Studies on factors which influence cyclic 3', 5'- adenosine monophosphate production in-vivo and in-vitro

Fraser, William Duncan (1995) Studies on factors which influence cyclic 3', 5'- adenosine monophosphate production in-vivo and in-vitro. MD thesis, University of Glasgow.

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Daily variations in plasma (PcAMP), urine (UcAMP), nephrogenous cAMP (NcAMP) and hormones known to affect cAMP metabolism were studied. Circadian variations in PTH (1-84) and NcAMP were observed. Both increased significantly overnight and early morning. PcAMP and glucagon decreased overnight and following meals. PcAMP had greater fluctuations over 24h than glucagon indicating several hormones have transient effects on PcAMP production.
A 7h shift of the sleep-wake cycle moved sleep-associated prolactin secretion but caused minimal alteration in PTH (1-84). NcAMP secretion overnight decreased, suggesting PTH (1-84) end organ responses may be modified by other hormones and effectors when sleep is disturbed. PcAMP decreased overnight reaching a nadir corresponding to the lowest glucagon concentration. PcAMP fluctuations were greater when compared to normal 24h profiles.
Following a 96h fast serum phosphate, PTH (1-84) and NcAMP circadian rhythms were attenuated. Mean serum calcium and NcAMP production were significantly decreased. Fasting increased variability in PTH (1-84) secretion which may be an important signalling mechanism inducing bone resorption. Serum phosphate fluctuations may play an important role in the genesis of PTH (1-84) and NcAMP circadian rhythms. Mean glucagon concentration was increased with attenuation of the circadian rhythm and mean PcAMP concentration increased. Dissociation of bone and kidney effects of PTH (1-84) in fasting may be due to acute acidosis.
In 1°HPT the concerted overnight increase in PTH (1-84) and NcAMP was absent. Parathyroid adenoma resection restored phosphate, PTH (1-84) and NcAMP circadian rhythms. The phosphate increase may stimulate PTH (1-84) secretion which then influences the phosphate rhythm. Mean PcAMP was increased in 1°HPT but 24h PcAMP profiles differed little pre and post surgery.
24h studies indicate control of the circadian rhythms of PTH (1-84), NcAMP and PcAMP is complex. Several hormones act in concert to regulate cAMP generation and end organ responses.
Thyrotoxic patients had mean PTH (1-84) and NcAMP lower than normals, hypothyroid and treated hypothyroid patients. Increased serum calcium decreases PTH (1-84) and subsequently NcAMP in thyrotoxicosis. In hypothyroid patients PTH (1-84) and NcAMP are dissociated with mean PTH (1-84) increased compared to thyrotoxic patients but NcAMP remained similar to euthyroid subjects indicating resistance to PTH (1-84) . Thyroxine treated patients have mean PTH (1-84) concentrations lower than euthyroid controls with a relatively increased NcAMP. PcAMP is increased in thyrotoxic and thyroxine treated patients and decreased in hypothyroid patients.
Synacthen increased PcAMP with the pattern similar to the cortisol profile. This was either due to simultaneous release of cAMP and cortisol following adrenal stimulation or was a direct effect of cortisol. Dexamethasone decreased NcAMP and UcAMP with a variable effect on PcAMP. Variability in the PcAMP response is probably due to combined effects of dexamethasone and cortisol whereas decreased UcAMP and NcAMP production may represent altered kidney cell sensitivity to PTH (1-84) caused by decreased cortisol.
In 1°HPT patients a significant correlation between PTH (1-84) and NcAMP exists (r=0.66, p<0.001). A subgroup of 1°HPT patients have NcAMP inappropriately high for the measured PTH (1-84). Two patients had increased PTHrP but in the majority PTHrP was low and the discrepancy was unexplained. In three patients PcAMP concentration was lower than expected. Twelve 1°HPT patients, divided equally, received atenolol or placebo and were studied prospectively for 6 months. No significant effect of atenolol on any parameter was observed.
In patients with Paget's disease treated with bisphosphonate (APD) calcium decreased, PTH (1-84) and NcAMP increased significantly. Patients with HCM prior to therapy with APD had a suppressed or low PTH (1-84) . A high percentage had elevated NcAMP indicating the presence of a circulating humoral factor, probably PTHrP, causing hypercalcaemia. APD therapy in HCM patients resulted in a significant reduction in serum calcium. No patient developed significant hypocalcaemia. PTH (1-84) secretion was stimulated as serum calcium decreased and PTH (1-84) was detectable or elevated whilst some patients remained hypercalcaemic. The absolute concentration rather than rate of decrease of serum calcium was more important in regulating PTH (1-84) secretion. NcAMP correlated poorly with PTH (1-84) in treated HCM patients. An increase then decrease in NcAMP was observed associated with a decrease then increase in TmP04 indicating PTHrP may increase transiently following APD therapy.
PTHrP measurement confirmed the hormone's role in the aetiology of HCM and a strong correlation of PTHrP with NcAMP. In HCM patients with breast, lung and kidney malignancy PTHrP secretion commonly results in elevated NcAMP. Patients with HCM and haematological, gastrointestinal and ear-nose-throat malignancies have low circulating PTHrP and normal or low NcAMP. Squamous cell carcinomas commonly secrete PTHrP. 78% of normals had detectable PTHrP which may reflect sample collection into protease inhibitor tubes, age distribution of the normal,population, assay sensitivity and properties of the assay antibodies. Alternatively the assay may be subject to nonspecific binding effects or matrix effects. A subgroup of patients was detected with low PTHrP and low PTH (1-84) with inappropriate or elevated NcAMP. These patients may be producing a factor which stimulates NcAMP production. This factor (s) may be a fragment of PTHrP or PTH not recognised by the IRMAs or may be a new molecule causing hypercalcaemia mediated via cAMP.
Superfusion of rat renal tubules was used to study cAMP production in response to PTH stimulation. Optimal conditions producing a consistent, reproducible cAMP response were identified. Tubules were equilibrated on columns for 90 min, prior to stimulation with 2.5 units bovine PTH (1-84) at 10 min intervals. A significant decrease in cAMP production in response to PTH was observed when tubules were perfused with buffer acidic (pH 7.1) relative to normal physiological pH (pH 7.4). Perfusion with buffer relatively alkalotic (pH 7.65) had no significant effect on PTH stimulated cAMP production. 7mM arginine hydrochloride in the perfusate significantly decreased cAMP production. Superfusion of rat renal tubules was a reliable, reproducible method for studying the action of PTH.

Item Type: Thesis (MD)
Qualification Level: Doctoral
Subjects: Q Science > QH Natural history > QH345 Biochemistry
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: O'Reilly, Dr. Denis StJ
Date of Award: 1995
Depositing User: Alastair Arthur
Unique ID: glathesis:1995-74310
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 25 Jul 2019 08:46
Last Modified: 25 Jul 2019 08:46

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