Daily water and electrolyte balance in chronically hyperprolactinaemic rats

Effect of splenic extract on plasma volume and renal function in the rat

A hypotensive and natriuretic factor has recently been extracted from the rat spleen. Experiments were designed to investigate the mechanisms underlying the increase in urine output caused by splenic extract. Rat spleens were homogenized in phosphate buffered saline (PBS), centrifuged, subjected to ultrafiltration (mol. wt. cutoff 10,000), extracted on C18 affinity columns and dried. After reconstitution in isotonic saline, the extract was injected IV into conscious rats. Splenic extract caused a decrease in plasma volume (17.4+/-1.1 to 15.8+/-1.0 ml at 1 hr), and a delayed increase in urine output (1.8+/-0.2 to 4.0+/-0.4 ml/hr at 2 hr). There were no such changes in the muscle-injected control group. The increase in urine output was accompanied by an increase in glomerular filtration rate (splenic extract, 2.2+/-0.2 to 5.9+/-1.6 ml/min; muscle extract, 2.9+/-0.4 to 3.1+/-0.6 ml/min). Renal blood flow in the splenic extract-injected group fell during the course of the experiment so that, at 120 min., it was significantly lower both with respect to its baseline value and the muscle control group (splenic extract 22.1+/-0.2 to 17.5+/-2.2 ml/min; muscle extract 24.4+/-4.1 to 23.3+/-3.8 ml/min). During this same period, mean arterial pressure in the splenic extract group also fell from 98+/-2 to 91+/-4 mmHg. Renal vascular conductance therefore did not change. In conclusion, splenic extract causes a primary decrease in plasma volume and a delayed increase in urine output that is mediated, at least in part, by an increase in glomerular filtration rate. It is suggested that the splenic factor(s) probably achieves this by differential vasodilatation of the afferent glomerular arteriole and constriction of the efferent glomerular arteriole.

Differential effect of CCl4 on renal function in cirrhotic and non-cirrhotic rats

The pathogenesis of renal function alteration associated with liver disease remains to be elucidated. Although different experimental animal models have been utilized in order to explain such pathophysiological state, none of them have completely explained the mechanisms involved. In this study we performed differential hemodynamic, hepatic and renal function alteration studies after induction of acute liver damage via intragastric administration of a single dose of CCl4 to cirrhotic and non-cirrhotic rats. Cirrhotic rats with acute liver damage exhibited a significant decrease in mean arterial pressure followed by a decreased glomerular filtration rate, urinary sodium concentration and an induction of plasma renin concentration and activity. At the same time, a significant association between oliguria and mortality was observed. The renal histopathological studies revealed glomeruli with mesangial hypercellularity and thickening of capillary wall, but not tubular epithelial injury. All these alterations were not detected in the control group, i.e. by non-cirrhotic rats with acute liver damage. This study suggests that the effect of CCl4 on kidney structure and function depends on the functional state of the liver. Since this experimental model of acute liver damage in cirrhotic rats presents hemodynamics and renal function alterations similar to those observed in the hepatorenal syndrome in man, it could be utilized to study the pathogenesis of renal function alterations associated with liver damage.

Clearance of IGFs and insulin from wounds: effect of IGF-binding protein interactions

We have examined the role binding proteins have in regulating the clearance of exogenous growth factors from wounds. Hunt-Schilling chambers were subcutaneously implanted in rats, and the clearance of insulin-like growth factor (IGF) I from the chamber wound fluid was compared with IGF-II, LR3-IGF-I, which binds poorly to IGF-binding proteins (IGFBP), or insulin. Elimination rate constants of the slow phase of the decay curves did not differ between IGF-I and IGF-II. However, LR3-IGF-I and insulin were cleared more rapidly from wound fluid than IGF-I so that the half-lives for IGF-I, IGF-II, LR3-IGF-I, and insulin were 872, 861, 563, and 324 min, respectively. In wound fluid, minimal degradation of the IGFs occurred, whereas insulin was degraded considerably. The increased clearance of LR3-IGF-I and insulin equated with a reduced association with wound fluid IGFBPs, and increased amounts of radioactivity of these peptides were detected in the circulation and urine. These results show that this model of wound repair may be of use in examining the kinetics of growth factors and other bioactive molecules in extravascular spaces and support the hypothesis that IGFBPs can be significant regulators of IGF bioavailability in vivo.

Neurohypophysial hormone secretion in hyperprolactinaemic women

Prolactin (PRL) has been reported to promote antidiuresis and increase intestinal water-electrolyte absorption, whereas osmolar changes have been shown to influence PRL secretion. However, the mechanisms of action of PRL on the salt-water balance remain unclarified. The present clinical study targeted the effects of hyperprolactinaemia on the secretion of arginine-8-vasopressin (AVP), oxytocin (OXT) and cortisol. Plasma AVP and OXT were measured by radioimmunoassay, and cortisol by fluorimetry. In healthy women (21-39 y, n=6), an oral water load (OWL, 20 ml/bw) significantly suppressed the plasma levels of AVP, OXT and cortisol, and the PRL level too tended to decrease. In hyperprolactinaemic females (22-41 y, n=6, three with pituitary adenomas), water retention was registered following an OWL, together with paradoxical AVP and OXT level increases, whereas the cortisol response remained normal, and the PRL level did not change at all. Histamine (0.5 mg sc) stimulated the release of AVP, OXT and cortisol in the control and hyperprolactinaemic groups alike. These data suggest that alterations in AVP and OXT hypersecretion may contribute to the water retention in hyperprolactinaemia.

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.

Role of prolactin and gonadal steroids in regulation of oxytocin mRNA during lactation

The role of suckling, prolactin, and gonadal steroids in regulating hypothalamic oxytocin (OT) mRNA content during the 1st and 2nd wk of lactation was evaluated. On day 4 of lactation, OT mRNA content decreased in rats removed from their litter for 24 h compared with suckled controls. Either suckling (in the absence of prolactin release) or prolactin (in the absence of suckling) maintained OT mRNA content at this stage of lactation. In contrast, at day 11 of lactation, OT mRNA content remained unchanged in rats deprived of pups for 24 h. Ovariectomy did not compromise the ability of day 11 nonsuckled rats to maintain OT mRNA content; however, the increased water intake, plasma prolactin, plasma vasopressin (VP), plasma renin concentration, and hypothalamic VP mRNA content at day 11 compared with day 4 suggest a role for fluid balance in determining OT mRNA content during the 2nd wk of lactation. Thus, at day 4, suckling is a major determinant of OT mRNA content as a result of both direct activation of neuronal afferents to the OT neurons and stimulation of prolactin release. This is in contrast to day 11, when fluid balance may predominate in regulation of OT mRNA.

Lack of an inhibitory effect of hyperprolactinemia on androgen-dependent marking

An experiment was performed to determine if hyperprolactinemia (chronically elevated serum prolactin levels), which inhibits testosterone-activated male sexual activity, also affects other androgen-dependent behaviors. Thus defecation and urine marking in response to a novel environment were examined in sham-operated and pituitary-grafted (hyperprolactinemic) male rats that had been castrated or castrated and given subcutaneous testosterone implants. Both castration and pituitary grafting significantly inhibited defecation, with the inhibitory effects of hyperprolactinemia being most pronounced in the castrated non-testosterone-treated animals. In contrast, castration significantly reduced the amount of urine marking observed, but pituitary grafting was without effect on this behavior. Thus, although hyperprolactinemia may inhibit sexual activity through an antagonism of the activational effects of testosterone, these results suggest that this effect is specific to sexual behavior and does not involve a more generalized inhibition of the effects of testosterone on androgen-dependent behaviors.

Altered renal function in chronically hyperprolactinaemic rats

1. Standard renal clearance techniques were used to investigate the effects of chronic hyperprolactinaemia on kidney function in male, female and ovariectomized female rats. 2. All hyperprolactinaemic rats showed a significantly increased glomerular filtration rate (G.F.R.) compared to controls. Values were (microliter min-1) 2738 +/- 146 vs. 2299 +/- 99 for males (P less than 0.05), 2236 +/- 79 vs. 1865 +/- 74 for females (P less than 0.01) and 2200 +/- 76 vs. 1941 +/- 62 for ovariectomized females (P less than 0.05). 3. Hyperprolactinaemic rats in all groups also showed a significant increase in absolute tubular reabsorption of water, sodium and chloride compared to their respective controls. Increases here averaged 19%. 4. There was a significantly greater fractional tubular reabsorption of fluid and solutes in hyperprolactinaemic male rats compared to controls. Values were (%) 92.9 +/- 0.6 vs. 90.3 +/- 0.7 for water, 93.0 +/- 0.4 vs. 91.0 +/- 0.6 for sodium and 89.9 +/- 0.7 vs. 86.5 +/- 0.9 for chloride. In each case P less than 0.05. 5. These results imply an osmoregulatory role for prolactin which is not specific to pregnancy or related female reproductive states.

Effect of acute administration of bromocriptine on isoproterenol- and angiotensin II-induced water intake in estrogen-treated rats

Chronic administration of an estrogenic agent is well known to attenuate the drinking response of rats to treatment with a variety of dipsogenic agents, and to increase plasma concentration of prolactin. Treatment with prolactin is also known to reduce the drinking response to administration of the dipsogenic agent, isoproterenol. Hence, a possibility existed that the antidipsogenic effect of chronic treatment with estrogen was mediated by an increased plasma prolactin concentration. Since bromocriptine, a dopaminergic agonist, is known to reduce plasma prolactin concentration in estrogen-treated rats, it was administered (1.0 mg/kg, IP) 15 min prior to treatment with either isoproterenol (25 micrograms/kg, SC) or angiotensin II (200 micrograms/kg, SC). The results suggest that the antidipsogenic effect of chronic treatment of rats with estradiol benzoate (30.4 and 45.7 micrograms/kg/day) can be reversed, at least partially, by acute administration of bromocriptine.

Effects of chronic estrogen treatment on water exchange in rats

Intact female rats implanted subcutaneously with Silastic tubes containing estradiol benzoate (EB) (28.7 micrograms X kg-1 X day-1) for 28 wk had a significantly greater daily intake of water, a higher water-to-food intake ratio, and a greater urine output than untreated control rats. Ovariectomized (OVX) rats also implanted for 14 wk with EB tubes (15 and 36 micrograms X kg-1 X day-1) showed identical results. Dipsogenic responses of the EB-treated rats to isoproterenol (25 micrograms/kg sc), angiotensin II (200 micrograms/kg ip), and hypertonic saline (1 M, 1% of body wt ip) were significantly attenuated. Both intact and OVX rats were subjected to a 24-h dehydration to assess renal concentrating ability. EB-treated rats lost significantly more weight and excreted significantly more urine of lower osmolality than controls. Administration of vasopressin to volume-loaded, EB-treated rats revealed no abnormalities in the ability to concentrate urine to the level of controls. Thus, in spite of a reduced responsiveness to several dipsogenic stimuli, EB-treated rats have an increased daily water turnover apparently related to an inability to concentrate their urine. This in turn may be related to abnormalities in either synthesis or release of antidiuretic hormone or both.

Plasma prolactin during the body fluid and electrolyte changes of dehydration and sodium depletion in steers

The effect of dehydration and sodium depletion on plasma prolactin levels in steer calves is very different from the changes seen in the rat and possibly in man. Removal of drinking water was followed by progressive dehydration for 96 h during which time packed cell volume (PCV) increased from 39.9% to 44.7% and plasma osmolarity (pOsm) rose from 303.3 mOsm to 342.0 mOsm/l with hypernatraemia. At the same time plasma prolactin ( pPRL ) was rapidly reduced from a basal value of 2.3 ng/ml to barely measurable amounts and remained low during dehydration. Restoration of ad lib drinking water was followed by rapid reduction of PCV and pOsm to sub-basal levels during which time the pPRL increased significantly to persist at 15 ng/ml. Sodium depletion was produced by continuous loss of sodium-rich saliva from unilateral fistulation of a parotid duct. During sodium deficiency PCV increased from 38.6% to 45.6% but pOsm fell significantly from 299.9 mOsm/l to 286 mOsm/l with hyponatraemia. As in dehydration, during sodium depletion pPRL was suppressed, and after 7 days was reduced from a basal level of 5.4 ng/ml to 0.5 ng/ml. The sodium depleted steers when given 0.3M NaHCO3, which they consumed readily to restore sodium homeostasis, restored the deficiency gradually in 5 days when pPRL , pOsm and PCV all returned to basal levels without any 'overshoot' or hypersecretion of pPRL . Our finding indicate that extracellular fluid volume changes, not electrolyte content, affect pPRL . This is in agreement with results obtained in the rat, and possibly in man, but the fact that in the steer, the endogenous changes in prolactin level show a profound reduction provides an extreme example of species difference. The means whereby both divergent physiological processes of dehydration and sodium depletion generate stimuli which inhibit prolactin secretion and the relevance of this response in fluid balance homeostasis requires further research.

Effects of elevated prolactin and its normalization on thyroid hormone, cardiac beta-adrenoreceptor number and beta-adrenergic responsiveness

Subcutaneous inoculation of the prolactin secreting MtTW15 adenoma in male Wistar Furth rats for 4 weeks produced a significant increase in serum prolactin and a corresponding decrease in peripheral beta-adrenergic responsiveness. Both the isoproterenol induced drink and heart rate responses used to assess the beta-adrenergic responsiveness were significantly reduced in the hyperprolactinemic rat. Serum T3 and T4 levels were measured as was cardiac beta-adrenergic receptor number to ascertain if an alteration of thyroid hormone and a resultant decrease in beta-adrenergic receptor number was responsible for the attenuated beta-adrenergic responsiveness. Serum T4 was significantly reduced in the hyperprolactinemic group (1.9 +/- 0.3 microgram%) as compared to the control group (6.4 +/- 0.l5 microgram%). However there was no significant difference in serum T3 or in cardiac beta-adrenergic receptor number between the two groups. Removal of the MtTW15 adenoma resulted in a normalization of serum prolactin, T4, and in the responsiveness of the peripheral beta-adrenergic system within 4-6 weeks. These results indicate that the attenuated beta-adrenergic responsiveness associated with hyperprolactinemia is reversible and not dependent on a reduction in beta-adrenergic receptor number.

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.

Effects of chronic hyperprolactinaemia on experimentally induced thirsts in male rats

Chronic hyperprolactinaemia was induced in ten male Wistar rats by subcutaneous injection of tissue fragments of the MtTW 15 prolactin-secreting adenoma. Serum prolactin was elevated significantly by 3 weeks after adenoma inoculation and increased progressively thereafter. An additional ten Wister-Furth rats were inoculated with the MtTW 15 adenoma and an equal number of age-matched controls were used to evaluate the drinking response to several dipsogenic agents. The hyperprolactinaemic rats consumed twice as much water as did control rats over a 24 hr period. Administration of the beta-adrenergic agonist, isoprenaline (25 micrograms/kg, s.c.), resulted in a similar increase in water intake in both groups prior to an elevation in serum prolactin. However, as serum prolactin increased there was a corresponding significantly attenuated dipsogenic response to isoprenaline. Chronic hyperprolactinaemia did not alter the drinking response to angiotensin II (200 micrograms/kg, s.c.), to 24 hr of dehydration or to administration of hypertonic (1 M) saline. These results indicate that chronic hyperprolactinaemia selectively attenuates the dipsogenic response to beta-adrenergic stimulation and support an involvement of prolactin in modifying the responsiveness of the beta-adrenergic system.

Acute and chronic effects of progesterone and prolactin on renal function in the rat

The renal effects of acute and chronic progesterone and prolactin administration were investigated in rats. Acute progesterone treatment caused a reduced urinary potassium excretion compared to controls (0.80 +/- 0.03 vs. 1.30 +/- 0.04 mumole min-1; P less than 0.01). No other renal changes were apparent. Acute prolactin administration produced no significant changes in renal function. Chronic progesterone treatment reduced urinary potassium output by increasing tubular reabsorption. Absolute reabsorption (mumole min-1) for controls was 4.70 +/- 0.42 and for progesterone treatment, 6.40 +/- 0.57 (P less than 0.05 comparing the two). No other renal changes were apparent. Animals made pseudopregnant by chronic prolactin administration showed a significant (16%) elevation in glomerular filtration rate, similarly enhanced fluid and solute reabsorption, and a significant (16%) increase in proximal tubule length. This would imply a role for prolactin in early rat pregnancy and pseudopregnancy when similar renal changes are evident and circulating hormone levels high.

Chronic hyperprolactinemia reduces peripheral beta-adrenergic responsiveness in male rats

The MtTW15 prolactin (PRL) secreting adenoma elevated serum PRL concentrations over controls within 5 weeks after tissue implantation. This treatment resulted in a significant reduction of peripheral beta-adrenergic responsiveness as assessed by three different parameters. Isoproterenol-induced thirst was significantly attenuated in the MtTW15 rats. The decrease in the thirst response was proportional to the increase in serum PRL. Unanesthetized heart rates of both groups were not significantly different before isoproterenol was administered. However, following administration of the beta-adrenergic agonist the heart rate response in rats with elevated PRL was significantly attenuated when compared to the controls. Elevation of serum PRL virtually abolished the elevation of tail skin temperature response associated with administration of isoproterenol. Collectively, these results suggest hyperprolactinemia reduces peripheral beta-adrenergic responsiveness; however the mechanism for this reduced response remains to be elucidated.

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.