Circulatory changes following prolactin administration

Relative Postpartum Retinal Vasoconstriction Detected With Optical Coherence Tomography Angiography

Purpose

To characterize changes in retinal perfusion during pregnancy and the postpartum period using optical coherence tomography angiography (OCTA).

Methods

A nonmydriatic OCTA camera was used to image healthy women who were pregnant or in the postpartum period along with nonpregnant controls. Perfusion density (PD) and vessel length density (VLD) in the superficial capillary plexus (SCP), intermediate capillary plexus (ICP), and deep capillary plexus (DCP) were evaluated.

Results

A total of 16, 15, and 13 eyes from nonpregnant, pregnant, and healthy postpartum subjects, respectively, were evaluated. When compared to controls, there were significant increases in ICP PD during the second and third trimester of pregnancy, along with significant decreases in both PD and VLD in SCP, ICP, and DCP up to 14 weeks postpartum.

Conclusions

During pregnancy, vascular changes consistent with retinal vasodilation were noted in the ICP. During the postpartum period, changes in retinal vasculature suggest relative vasoconstriction involving all three layers when compared to both the pregnant and nonpregnant states.

Translational Relevance

Detecting postpartum changes in retinal vasculature could offer important insights into postpartum physiology throughout the body.

Power spectral analysis of plethysmographic pulse waveform in pregnant women

OBJECTIVE

To investigate the effect of pregnancy on the pulse wave of the mother.

PATIENTS AND METHODS

Pulse waveforms recorded via a pulse oximeter from the left index finger of non-pregnant women and pregnant women in the three trimesters of pregnancy were Fourier transformed into power spectra. Spectral indices of the power spectra of pulse waveform were obtained and compared among non-pregnant women and pregnant women in the three trimesters of pregnancy.

RESULTS

The power of harmonics of pulse wave decayed exponentially with respect to the order of harmonics. The exponent and initial value of exponential decay for the power of harmonics and the power of the 2nd harmonic were increased, whereas the total power of pulse and the powers of higher order harmonics were decreased during pregnancy.

CONCLUSION

The power of harmonics of pulse wave can be described by an exponential decay function with respect to the order of harmonics in both non-pregnant and pregnant women. The effects of pregnancy on the pulse wave are the reduction in the total power of pulse and the power of higher order harmonics, and the increase in the power of lower order harmonics in the power spectrum of pulse wave. This effect of pregnancy on the pulse wave might be caused by the decrease in vascular resistance during pregnancy, the increase in workload on the heart due to increased demand of the growing fetus, and the aortocaval compression caused by the progressively enlarged gravid uterus and fetus.

Update on peripartum cardiomyopathy

Although multiple mechanisms have been postulated, peripartum cardiomyopathy (PPCM) continues to be a cardiomyopathy of unknown cause. Multiple risk factors exist and the clinical presentation does not allow differentiation among potential causes. Although specific diagnostic criteria exist, PPCM remains a diagnosis of exclusion. Treatment modalities are dictated by the clinical state of the patient, and prognosis is dependent on recovery of function. Randomized controlled trials of novel therapies, such as bromocriptine, are needed to establish better treatment regimens to decrease morbidity and mortality. The creation of an international registry will be an important step to better define and treat PPCM. This article discusses the pathogenesis, risk factors, diagnosis, management, and prognosis of this condition.

Aetiology and risk factors of peripartum cardiomyopathy: a systematic review

BACKGROUND

Peripartum cardiomyopathy (PPCM) is a disorder of unknown aetiology in which heart failure due to left ventricular dysfunction occurs between the last month of pregnancy and first five months post-partum. Theories abound concerning the specific cause and risk factors for PPCM, but none have been accepted universally. The primary objective of this review was to summarize the state of knowledge on the pathogenesis of PPCM, especially in light of recent studies.

METHODS

We searched MEDLINE (January 1966-September 2007), OVID, and reference lists of articles for studies containing information on the aetiology and risk factors for PPCM, and published in English.

RESULTS

The literature reveals a wealth of articles proposing various mechanisms for aetiology and risk factors of PPCM. There is conflicting evidence on the pathogenetic role of viral myocarditis, abnormal immune response to pregnancy, abnormal response to the haemodynamic stress of pregnancy, accelerated myocyte apoptosis, cytokine-induced inflammation, malnutrition, genetic factors, excessive prolactin production, abnormal hormonal function, increased adrenergic tone, and myocardial ischaemia. A number of factors are postulated to increase the risk of the development of PPCM. These include non-Caucasian ethnicity, advanced maternal age, multiparity, poor socioeconomic status, multiple pregnancy and prolonged tocolytic use. The authors call for a strict definition of PPCM that excludes known causes of heart failure, such as the pregnancy-induced hypertensive spectrum of disorders.

CONCLUSION

The aetiology and risk factors for PPCM are poorly defined. There is a need for large-scale multi-centre epidemiological studies and registries to delineate the aetiology and pathogenesis of PPCM.

Clinical and immunologic characteristics in peripartum cardiomyopathy

Peripartum cardiomyopathy (PPCM) is a rare disorder of dilated cardiomyopathy and left ventricular dysfunction occurring in the last month of pregnancy or within 5 months postpartum. Outcome of PPCM is highly variable, comprising clinical improvement and rapid deterioration unresponsive to medical treatment requiring heart transplantation or even death. In this study, we report the clinicopathologic findings of 10 patients with PPCM who were retrospectively identified in our cardiomyopathy registry. During a follow-up of 69+/-27 months, no patient died or required orthotopic heart transplantation. Left ventricular ejection fraction was 38+/-7% at the time of diagnosis and 53+/-7% during follow-up. While all patients had sinus rhythm at the time of diagnosis, three patients presented with left bundle branch block. We found no evidence of viral infection in endomyocardial biopsy samples of seven patients by PCR. Histopathologic findings revealed the presence borderline myocarditis in two of seven patients (29%). Circulating autoantibodies to cardiac tissue of any kind were observed in all patients. In conclusion, in our retrospective observational study, no patient diagnosed with PPCM died or received orthotopic heart transplantation. Improvement of left ventricular ejection fraction was present in eight patients (80%), while LV dysfunction persisted in four patients. Our findings support the hypothesis of an underlying autoimmune pathomechanism in this rare disease.

Stimulatory effects of hyperprolactinemia on aldosterone secretion in ovariectomized rats

BACKGROUND

To evaluate the effects of hyperprolactinemia on aldosterone secretion and its mechanisms of action in ovariectomized (OVX) rats.

METHODS

Hyperprolactinemia was induced by the transplantation of rat anterior pituitary (AP) glands under the kidney capsule for 6 weeks in female rats. Control rats underwent cerebral cortex (CX) transplantation. Four weeks after transplantation, the rats were OVX 2 weeks before decapitation. After decapitation, the trunk blood was collected, and the adrenal glands of CX- and AP-grafted rats were prepared as zona glomerulosa (ZG) cells for in vitro study.

RESULTS

Plasma prolactin and aldosterone in the rats were increased by AP gland transplantation. In the in vitro study, the basal aldosterone secretion by the adrenal ZG cells was higher in AP-grafted rats than in CX-grafted rats. The AP-grafted group showed increased responsiveness to angiotensin II (10(-8) M), KCl (8 x 10(-3) M), or 8-bromo-adenosine 3',5'-cyclic monophosphate (8-br-cAMP; 10(-4) M, a membrane-permeable analogue of cAMP) with regard to aldosterone secretion as compared with the CX-grafted group. N-(2-[p-Bromocinnamylamine]ethyl)-5-isoquinolinesulfonamide (H89; 10(-6), 10(-5) M, a protein kinase A inhibitor) or tetrandrine (10(-5) M, a blocker for both L-type and T-type Ca2+ channels) induced a greater suppression of aldosterone secretion in the AP-grafted group than in the CX-grafted group. No significant differences between the CX- and AP-grafted groups were observed, however, with regard to the adrenocorticotropichormone (10(-9) M)-, forskolin (10(-5) M, an adenylyl cyclase activator)-, or nifedipine (10(-5) M, an L-type Ca2+ channel blocker)-induced responsiveness of aldosterone secretion. In addition, there was no difference in the expression of desmolase (i.e., cytochrome P450 side-chain cleavage enzyme) in ZG cells between AP- and CX-grafted rats. The conversions of 25-OH-cholesterol into pregnenolone in the presence of trilostane (an inhibitor of 3beta-hydroxysteroid dehydrogenase) and corticosterone into aldosterone, as well as the expression of the steroidogenic acute regulatory protein in ZG cells, were greater in AP-grafted rats than in CX-grafted rats.

CONCLUSIONS

These results suggest that hyperprolactinemia increases basal, angiotensin II- and KCl-stimulated aldosterone secretion by ZG cells in OVX rats through activation of T-type Ca2+ channels, the post-cAMP and protein kinase A pathway, cytochrome P450 side-chain cleavage enzyme, and aldosterone synthase, as well as by causing increased expression of steroidogenic acute regulatory protein in ZG cells.

Aetiopathogenesis of peripartum cardiomyopathy: prolactin-selenium interaction?

The aetiology of peripartum cardiomyopathy is unknown. Fragmentary evidence from the published literature are synthesised to suggest a hypothesis that prolactin-selenium interactions resulting in selenium deficiency and/or autoimmunity are responsible for peripartum cardiomyopathy. This hypothesis best explains the various known facts about the disease. The possible link between prolactin and selenium should be explored.

Peripartum cardiomyopathy

Congestive heart failure is an uncommon complication of pregnancy with potentially life-threatening consequences. Peripartum cardiomyopathy is a disease of unknown cause in which severe left ventricular dysfunction occurs during late pregnancy or the early puerperium. In the past, the diagnosis of this entity was made on clinical grounds; however, modern echocardiographic techniques have allowed more accurate diagnoses by excluding cases of diseases that mimic the clinical symptoms and signs of heart failure. Risk factors for peripartum cardiomyopathy include advanced maternal age, multiparity, African descent, twinning, and long-term tocolysis. An extensive search for the causes of peripartum cardiomyopathy has been unrevealing. Treatment includes digitalis, diuretic agents, and vasodilators. Anticoagulation is strongly recommended, especially if ventricular function is persistent. The prognosis of peripartum cardiomyopathy is related to the recovery of ventricular function. Caution is advised in recommending subsequent pregnancy, especially if left ventricular dysfunction is persistent.

Maternal and fetal prolactin in pregnancy-induced hypertension

In plasma from 35 women with pregnancy-induced hypertension (PIH) and 35 normal pregnant women both at 39 weeks of gestation, plasma prolactin levels were measured at 8.30 a.m. (PRL1) and 9.30 a.m. (PRL2) under basal conditions. At delivery umbilical cord blood samples were taken for measurement of fetal prolactin (PRLF). PRL1 and PRL2 were higher in women with PIH, but no significant relations were found between PRL1/PRL2 and blood pressure. PRLF did not differ when infants of mothers with PIH and infants of normal pregnant women were compared, but PRLF had a significant direct independent relation with PRL2. The latter relation may be due to the increase in placental oestrogens during pregnancy, which stimulate both the maternal and fetal hypophyses and their prolactin secretion. PRLF did not show any relation with neonatal morbidity, but PRL1 showed a significant direct relation with the Apgar score at 5 min.

Prolactin stimulation of protein kinase C activity in rat aortic smooth muscle

Prolactin (PRL) activated protein kinase C (PKC) in a dose dependent manner in rat aortic smooth muscle. Aortic strips incubated with sub-nanomolar concentrations of ovine PRL for 25 min. at 37 degrees C showed a significant stimulation of PKC activity in both cytosolic and particulate fractions. This activation could be blocked using either anti-PRL antibodies or 1-(5- isoquinolinesulfonyl)-2-methylpiperazine (H-7), a PKC inhibitor. The results further support the role of PKC in the signal transduction pathway for PRL action and suggest that this activation may be involved in vascular smooth muscle function.

Relationship between serum prolactin concentration, vascular angiotensin sensitivity and arterial blood pressure during third trimester pregnancy

Prolactin may play an important role in the pathogenesis of pregnancy-induced hypertension (PIH) and preeclampsia. In 105 normotensive nulliparous women at 28 to 32 weeks of gestation, the relationship between serum prolactin concentration (PRL) and blood pressure behaviour was examined under standardized conditions. Neither postural change from left lateral to supine recumbency nor the infusion of low doses of angiotensin-II-amide had an effect on PRL levels. Similar mean PRL levels were found in pregnant women with a low angiotensin pressor dose (ADP less than 10 ng x kg-1 x min-1) or "angiotensin sensitivity", a positive supine pressor response (delta pd greater than or equal to 20 mmHg) or an increased serum uric acid concentration (greater than 3.6 mg/dl), which are criteria for an increased risk of developing hypertensive complications. However, in the group of subjects with angiotensin sensitivity, a significant correlation was found (a) between PRL levels and the APD and (b) between PRL levels and diastolic blood pressure increase after 5 min of supine recumbency. These results may reflect diminished dopaminergic activity in the central nervous system, which could influence both blood pressure and prolactin secretion.

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.

Role of prolactin in pregnancy hypertension

In 51 patients with pregnancy hypertension (H) and 51 normotensive gravid women (N), matched for age of gestation, plasma prolactin was measured at 8.30 am (PRL1) and 9.30 am (PRL2) in basal conditions and after 10 minutes of upright posture (PRL3). While in N there was a fall from PRL1 to PRL2 which was nonsignificant, in H there was a significant fall from PRL1 to PRL2. With upright posture there was a further decrease in prolactin in N and a significant increase in H. With multiple regression analysis, systolic and diastolic blood pressure did not show any independent relations with PRL1, PRL2 and PRL3, while serum proteins and proteinuria showed a significant relation with PRL1, as did serum proteins, serum potassium and serum urate with PRL2 and serum urate with PRL3. As has been suggested in primary hypertension, a certain increase in peripheral sympathetic tone, dependent on a decreased central dopaminergic activity, may be present in patients who develop pregnancy hypertension compared to normotensive pregnant controls and may be involved in the pathogenesis of pregnancy hypertension.

Prolactin in severe toxemia of pregnancy

Plasma PRL levels were measured in 111 normal pregnant women and in 21 patients with severe toxemia of pregnancy. Twelve of 21 patients with severe toxemia of pregnancy showed high PRL levels in zone A (greater than mean value + 1 S.D. of PRL values in normal pregnancy). These 12 were significantly lower (P less than 0.02) in the Ccre rate, at 70.2 +/- 19.2 ml/min, than 5 toxemia patients (101.4 +/- 26.7 ml/min) in zone B (mean + 1 S.D. approximately mean) and 4 toxemia patients (110.0 +/- 35.3 ml/ml) in zone C (mean approximately mean -1 S.D.). Also, BUN, proteinuria and uric acid levels in zone A patients were higher than in those in zone B and C. However, no correlation was found between PRL levels and mean diastolic and systolic blood pressure. These results suggest that high PRL concentrations in toxemia of pregnancy may be associated with renal dysfunction.

Paracrine interactions in the anterior pituitary

The topographical affinity between certain cell types in rat anterior pituitary as well as the presence of biogenic amines, neuropeptides, growth and tissue factors in specific cell types suggest participation of paracrine control mechanisms in the regulation of anterior pituitary hormone secretion. Due to the recent advances in the separation of pituitary cell types and the development of three-dimensional cell cultures, direct experimental evidence for control by intercellular messengers has become available. The stimulation of PRL release from superfused pituitary cell aggregates by LHRH has been shown to be mediated by gonadotrophs. Gonadotrophs appear to secrete a factor with PRL-releasing activity. Gonadotrophs also modulate the stimulation of PRL release by angiotensin II. Interaction of somatotrophs with an unknown small-sized cell type strongly amplifies the GH response to adrenaline, GRF and VIP. The latter phenomenon requires the permissive action of glucocorticoids. Some of these in vitro observations can be correlated with recently reported in vivo actions of LHRH, PRL and angiotensin II and with pathophysiological changes in the pituitary.

Interaction of prolactin with adrenal hormones in blood pressure regulation in rats

This study examined the role of the adrenals in the systolic blood pressure (BP) response to ovine prolactin (oPRL) administration as well as the interaction of oPRL with epinephrine (EPI), norepinephrine (NOR), and corticosterone (CORT) on BP in the rat. After intraperitoneal implantation of 7-day osmotic minipumps releasing 0.6 micrograms oPRL/h in 0.9% NaCl on day 1, BP was measured via tail cuff on days 1, 4, and 7 in adrenalectomized (ADX) and sham-ADX male rats (n = 8/group). BP was also measured in ADX rats receiving CORT (27.0 micrograms/h), EPI (1.7 micrograms/h), NOR (5.0 micrograms/h), CORT + oPRL, EPI + oPRL, and NOR + oPRL by osmotic pump (n = 8/group). All ADX rats not receiving CORT were given 1.0% NaCl replacement for drinking water. oPRL increased BP over 17 days in sham-ADX rats (P less than 0.01) but had no effect in ADX rats. BP also increased over 7 days in ADX animals receiving CORT, EPI (P less than 0.01), and NOR (P less than 0.01). BP responses to CORT + oPRL and EPI + oPRL were similar to those of CORT and EPI alone, respectively. However, the BP response to NOR + oPRL on day 4 was 250% (P less than 0.01) that to NOR alone and similar to NOR alone on day 7. Replacement of CORT + NOR + oPRL did not prolong the oPRL-induced sensitization to NOR. In no instance did oPRL, by itself, alter heart rate. These data suggest that the pressor response to oPRL requires the presence of the adrenal glands and that oPRL transiently potentiates the pressor response to NOR in vivo.

Zinc and vascular reactivity in rat mesenteric vessels: possible altered dihomo-gamma-linolenic acid metabolism in spontaneously hypertensive rats

Zinc at a concentration of 0.4 microgram/ml potentiated pressor responses to norepinephrine in isolated perfused mesenteric vessels of SHR and WKY. At a higher concentration, 3.2 micrograms/ml, it inhibited responses to norepinephrine in WKY but produced no such inhibition in SHR. However, a transient potentiation was observed in SHR with the higher concentration. Pressor responses to potassium in WKY were not affected by zinc at either concentration. In SHR, however, the higher dose of zinc inhibited pressor responses to potassium. The low dose had no effect. Since effects of zinc may be mediated by release of DGLA, we suggest that in SHR DGLA release may be impaired.

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.

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.

A prospective study of circulating prolactin during primigravid pregnancy

A prospective, cross-sectional study of 164 primigravid patients was conducted to determine the role of prolactin in the pathogenesis of pregnancy induced hypertension. Clinically normal patients had peripheral venous blood sampled from the lateral and recumbent positions monthly in the morning during their last two trimesters in labor and six weeks postpartum. One-third of the patients had 24 hour urine collections. Homologous double antibody radioimmunoassays were performed to determine prolactin levels. The data were analyzed according to pregnancy outcome: pregnancy-induced hypertension or normotensive throughout pregnancy. Acute positional change did not influence prolactin level. Prolactin levels were significantly elevated in the hypertensive outcome group only at 37-39 weeks and were not correlated with sodium excretion. We conclude that circulating prolactin does not play a significant role in pathogenesis of pregnancy-induced hypertension, but perhaps the elevated levels may be reflecting pathophysiologic changes.

Renal and vascular activity of prolactin preparations. Contamination of prolactin preparations with ADH and implications on renal and vascular prolactin research

Prolactin, as a "broad spectrum hormone", has been described to exert also vascular and renal actions in laboratory animals and in humans. However, prolactin preparations of various species are contaminated with neurohypophysial hormones (ADH, oxytocin) which possess vascular and renal activities. Antisera against ADH, oxytocin and prolactin are rather specific inactivators of the biologic activity of the respective hormone; the oxytocinasevasopressinase system of pregnancy plasma destroys ADH and oxytocin. Incubation-identification procedures with antisera against ADH, oxytocin and prolactin and with pregnancy plasma revealed that changes in blood pressure, urine flow and urinary osmolarity cannot be ascribed to prolactin per se but to the ADH impurity of prolactin preparations. Furthermore, recent metabolic studies in normally hydrated, overhydrate and dehydrated animals and humans have shown that prolactin does not affect renal water and electrolyte excretion. Thus, earlier reports on vascular and renal activity of prolactin in laboratory animals and humans should be viewed with great caution. Elimination of neurohypophysial hormone impurities of prolactin preparations by incubation with either ADH and oxytocin antisera or with pregnancy plasma provides techniques for better assessment of the real biologic effects of the prolactin molecule.

Maternal plasma and amniotic fluid prolactin levels in normal and hypertensive pregnancy

Prolactin levels in the plasma and amniotic fluid of 121 normal pregnant women, 78 women with pre-eclampsia and 30 women with essential hypertension complicating pregnancy were determined by radioimmunoassay. Mean prolactin levels in plasma, but not in amniotic fluid, were significantly lower than controls in the group with pre-eclampsia (P less than 0.01) and in the group with essential hypertension (P less than 0.05). These findings suggest altered production and/or clearance of prolactin from the maternal compartment in these patients and may explain their increased response to pressor agents.

Prolactin and hypertension

Progesterone (2.5 mg per kilogram) caused sustained hypertension in rabbits. When the same dose of progesterone was administered together with prolactin (1.25 mg. per kilogram), there was no increase in the blood pressure. In rabbits with progesterone-induced hypertension, the addition of prolactin caused a sharp drop in blood pressure. It is suggested that prolactin acts by reducing the sensitivity of the blood vessels to circulating pressor substances and further that a reduced prolactin response may be the cause of heightened sensitivity to pressor substances observed in pre-eclampsia.