Parathyroid hormone secretion in the absence of extracellular free Ca2+ and transmembrane Ca2+ influx

Nifedipine, but not verapamil, acutely elevates parathyroid hormone levels in premenopausal women

BACKGROUND

Calcium channel antagonist therapy in humans has been associated with changes in anterior pituitary, thyroid and adrenal hormone secretion. Human studies assessing effects of calcium channel antagonists on calciotropic hormones have been few and typically involved small numbers of subjects studied for short periods of time. Few of these previously published studies have included women. The endocrine effects of calcium channel antagonists in women have become increasingly important as more women are taking these agents for diseases such as hypertension, angina, Raynaud's phenomenon and migraine.

OBJECTIVE

To assess both acute and chronic effects of calcium channel antagonists on calciotropic hormones in women.

DESIGN

A four-week prospective, randomized trial.

SUBJECTS

Twenty-nine premenopausal women, randomly assigned to receive either 240 mg of sustained release verapamil or 30 mg of sustained release nifedipine daily. LABORATORY END-POINTS: Total and ionized serum calcium, phosphate, creatinine, parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrP) and calcitonin, measured at baseline, after 24 hours, and 28 days of treatment.

RESULTS

Total and ionized calcium, phosphate, creatinine, PTHrP and calcitonin levels were not altered significantly after 24 hours or 28 days in any of the subjects, when compared to baseline. There were no significant differences in PTH levels after 24 hours or 28 days of verapamil treatment. There was a significant increase in serum PTH levels after 24 hours of nifedipine therapy; however, these differences were not evident after 28 days of therapy.

CONCLUSIONS

The short-term administration of nifedipine results in increased release of parathyroid hormone; however, long-term administration has no significant effect on the concentrations of calciotropic hormones.

Calcium binding by parathyroid cell plasma membranes

Parathyroid hormone (PTH) secretion from parathyroid glands is controlled mainly by extracellular calcium both in vivo and in vitro. In this study, the Ca2+ binding properties of bovine parathyroid cells have been investigated on a highly purified plasma membrane preparation with flow dialysis techniques. Scatchard plot analysis of the data shows the existence of at least two different binding sites: the high affinity have an apparent Kd1 of 6.6 X 10(-5) M and a capacity (n1) of 20.1 nmol/mg protein. The low affinity sites have an apparent Kd2 of 2.6 X 10(-4) M and a capacity (n2) of 37.7 nmol/mg protein. Furthermore, at higher total Ca2+ concentrations, additional bindings with an apparent Kd3 in the millimolar range and a capacity (n3) of 118 nmol/mg protein are detectable. Neither the apparent Kd's nor n's of these calcium binding sites was affected by isotonic substitution of the medium with NaCl or KCl, while the number of binding sites increased in Choline Cl. On the other hand, the presence of 5-10 mM LiCl in isotonic Na-K medium caused a marked decrease in calcium binding from these sites. LiCl is the only monovalent cation that both in vitro and in vivo is able to enhance PTH release regardless of high or low extracellular Ca2+ concentrations. Therefore the observed displacement of calcium from calcium binding sites seems to be rather specific and could correlate with the observed enhanced PTH secretion in parathyroid cells. It is proposed that the first event in the regulation of PTH secretion by extracellular Ca2+ is the binding of Ca2+ to the low affinity, high capacity sites present on the surface of parathyroid cells and that calcium occupancy of those sites is the necessary event to initiate intracellular signals leading to inhibition of PTH secretion.

Ouabain and low extracellular potassium inhibit PTH secretion from bovine parathyroid cells by a mechanism that does not involve increases in the cytosolic calcium concentration

We have previously found that high extracellular calcium (Ca++) concentrations inhibit PTH release in association with a threefold to fourfold rise in cytosolic Ca++ concentration. Recent data have also shown that low extracellular potassium (K+) concentration or ouabain also inhibits PTH release to an extent comparable to that seen with high Ca++ and produce a marked rise in the intracellular sodium (Na+) content. These results suggested that low K+ and ouabain might modulate PTH release through increases in cytosolic Ca++ related to alterations in Na+-Ca++-exchange. In the present studies, we have examined further the mechanism(s) by which inhibition of the Na+-K+-ATPase regulates PTH release. Exposure of cells loaded with the Ca++-sensitive dye QUIN-2 to low K+ produced a 10% to 17% increase in cytosolic Ca++ at 0.5 to 1.0 mmol/L extracellular Ca++, which was statistically significant only at 0.75 mmol/L Ca++. In contrast, low K+ caused a statistically significant decrease in cytosolic Ca++ at 1.5 to 2 mmol/L Ca++, while ouabain lowered cytosolic Ca++ significantly by 23% to 46% at all Ca++ concentrations examined (0.5 to 2 mmol/L). Low K+ or ouabain had no effect on cellular levels of ATP or GTP or intracellular pH measured using the pH-sensitive dye BCECF [2', 7'-bis(carboxyethyl)-5,6-carboxyfluorescein]. The inhibition of secretion by low K+ or ouabain, unlike that due to high extracellular Ca++, was not reversed by TPA (12-O-tetradecanoyl phorbol 13-acetate), an activator of protein kinase C. Low K+ did produce a modest (30% to 40%) lowering of agonist-stimulated but not basal cAMP content.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus-secretion coupling of parathyroid hormone release: studies of 45Ca and 86Rb fluxes

Pieces of rat parathyroid glands were used to study fluxes of 45Ca and 86Rb. The uptake of 45Ca increased with the extracellular Ca2+ concentration up to at least 5 mM. A rise of extracellular Ca2+ had dual effects on 45Ca efflux in terms of an initial stimulation and a subsequent inhibition. However, K+ depolarization neither affected the uptake nor the efflux of 45Ca indicating a lack of voltage-dependent Ca2+ channels. The depolarization obtained with exposure to Ca2+ cannot be attributed to a decreased K+ permeability, since the 86Rb concentrating ability diminished and the efflux of the isotope increased when parathyroid pieces were exposed to a raised Ca2+ concentration. A stimulation of 86Rb efflux by the Ca2+ ionophore A-23187 indicated that the parathyroid cells possess a K+ permeability activated by cytoplasmic Ca2+. It is suggested that Ca2+ fluxes through channels sensitive to activation by Ca2+ are important both for the membrane potential and the cytoplasmic Ca2+ activity.

Defective regulation of the cytosolic Ca2+ activity in parathyroid cells from patients with hyperparathyroidism

The parathyroid hormone (PTH) release and cytosolic Ca2+ activity were determined in normal bovine parathyroid cells and parathyroid cells obtained from patients with hyperparathyroidism (HPT). There was a sigmoid relation between the cytosolic Ca2+ activity and the extracellular calcium concentration between 0.5 and 6.0 mmol/l. The PTH release was inhibited in parallel with the rise in the cytosolic Ca2+ activity. Both the hormone release and the cytosolic Ca2+ activity were lower in cells from human adenomas and hyperplastic glands, and in comparison with the bovine preparations these cells had higher set points for the cytosolic Ca2+ activity and PTH release. There was a close correlation between the individual set points for the cytosolic Ca2+ activity and PTH release in a material containing both normal and pathological cells. The results indicate that the abnormal PTH release characteristic of HPT is due to a defective regulation of the cytosolic Ca2+ activity.

Histamine release by pharmacological agents in the absence of external free Ca2+

The involvement of extracellular free Ca2+ in histamine release was investigated in rat peritoneal mast cells. Incubation of non-antigenized cells in a media with high extracellular potassium did not increase histamine release. Secretion induced by A23187 and compound 48/80 in the presence of Ca2+ requires metabolic energy. In the absence of external free Ca2+ (2.5 microM) histamine release induced by A23187 is reduced but not abolished. Secretion induced by compound 48/80 is independent of extracellular Ca2+. These results lead us to suggest that mast cell plasma membranes probably lack voltage-gated Ca2+ channels and that external Ca2+ may not be an absolute requisite for histamine secretion.

Biochemical and morphological characterization of a plasma membrane-enriched fraction from bovine parathyroid cells

A fraction of enriched plasma membranes from bovine parathyroid cells has been prepared by differential centrifugation. Biochemical characterization shows that this fraction has a specific activity enrichment of 7.2-fold in ouabain-sensitive Na+-K+ ATPase, and 3.5-fold in 5'-nucleotidase. Less than 4% of the total mitochondria and lysosomes are present within the plasma membranes, while microsomal contamination accounts for 14% of total specific activity. Parathyroid hormone radioimmunoassay also reveals the presence of some secretory granules within the plasma membrane fraction. The characteristic morphological aspect of the unusual surface membrane is shown by freeze-fracture electron microscopy. In the enriched pellets, vesicles identified as having a plasma membrane origin have variable sizes, and 50% show an inside-out conformation. Even though the plasma membrane fraction described herein is not absolutely free from contamination by other subcellular components, this protocol represents the first attempt to purify surface membrane from parathyroid tissue and provide the starting material for understanding, at a molecular level, the properties of extracellular Ca2+ regulation and its coupling with secretion of parathyroid hormone.

Potassium stimulates parathyroid hormone release in the absence of extracellular calcium

Potassium-stimulated release of many hormones requires the presence of extracellular calcium. At variance with this mechanism, potassium-evoked parathyroid hormone (PTH) release from perifused dispersed bovine parathyroid cells also occurred in calcium-free medium containing 1 mM EGTA. Tetraethylammonium, which presumably suppresses the efflux of potassium from parathyroid cells, also stimulated PTH secretion. Removal of sodium did not suppress potassium-stimulated PTH secretion, but inhibited the release of the hormone evoked by calcium removal and by isoproterenol. Ouabain, on the other hand, suppressed the release of PTH evoked by calcium removal and by potassium, but not by isoproterenol. Unlike high potassium and removal of calcium, isoproterenol caused a parallel increase in cAMP release. In conclusion, PTH secretion is reversibly stimulated by potassium in the absence of extracellular calcium. Our findings suggest that potassium stimulates PTH secretion by a unique mechanism the nature of which remains to be elucidated.