 and 0 mg/kg body wt. caused neither drinking nor a change in urine output in normal water replete male and female rats. 2. The water intake of male rats subjected to 48 hr water deprivation was substantially increased after injection of prolactin. 3. The water intake of male and female rats injected I.P. with a hyperoncotic solution of polyethylene glycol (20 M) was significantly increased after injection of prolactin. 4. Prolactin ws found to act synergistically with a subthreshold dose of angiotensin II amide to cause significant drinking and fluid retention. 5. The drinking responses of male and female rats injected I.V. with hypertonic saline were not modified by prolactin. 6. It is concluded that prolactin may act to increase the net water gain of animals suffering a deficit of the extracellular fluid space but is without effect on stimuli arising from deficits in the intracellular fluid space.