Physiological cortisol levels block the inhibition of vascular reactivity produced by prolactin

Regulation of Cytoplasmic Calcium: Interactions between Prostaglandins, Prostacyclin, Thromboxane A2, Zinc, Copper and Taurine

The regulation of cytoplasmic calcium is a key process in nerve tissue. Using a smooth muscle model we have shown that prostaglandine (PG) E2 probably regulates entry from extracellular fluid, whereas the release from intracellular stores depends on the interplay between thromboxane (TX) A2, PGEl and prostacyclin. Hormones and other agents interact with this system in the following ways: vasopressin, angiotensin and inositol mobilize arachidonic acid from membrane phospholipids and increase synthesis of PGE2 and TXA2, Cortisol blocks this action. Prolactin and zinc mobilize dihomo-γ-linolenic acid and increase synthesis of PGEL These effects can be blocked by Cortisol, lithium and taurine, three agents which on their own have no effect on basal PG production. Epileptogenic agents like penicillin and picrotoxin also stimulate PG synthesis, while diphenylhydantoin is a PG antagonist and diazepam is a TXA2 antagonist. The effects of all these agents occur at concentrations which are physiological in the case of the natural ones, and readily attained in human plasma in the case of the drugs. In view of recent evidence that calcium may be important in demyelination and considering the established role it plays in nerve conduction and synaptic transmission, we suggest that these observations may be of significance in understanding Friedreich's ataxia.

Release of fatty acids by perfused vascular tissue in normotensive and hypertensive rats

The release of fatty acids from perfused mesenteries of spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats (WKY) was studied. The release of the prostaglandin precursors dihomogammalinolenic acid, arachidonic acid, and eicosapentaenoic acid was reduced in SHR when compared with age-matched WKY. The release of all other fatty acids detected in the effluent was also reduced. The differences in fatty acid release were evident even when tissue levels of the fatty acids were similar or higher in SHR than in controls. The addition of evening primrose oil and fish oil into the diet partially corrected these defects. Evening primrose oil and fish oil both attenuated increases in blood pressure, but fish oil was more potent than primrose oil. Although both diets reduced vascular reactivity, primrose oil was more effective with lower doses of norepinephrine whereas fish oil blunted the effects of both low and high doses of norepinephrine. The possible mechanisms for the effects of primrose oil and fish oil on vascular reactivity are briefly discussed.

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.

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.

A possible mechanism of inotropic action of prolactin on rat heart

Prolactin possesses positive inotropic actions in isolated heart preparations although the mechanism of this influence is not understood. Our study was designed to investigate the mechanism of this effect on the rat heart. Prolactin (50 ng/ml) produced a time-dependent increase (60%) in contractile force that reached maximum after 30 min and remained steady for a further 30 min. A similar time-dependent phenomenon was seen with 200 ng/ml prolactin although the maximum inotropic effect was reduced. Indomethacin (30 micrograms/ml) significantly reduced the inotropic effect of both prolactin concentrations although the effect of the hormone was not related to the release of 6-keto-PGF1 alpha, the prostacyclin metabolite. Propranolol (1-20 micrograms/ml) significantly reduced the positive inotropic effect of prolactin. Prolactin however had no influence on myocardial adenylate cyclase activity. Hearts that were removed from animals pretreated with 1.25 or 2.50 mg/kg reserpine did not respond to prolactin administration. It is suggested that the inotropic influence of prolactin is mediated by endogenous catecholamine liberation.

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.

Spironolactone inhibits vascular reactivity by a prostaglandin related mechanism unconnected with aldosterone

Effects of aldosterone and spironolactone on the vascular responses to noradrenaline and potassium were studied in the perfused rat mesenteric vascular bed. Aldosterone did not modify the response to either vascular agent. Spironolactone inhibited the vascular response to both pressor agents in a dose-dependent manner. The inhibitory effect of spironolactone was not altered by various concentrations of aldosterone and ouabain. However it was not apparent in preparations in which endogenous prostaglandin synthesis had been abolished in indomethacin. The observations suggest that spironolactone has actions on vascular reactivity which are not related to aldosterone or to sodium/potassium pumping. They may depend on modification of prostaglandin biosynthesis.

Interactions between dopamine and prostaglandins on vascular reactivity to noradrenaline: dopamine inhibits the action of PGE1

Interactions between dopamine and responses to noradrenaline as modulated by prostaglandins (PGs) were studied in the perfused rat mesenteric vascular bed. When perfused alone dopamine up to a concentration of 10(-7)M neither changed baseline pressure nor modified the pressor response to noradrenaline. Dopamine at 10(-9) to 10(-7)M significantly inhibited responses to noradrenaline which had been enhanced by the presence of 10(-10) to 10(-8) M PGE1. In preparations in which vascular responses to noradrenaline had been abolished by indomethacin and restored by adding PGE1, 10(-9) to 10(-7)M dopamine significantly inhibited the restored responses. Dopamine also attenuated the inhibitory effects of prostacyclin on pressor responses to noradrenaline but it did not change the actions of either PGE2 or PGF2 alpha. Pimozide, a mainly centrally acting dopamine antagonist, did not interfere with these peripheral actions of dopamine. The dopamine effects were blocked by another dopamine antagonist, metoclopramide. Dopamine can inhibit the effects of PGE1 and prostacyclin in the rat mesenteric vascular bed, possibly by interacting with specific dopamine receptors.

A new concept of lifestyle-related cardiovascular disease: the importance of interactions between cholesterol, essential fatty acids, prostaglandin E1 and thromboxane A2

A proposal to account for the inter-relationships between established risk factors and cardiovascular disease is presented. In this concept, the critical substance is prostaglandin (PG) E1 which is a vasodilator, an inhibitor of platelet aggregation, an inhibitor of cholesterol and collagen biosynthesis and an inhibitor of smooth muscle proliferation. PGE1 biosynthesis is enhanced by the essential fatty acid, linoleic acid, by the platelet aggregating agent thromboxane (TX) A2, by cholesterol and by melatonin. These factors may participate in a negative feedback control loop. As a result of the operation of this loop, any tendency for PGE1 levels to fall is followed by increased cholesterol and TXA2 biosynthesis, and enhanced platelet aggregation, vasoconstriction, smooth muscle proliferation and collagen biosynthesis. Enhancement of PGE1 biosynthesis will have the opposite effects. Factors known to increase PGE1 biosynthesis include essential fatty acids, vitamin C, ethanol, pyridoxine, zinc and probably niacin, all of which are known to have some protective effects against cardiovascular disease. The hypothesis predicts that lowering of cholesterol biosynthesis by any method other than enhanced PGE1 formation, while reducing the risk of cardiovascular disease, will increase the risk of other disorders. The hypothesis suggest new approaches to treatment and new ways of combining existing treatments. Colchicine, which at low concentrations may imitate and action of melatonin, has particularly interesting possibilities. Colchicine and related compounds have already been shown to have potent cholesterol-lowering and anti-atherogenic actions in both humans and animals.

Lithium treatment of hypothermia caused by electric shock

Prostaglandins have hyperthermic as well as hypothermic effects, and lithium, which inhibits the conversion of dihomogammalinolenic acid to PGE1, can produce hyperthermia. Electroconvulsive shock raises prostaglandin F in rat cerebral cortex and in the case presented here lithium normalized temperature in a patient who became hypothermic after she accidentally received an electric shock.

Possible role of prostaglandin E1 in the affective disorders and in alcoholism

Prostaglandin (PG)E1 may play an important part in the affective disorders, with an excess being present in mania and a deficiency in depression. Platelets from manic patients produce more PGE1 than normal while those from depressive patients produce less. Ethyl alcohol stimulates PGE1 production whereas lithium inhibits it. Alcoholics will tend to have raised PGE1 concentrations while drinking, but, because precursor supplies are limited, when alcohol concentrations fall PGE1 concentrations may fall sharply leading to depression. PGE1 biosynthesis may be affected by nutritional factors including essential fatty acids, pyridoxine, vitamin C, and zinc. Nutritional approaches may be of value in both depression and alcoholism.

The nutritional regulation of T lymphocyte function

Prostaglandin (PG) E1 plays a major role in the regulation of thymus development and T lymphocyte function and the evidence for this is reviewed. The production of PGE1 is dependent on nutritional factors with linoleic acid, gamma-linolenic acid, pyridoxine, zinc and vitamin C playing key roles. Inadequate intake of any one of these will lead to inadequate PGE1 formation and defective T lymphocyte function. Megadoses of any one are likely to be only minimally effective in the absence of adequate intakes of the others. By careful attention to diet it should be possible to activate T lymphocyte function in the large number of diseases including rheumatoid arthritis, various auto-immune diseases, multiple sclerosis, and cancer in which such function is defective. It is possible that T lymphocytes may require both endogenous and exogenous PGE1 in order to function adequately. It is therefore of particular interest that many cancer cells and virally infected cells are unable to make PGE1 because they cannot convert linoleic acid to gamma-linolenic acid. The direct provision of gamma-linolenic or dihomo-gammalinolenic acids in these situations is worthy of full investigation.

The regulation of prostaglandin E1 formation: a candidate for one of the fundamental mechanisms involved in the actions of vitamin C

Vitamin C stimulates the formation of PGE1 in human platelets. The effect occurs over the physiologically relevant range of concentrations. PGE1 is required for T lymphocyte function and plays a major part in the regulation of immune responses. PGE1 is also important in the regulation of collagen and ground substance metabolism, in cholesterol metabolism and in regulation of responsiveness to insulin. It is proposed that defective formation of PGE1 could account for many of the features of scurvy and for many of the reported therapeutic effects of vitamin C. If correct, vitamin C will be of value only in conjunction with an adequate supply of dihomogammalinolenic acid, the precursor of PGE1. Essential fatty acids, pyridoxine and zinc are all required to achieve this.

Cellular basis of prolactin action: involvement of cyclic nucleotides, polyamines, prostaglandins, steroids, thyroid hormones, Na/K ATPases and calcium: relevance to breast cancer and the menstrual cycle

The mechanisms of prolactin action are uncertain. Prolactin effects are characterised by great variability in the prolactin concentration required to produce a response and by the frequent occurrence of "bell-shaped" dose response curves. Relationships between prolactin and thyroid hormones and steroids are difficult to understand: both steroids and thyroid hormones may potentiate or inhibit prolactin effects in different situations. At the second messenger level, claims have been made that cyclic nucleotides, polyamines, Na/K ATPases and prostaglandins are involved. There is evidence that prostaglandin E1 may be an important second messenger and it is proposed that this is the clue to understanding the complexity. At low concentrations PGE1 enhances intracellular calcium release and at high concentrations PGE1 inhibits calcium release. The other second messengers proposed are all probably dependent upon calcium. Many prolactin effects may involve both PGE1 and another second messenger: at low prolactin concentrations PGE1 will potentiate the other messenger whereas at high concentrations the PGE1 effect will be inhibitory leading to a bell-shaped response. Thryoid hormones seem to enhance while glucocorticoids inhibit PGE1 synthesis. PGE1 dependent effects will thus be enhanced by thyroid hormone and blocked by cortisol. The reverse will be true of effects dependent on other second messengers.

Plasma prolactin in pregnancy induced hypertension

In a prospective study, plasma prolactin concentration at 37 to 41 weeks gestation in 45 primigravidae showed a significant correlation with maximal rise in diastolic blood pressure between levels at 7 to 16 weeks and those measured between 28 weeks and delivery.

The roles of prostaglandins and calcium accumulation in muscular dystrophy

There is good evidence that abnormal calcium accumulation may be a final common pathway of muscle degeneration in the muscular dystrophies. Prostaglandins are able to promote calcium entry into cells and excess prostaglandin activity coupled with a defect in intracellular calcium release could cause toxic accumulations of calcium in intracellular organelles such as mitochondria. Serotonin stimulates prostaglandin synthesis while tricyclic antidepressants inhibit calcium release from intracellular organelles thus possibly accounting for the models of muscular dystrophy reported using this combination. The prostaglandin/calcium hypothesis can account for the effects of vitamin E, steroids and local anaesthetic-like drugs in muscular dystrophy. Since many drugs already in clinical use for other purposes can be used to control prostaglandin synthesis or action this hypothesis has immediate potential clinical applications.

Schizophrenia as a prostaglandin deficiency disease

Evidence that schizophrenia may be a prostaglandin deficiency disease comes from three main sources: (1) all effective antischizophrenic drugs stimulate prolactin secretion and prolactin is a potent stimulator of prostaglandin synthesis; (2) schizophrenics are resistant to pain and inflammation and are free of rheumatoid arthritis and there is increasing evidence that prostaglandins play important roles in pain, inflammation, and rheumatoid arthritis; (3) high doses of drugs recently shown to be prostaglandin antagonists cause schizophrenia-like syndromes. The hypothesis is not necessarily inconsistent with current transmitter theories of schizophrenia since prostaglandins modify transmitter secretion and action. It does indicate radically new approaches to investigation, treatment, and drug design not suggested by the transmitter concepts.

Roles of prostaglandins suggested by the prostaglandin agonist/antagonist actions of local anaesthetic, anti-arrhythmic, anti-malarial, tricyclic anti-depressant and methyl xanthine compounds. Effects on membranes and on nucleic acid function

We have recently found that local anaesthetic, anti-malarial, anti-arrhythmic, tricyclic antidepressant and methyl xanthine compounds behave as prostaglandin antagonists at drug concentrations readily attainable in human body fluids. We have also found that various hormones, including prolactin, vasopressin and angiotensin when present in physiological concentrations can be potent stimulators of prostaglandin synthesis. Cortisol has no effect on basal prostaglandin systhesis but in physiological concentrations is able to reverse the effects of other hormones. The drugs are thought to act mainly by interfering with either calcium interactions with membranes, or with cyclic nucleotide synthesis or degradation or with nucleic acid function. We propose that prostaglandins play crucial roles at all three points. The hypothesis leads to new approaches to many aspects of cell regulation, to the functions of many systems, especially the neuromuscular, cardiovascular, renal and respiratory systems. It also has profound implications for drug design and evaluation and for the understanding of many clinical disorders.

Chloroquine, quinine, procaine, quinidine, tricyclic antidepressants, and methylxanthines as prostaglandin agonists and antagonists

Chloroquine, quanine, procaine, quinidine, clomipramine, theophylline, and caffeine have been shown to be strong prostaglandin antagonists and weak agonists. The antagonist effect is clearly demonstrable at concentrations reached in human plasma when the drugs are used therapeutically. This suggests that prostaglandins are important in several situations in which their role has hitherto been unsuspected. New approaches to the development of prostaglandin antagonists and new uses for established drugs are indicated. In a preliminary study chloroquine has been successfully used to close patent ductus arteriosus in three infants.

Cortisol in physiological concentrations acts within minutes to modify effects of prolactin and growth hormone on prostaglandin secretion: importance of effect in modulating cellular responses to calcium and cyclic nucleotides

We propose that intracellular prostaglandins (PGs) are essential for the final expression of the effects of second messengers in most cells. We suggest that the amounts of PGs required are very small (in the picomolar range) and are much lower than those used in most current PG studies. We suggest that while therapeutic levels of inhibitors of PG synthetase may be adequate to block the overflow of PGs from cells, they are in most cases unlikely to reduce intracellular PGs sufficiently to test the role of such PGs. We propose that there is a basal level of PG synthesis unaffected by hormones but that above this level PG synthesis is regulated by the interplay between physiological levels of cortisol, prolactin, growth hormone and thyroid hormones. For the most part prolactin seems to stimulate PG synthesis and cortisol to inhibit it: cortisol has, however, no inhibitory effect on basal PG synthesis. In reducing prolactin-stimulated PG synthesis cortisol is 1000-2000 times more potent than indomethacin on a molar basis. We suggest that the regulation of intracellular PG levels is the mechanism of the so-called "permissive" actions of these hormones. These concepts could prove important in the understanding of many aspects of physiology and pathophysiology including diurnal and seasonal changes in hormone responsiveness. They are also relevant to the use of established drugs and the design of new ones.