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

Some theoretical questions of the peptide and steroid hormone regulation. Part III.: The RLVC hypothesis as a possible mode for the interaction of the peptide and steroid hormone regulation

My previous models for peptide and steroid hormone actions, in accordance with their biological and molecular-informational requirements, suggested possible mechanisms for the subcellular interactions of the two regulatory systems. In my hypothesis the homogeneous or heterogeneous multiligand binding-proteins (receptors, characterized previously) have the ability to bind both steroids and (cyclic) nucleotide phosphates in a unique distribution and sequence. The specificity of such a Receptor-Ligand Variable Complex (RLVC), the pattern of the steroid and/or (cyclic) nucleotide phosphate ligands, is determined by the equilibrium between the actually acting steroids and peptides on the one hand and by the binding possibilities of the receptor on the other.

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.

The relationships between cyclic AMP, calcium and prostaglandins as second messengers

There is considerable dissatisfaction with present second messenger hypotheses involving cyclic nucleotides and calcium. The recent findings that methyl xanthines and adenosine are prostaglandin antagonists casts doubt on much of the evidence in favour of the cyclic AMP hypothesis. There is evidence that allosteric sites may modify the binding of calcium and cyclic nucleotides to key cellular regulators and that a range of substances including steroids, adenosine and prostaglandins may occupy those sites. This concept introduces much needed flexibility into the second messenger concept, allows many experiments to be reinterpreted and has major implications throughout the biomedical sciences.

Prostaglandin E1 modifies nerve conduction and interferes with local anaesthetic action

Prostaglandin E1 (PGE1) altered both the amplitude and conduction velocity of the compound action potential in frog sciatic nerve. Concentrations up to 1 ng/ml increased both amplitude and conduction velocity but at higher concentrations both effects were reversed. Procaine, chloroquine, indomethacin and SC19220 all reduced action potential amplitude and conduction velocity. These local anaesthetic type actions could be partially or completely prevented by PGE1.

Adenosine as a natural prostaglandin antagonist in vascular smooth muscle

Adenosine has actions on smooth muscle similar to those of prostaglandin (PG) antagonists. Like some PG antagonists it is a phosphodiesterase inhibitor and seems to interfere with calcium effects. It has agonist/antagonist interactions with theophylline, a PG antagonist. In rat mesenteric vascular smooth muscle adenosine blocked responses to noradrenaline which depend on release of intracellular calcium but not those to potassium ions which depend on calcium entry from extracellular fluid. Partial inhibition of endogenous PG synthesis by indomethacin enhanced the adenosine effect. In preparations in which vascular reactivity had been abolished by indomethacin and then partly restored by 1 or 5 ng/ml PG2, adenosine also inhibited responses to noradrenaline: the curve for the 5 ng/ml PG2 concentration was to the right of and parallel to the 1 ng/ml curve consistent with a competitive interaction between adenosine and PG2. Similar interactions between adenosine and PG2 were shown in human lymphocytes in which activation also depends on calcium release. These findings suggest how calcium-dependent metabolic responses may be controlled and indicate further reasons for caution in the interpretation of cyclic AMP experiments.

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.