Evidence that parathyroid hormone-mediated calcium transport in rat brain synaptosomes is independent of cyclic adenosine monophosphate

Epidemiology, Pathophysiology, and Genetics of Primary Hyperparathyroidism

In this narrative review, we present data gathered over four decades (1980-2020) on the epidemiology, pathophysiology and genetics of primary hyperparathyroidism (PHPT). PHPT is typically a disease of postmenopausal women, but its prevalence and incidence vary globally and depend on a number of factors, the most important being the availability to measure serum calcium and parathyroid hormone levels for screening. In the Western world, the change in presentation to asymptomatic PHPT is likely to occur, over time also, in Eastern regions. The selection of the population to be screened will, of course, affect the epidemiological data (ie, general practice as opposed to tertiary center). Parathyroid hormone has a pivotal role in regulating calcium homeostasis; small changes in extracellular Ca++ concentrations are detected by parathyroid cells, which express calcium-sensing receptors (CaSRs). Clonally dysregulated overgrowth of one or more parathyroid glands together with reduced expression of CaSRs is the most important pathophysiologic basis of PHPT. The spectrum of skeletal disease reflects different degrees of dysregulated bone remodeling. Intestinal calcium hyperabsorption together with increased bone resorption lead to increased filtered load of calcium that, in addition to other metabolic factors, predispose to the appearance of calcium-containing kidney stones. A genetic basis of PHPT can be identified in about 10% of all cases. These may occur as a part of multiple endocrine neoplasia syndromes (MEN1-MEN4), or the hyperparathyroidism jaw-tumor syndrome, or it may be caused by nonsyndromic isolated endocrinopathy, such as familial isolated PHPT and neonatal severe hyperparathyroidism. DNA testing may have value in: confirming the clinical diagnosis in a proband; eg, by distinguishing PHPT from familial hypocalciuric hypercalcemia (FHH). Mutation-specific carrier testing can be performed on a proband's relatives and identify where the proband is a mutation carrier, ruling out phenocopies that may confound the diagnosis; and potentially prevention via prenatal/preimplantation diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Sequence of cellular events in pancreatic islets leading to impaired insulin secretion in chronic kidney disease

OBJECTIVES

In chronic renal failure (CRF), a multitude of metabolic derangements occur in the pancreatic islets, resulting in impaired glucose-induced insulin secretion. These abnormalities include a rise in the basal level of cytosolic calcium ([Ca(2+)]i) in the islets, a decrease in their basal and stimulated adenosine triphosphate (ATP) and adenosine diphosphate (ADP) content, a reduction in the V(max) of Ca(2+) ATPase and Na(+)-K(+) ATPase, and an impaired glucose-induced calcium signal. The sequence of events that leads to these derangements and to the impairment in insulin secretion during the evolution of CRF has not been defined. This study examined this particular issue by measuring the metabolic profiles of pancreatic islets weekly during the evolution of CRF over a period of 6 weeks.

RESULTS

The results showed that serum levels of parathyroid hormone (PTH) begin to rise during the first week of CRF. The V(max) of Ca(2+) ATPase and Na(+)-K(+) ATPase increased during weeks 1 to 3 of CRF but decreased to low levels thereafter. At week 3 of CRF, the basal level of [Ca(2+)]i began to rise, whereas basal and stimulated ATP and ADP content started to fall. Glucose-induced calcium signal, Δ[Ca(2+)]i, and insulin secretion became abnormally low between weeks 3 and 6 of CRF.

CONCLUSION

The data obtained allow for the inference of the following formulation: as serum levels of PTH begin to rise, calcium entry into islets is augmented, which in turn will stimulate the activity of Ca(2+) ATPase and the Na(+)-Ca(2+) exchanger, and therefore, calcium extrusion out of the islets is increased. Thus, [Ca(2+)]i remains normal during the first 2 weeks of CRF. Activation of the Na(+)-Ca(2+) exchanger may result in accumulation of sodium in the islets, an event that would activate the Na(+)-K(+) ATPase. Because calcium entry is further augmented by the progressive rise in serum PTH levels, mitochondrial oxidation and ATP production would be reduced, resulting in lower ATP content. This fall in ATP causes a reduction in the V(max) of Ca(2+) ATPase and Na(+)-K(+) ATPase, and therefore calcium extrusion out of the islets is reduced; consequently, [Ca(2+)]i rises. With the decrease in ATP content and the rise in [Ca(2+)]i, glucose-induced insulin secretion is impaired because of alterations in the closure of ATP-dependent potassium channels and reduction in the glucose-induced calcium signal (Δ[Ca(2+)])i.

Depressed cerebral oxygen metabolism in patients with chronic renal failure: a positron emission tomography study

To elucidate brain oxygen metabolism in uremic patients, regional cerebral blood flow (rCBF), oxygen extraction (rOEF), and oxygen metabolism (rCMRO(2)) were measured by positron emission tomography (PET) in 10 hemodialysis (HD) patients and 13 predialysis patients with chronic renal failure (CRF). Data were compared with 20 nonuremic patients (controls) without neurological abnormalities, congestive heart failure, history of cerebrovascular accident, diabetes mellitus, or symptomatic brain lesion on magnetic resonance imaging. In the hemisphere, rCMRO(2) in both HD (1.82 +/- 0.10 mL/min/100 g) and CRF patients (1.95 +/- 0.09 mL/min/100 g) showed significantly lower values compared with controls (2.23 +/- 0.05 mL/min/100 g; P < 0.01). Hemispheric rCBF in HD (35.6 +/- 2.1 mL/100 g/min) and CRF patients (36.1 +/- 2.1 mL/100 g/min) was not different from controls (31.8 +/- 1.4 mL/100 g/min). Hemispheric rOEF in CRF patients (45.7% +/- 1.6%) was significantly greater than that in controls (40.5% +/- 1.2%; P < 0.02), but rOEF in HD patients (43.7% +/- 1.9%) did not increase significantly. These tendencies were similar in all regions of interest, especially cerebral cortices. All PET parameters in frontal cortices tended to show the lowest values in patients with renal failure. For all HD patients, rCBF in both the frontal cortex and white matter correlated inversely with HD therapy duration (P < 0.05). In conclusion, brain oxygen metabolism is depressed in patients with renal failure on or before the start of HD therapy. The cause for depressed brain oxygen metabolism is considered to be either dysregulation of cerebral circulation or lower brain cell activity.

Central nervous dysfunction in uremia

The mechanisms of central nervous system dysfunction in uremia are multifactorial and only partially characterized. Studies using sealed presynaptic nerve terminals (synaptosomes) for in vitro ion transport and metabolism of neurotransmitter in chronic renal failure (CRF) neuronal cell culture and in vivo brain structure microdialysis generated significant new information. An increase in total calcium content of the cerebral cortex accompanied by increased levels of cytosolic calcium ([Ca(2+)]i) in synaptosomes are common findings in rats with CRF. Mechanisms leading to the increase in [Ca(2+)]i include increased calcium uptake mediated by parathyroid hormone and decreased activity of Na(+),K(+)-adenosine triphosphatase (ATPase) and Ca(2+)-ATPase of synaptosomes in CRF rats. Moreover, these synaptosomes respond inappropriately to depolarization, which can impair neurotransmitter metabolism. Brain gamma-aminobutyric acid content, norepinephrine, and acetylcholine release uptake and degradation are affected by uremia. These may lead to certain somatic, behavioral, and motor dysfunctions in uremia. Many derangements of the central nervous system in uremia appear to be mediated by secondary hyperparathyroidism of CRF because parathyroidectomy of animals with CRF prevented the increase in basal levels of [Ca(2+)]i and derangements in neurotransmitter metabolism. The role of other neurotoxins, such as guanidinosuccinic acid, are also reviewed.

Na-K-ATPase activity decreases with aging in female rat brain synaptosomes

To understand why elderly females are better able to tolerate hyponatremia, we measured brain Na-K-ATPase activity to determine whether this adaptive mechanism was affected by age. Using synaptosomes from 2-, 12-, and 19-mo-old female rats, we show in our results that Na-K-ATPase activity changes with age in female rats. Enzyme activity was significantly (P = 0.0026) reduced (17%) from 0.416 +/- 0.01 at 2 mo to 0.345 +/- 0.01 at 12 mo and reduced (P = 0.0001) (34%) to 0.274 +/- 0.02 micromol. min(-1). mg protein(-1) at 19 mo. To determine whether this decrease was due to reduced transport function of the Na-K-ATPase pump, we performed potassium transport using rubidium ((86)Rb+) as tracer. Ouabain-sensitive potassium uptake at 2 mo was 16.18 +/- 1.31 nmol/mg protein, was significantly (P = 0.0063) reduced (39%) to 9.79 +/- 1.44 nmol/mg at 12 mo, and was significantly (P = 0.0003) reduced (62%) to 6.12 +/- 1.05 nmol/mg protein at 19 mo. On the contrary, Na-K-ATPase activity remained elevated in males during aging. These data suggest that the Na-K-ATPase pump in female rat brain synaptosomes is decreased with increased age, and that this decrease is probably due in part to decreased potassium transport by the Na-K-ATPase pump.

Erythrocyte voltage-dependent calcium influx is reduced in hemodialyzed patients

BACKGROUND

Uremia displays increased cytosolic free calcium ([Ca2+]i) in many different cell types, supporting the hypothesis of an altered Ca2+ transport modifying the functional activity of calcium signaling pathway.

METHODS

Thirty-five hemodialyzed patients and 20 age-matched subjects were studied. Erythrocyte resting [Ca2+]i and Ca2+ influx were measured by the fluorescent Ca2+-sensitive dye fura-2.

RESULTS

We found an increase of resting [Ca2+]i in erythrocytes from uremic hemodialyzed patients compared with matched healthy controls (103 +/- 2.5 nM, N = 20, vs. 90 +/- 4, N = 20, P < 0.01). Moreover, we found an altered voltage-dependent Ca2+ influx showing a reduced transport rate (0.42 +/- 0.03 nM/second vs. 0.74 +/- 0.08, P < 0.01). High levels of plasma parathyroid hormone (PTH) were related to augmented Ca2+ entry (r = 0.511, P < 0.05), contributing to maintain a high level of [Ca2+]i. Hemodialysis had no effect on cell calcium level and Ca2+ influx indices. The therapy with Ca2+ antagonists did not modify the values of resting [Ca2+]i or Ca2+ influx indices, but the correlation between PTH and influx indices was lost.

CONCLUSIONS

In conclusion, we found evidence for an alteration of erythrocyte Ca2+ influx caused by uremic toxicity that could be related to some organ disorders in uremia. The chronic increase of cellular calcium may contribute to influx derangement.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Structure of a parathyroid hormone/parathyroid hormone-related peptide receptor of the human cerebellum and functional expression in human neuroblastoma SK-N-MC cells

Cloning and functional expression of a cDNA from the human cerebellum revealed a parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor protein of 593 amino acids, identical in sequence to the PTH/PTHrP receptor of the human kidney and an osteoblast-like cell line (Schipani et al., Endocrinology, 132 (1993) 2157-2165). Expression of mRNA hybridizing with the cloned cDNA, indistinguishable in size on Northern blots from a 2.3 kb transcript in kidney and liver, was detected in eight brain areas. In situ hybridization histochemistry in rat brain tissue sections revealed predominant signals in the Purkinje cell layer of the cerebellum and in the mesencephalic nucleus of the trigeminal nerve. In human neuroblastoma (SK-N-MC) cells, stably transfected with the cloned cDNA, hPTH(1-84) and hPTH(1-34) displaced binding of 125 pM [125I][Tyr36]chPTHrP(1-36) to the PTH/PTHrP receptor with IC50 values of 4.0 +/- 0.6 nM and 2.00 +/- 0.08 nM, and stimulated cyclic AMP accumulation with EC50 values of 0.19 +/- 0.06 nM and 0.09 +/- 0.01 nM, respectively. 16 out of 48 cells responded to 100 nM hPTH(1-34) with a 2-10-fold transient increase of cytosolic free calcium concentrations. In conclusion, a PTH/PTHrP receptor, identified in the human cerebellum, has the primary structure of the corresponding receptors of kidney and bone. Expression in human neuroblastoma SK-N-MC cells revealed functional properties indistinguishable from those of non-neuronal tissues. The widespread distribution of PTHrP and its receptor in brain implies biological functions remaining to be elucidated.

Parathyroid hormone stimulates the generation of inositol 1,4,5-triphosphate in brain synaptosomes

Parathyroid hormone (PTH) increases the levels of the second messenger, inositol 1,4,5 triphosphate (I1,4,5P3) in kidney and bone cells. It has been reported the I1,4,5P3 increases calcium uptake by brain synaptosomes. Because PTH also augments calcium entry in brain synaptosomes, it is possible that PTH induces the generation of I1,4,5P3 in these structures as well. The current study examined the effect of PTH-(1-84) on myoinositol turnover in vitro in rat brain synaptosomes. PTH-(1-84) in concentration of 10(-6)mol/L significantly (P < 0.01) increased the IP3 production (35 +/- 52%). The results indicate that PTH activates the phosphoinositol turnover in brain synaptosomes and that this pathway may be involved in the PTH-induced increase in [Ca2+]i in brain synaptosomes.

Function and metabolism of brain synaptosomes in chronic renal failure

Patients with advanced renal failure have derangements in the function of their nervous system. The available clinical and experimental data indicate that the state of the secondary hyperparathyroidism of renal failure plays a major role in the genesis of the nervous system dysfunction. The excess parathyroid hormone (PTH) mediates its deleterious effect by causing an elevation in cytosolic calcium of brain cells. This report reviews the evidence leading to the conclusion that PTH is a major uremic toxin.

Effect of parathyroid hormone and calcitonin on acetylcholine release in rat sympathetic superior cervical ganglion

The effects of parathyroid hormone (PTH) and calcitonin on acetylcholine release by rat superior cervical ganglion (SCG) were evaluated in vitro. SCG labeled with [3H]choline were exposed to four 5 min-long pulses of 40 mM K+, 35 min apart. PTH increased, and calcitonin inhibited, in a dose-dependent way, K(+)-elicited [3H]acetylcholine release, with apparent effective doses 50 of about 10(-9) M. The effect of PTH was inhibited by co-incubation with the PTH receptor antagonist NLe [8-18]-PTH (3-34) amide. Incubation of SCG for 120 min with PTH or calcitonin resulted in dose-dependent augmentation or inhibition of K(+)-induced increase of high affinity [3H]choline uptake, respectively, with a maximal effect at 10(-8) M concentration (PTH) and 10(-9) M concentration (calcitonin) and declining at higher concentrations. The increase in SCG [3H]choline uptake induced by PTH was blunted by preincubation with the PTH antagonist NLe [8-18]-PTH (3-34) amide. At 10(-7) M concentrations, PTH increased significantly the in vitro conversion of [3H]choline to [3H]acetylcholine, an effect inhibited by PTH receptor antagonist. Calcitonin did not modify SCG [3H]acetylcholine synthesis by rat SCG. The results indicate that, in vitro, PTH increases, and calcitonin inhibits, acetylcholine release in rat SCG.

Parathyroid hormone binding sites in the brain

Parathyroid hormone (PTH) has been shown to have actions within the brain, suggesting the presence of central PTH receptors. This possibility was examined by determining the binding of 125I-labeled [Nle8,18,Tyr34]bovine PTH to the plasma membranes of rat and rabbit brains. Specific binding of the tracer to membranes of the whole brain was time and tissue dependent, and was greater with membranes from the hypothalamus than with membranes from the cerebellum, cerebrum, or brain stem. The binding of the tracer to rat hypothalamic membranes was saturable and competitively displaced by unlabeled PTH(1-34), PTH(3-34), [Nle8,18,Tyr34]PTH(1-34), and by PTH-related protein, indicating the presence of a single class of high-affinity (dissociation constant = 2-5 nM), low-capacity (maximum binding capacity, Bmax = 110-250 fmol/mg protein) binding site. The binding of radiolabeled PTH to these sites was not displaced by unrelated peptides of comparable molecular size (calcitonin, calcitonin-gene related peptide, adrenocorticotropin). The binding of PTH to these sites did not, however, appear to stimulate adenylate cyclase activity, as in peripheral PTH target sites. Thus, although these results indicate the presence of PTH receptors in the brain, these binding sites have a lower affinity than those in peripheral tissues and may utilize a different signal transduction system.

Parathyroid hormone-induced dopamine turnover in the rat medial basal hypothalamus

The parathyroid hormone (PTH) gene is expressed and translated in the rat hypothalamus, and the possibility that PTH may modulate neural activity was therefore examined in anesthetized rats. Intracerebroventricular (ICV) injections of 1.0 or 10.0 micrograms rat, human, or bovine PTH(1-34) was followed 60 min later by increased concentrations of DOPAC (dihydroxyacetic acid) and the DOPAC:dopamine (DA) ratio in the medial basal hypothalamus (MBH), but not in other (brainstem, cerebral cortex, cerebellum) regions of the brain. Tissue concentrations of norepinephrine and serotonin were unchanged by ICV PTH administration, although MBH concentrations of 5-hydroxyindolacetic acid (5-HIAA) were increased following PTH administration. An increase in MBH DA turnover (as indicated by an increased DOPAC:DA ratio) was also induced by the ICV injection of 10 micrograms PTH-related protein [PTHrP(1-34)]. Pretreatment with the receptor antagonists PTH(7-34) or PTHrP(7-34) completely blocked the subsequent DOPAC response to ICV PTH or PTHrP, respectively. The DOPAC concentrations in hypothalamic extracellular fluid (ECF), sampled by microdialysis, were also increased within 20 min of PTH(1-34) perfusion, in the absence of changes in the ECF concentrations of 5-HIAA. These results demonstrate that PTH and PTH-like peptides specifically increase DA turnover in the rat MBH and suggest novel roles for these hormones in neural regulation.

Dopaminergic actions of parathyroid hormone in the rat medial basal hypothalamus in vitro

Parathyroid hormone (PTH) modulates dopamine (DA) metabolism in the rat medial basal hypothalamus (MBH) in vivo. Direct effects of PTH on MBH DA metabolism were therefore investigated in vitro. Incubation of rat MBHs for 60 min with 10(-7)-10(-5) M human PTH1-34 consistently reduced the tissue DA content and increased the DOPAC (dihydroxyphenylacetic acid) to DA ratio. This ratio was further increased in tissues incubated in 10(-5) M PTH1-34, as a result of an increase in DOPAC content. The tissue content of DOPAC and DA was unaffected by 10(-9) M PTH. The serotonin (5HT) content of the MBH was reduced by 10(-5) M PTH1-34, but concentrations of 5HT, 5-hydroxyindolacetic acid, and norepinephrine were otherwise unaffected by 10(-9)-10(-5) M PTH1-34. Concentrations of DA in the incubation media were reduced after exposure to 10(-6) or 10(-5) M PTH1-34. The uptake of 3H-labelled DA by incubated tissues was also reduced by 10(-6) M PTH1-34, as was the metabolism of 3H-labelled DA into tissue and media DOPAC. Monoamine oxidase (MAO) activities A and B were significantly increased after the incubation of the MBH with 10(-6) or 10(-5) M PTH1-34. These results further demonstrate neuromodulatory actions of PTH on dopaminergic neurons within the rat MBH in vitro, and suggest neural and/or neuroendocrine roles of PTH of central or peripheral origin.

The changes in contractile status of single vascular smooth muscle cells and ventricular cells induced by bPTH-(1-34)

Single smooth muscle cells from rat tail artery and ventricular myocytes from neonatal rat were isolated by repeated enzyme digestion. The change in cell area as determined photographically was used as an index of cell contraction. The photographic areas of single smooth muscle cells bathed in normal Tyrode solution were 403 +/- 22 (n = 13) square micra. Exposure of smooth muscle cells to a modified Tyrode solution containing 60 mM KCl induced cell contraction. This contraction was inhibited by bPTH-(1-34) at a concentration of 1 microM. The inhibitory effect of bPTH-(1-34) was time-dependent with maximum inhibition at 5 min after administration. The photographic areas of ventricular myocytes bathed in the culture medium without fetal calf serum were 516 +/- 47 (n = 29) square micra. At a concentration of 1 microM, bPTH-(1-34) produced a time-dependent contraction in ventricular myocytes as shown by the decrease in the photographic cell area (88 +/- 2% of the control value at 15 min, n = 9, p < 0.01). Furthermore, 1 microM nifedipine inhibited the effect of bPTH-(1-34) on the contraction of ventricular myocytes, indicating that bPTH-(1-34) might exert its action via a calcium channel related mechanism. In addition, bPTH-(1-34) increased the contraction frequency of single ventricular cells, which could also be inhibited by nifedipine. The present study suggests that bPTH-(1-34) directly relaxes precontracted single vascular smooth muscle cells and contracts single ventricular myocytes.

Chronic renal failure is a state of cellular calcium toxicity

Studies on the metabolic profile of many cells have shown that chronic renal failure (CRF) is associated with a significant elevation in the basal levels of cytosolic calcium ([Ca2+]i). This latter abnormality is, in major part, responsible for the organ dysfunction in CRF. The initial step leading to the eventual increase in the basal level of [Ca2+]i is parathyroid hormone (PTH)-mediated increased calcium influx into cells. This is followed by decreased extrusion of calcium out of cells due to reduced activity of the enzymes responsible for pumping calcium out of the cells. The combination of increased entry and decreased exit of calcium results in elevation of [Ca2+]i. Prevention of secondary hyperparathyroidism in CRF or blocking of the effect of PTH by a calcium channel blocker results in normalization of [Ca2+]i and restoration of cell function. Thus, the available data are consistent with the notion that CRF is a state of cellular calcium toxicity, which underlies many of the metabolic and functional derangements in CRF.

Impaired phagocytosis in chronic renal failure is mediated by secondary hyperparathyroidism

Patients with chronic renal failure (CRF) display impaired phagocytosis by the polymorphonuclear leucocytes (PMNL), and these cells have elevated basal levels of cytosolic calcium ([Ca2+]i) and reduced ATP content. It has been suggested that these changes in PMNL metabolism and function are mediated by the state of secondary hyperparathyroidism of CRF. To examine the role of excess PTH in these derangements of PMNL, we studied [Ca2+]i, ATP and phagocytic ability of PMNL in five groups of rats including: CRF, CRF normocalcemic parathyroidectomized (CRF-PTX), CRF and normal animals treated with verapamil (CRF-V), and normal-V, respectively. The level of [Ca2+]i in the PMNL of CRF rats (149 +/- 2.7 nM) was significantly (P less than 0.01) higher and the ATP content (4.2 +/- 0.17 nmol/5 x 10(6) PMNL) significantly lower (P less than 0.01) than in normal (108 +/- 2.4 nM; 9.5 +/- 0.15 nmol/5 x 10(6) PMNL), CRF-PTX (103 +/- 2.9 nM; 9.2 +/- 0.19 nmol/5 x 10(6) PMNL), CRF-V (107 +/- 2.2 nM; 9.0 +/- 0.2 nmol/5 x 10(6) PMNL) and normal-V (106 +/- 1.8 nM; 9.2 +/- 0.2 nmol/5 x 10(6) PMNL), despite sustained elevation in blood PTH in the CRF-V group. Phagocytosis was significantly (P less than 0.01) impaired in CRF animals (5.6 +/- 0.25 micrograms oil/10(7) PMNL/min) but was normal in CRF-PTX (9.3 +/- 0.21 micrograms oil/10(7) PMNL/min) and CRF-V (9.5 +/- 0.22 micrograms oil/10(7) PMNL/min) rats. The values of phagocytosis in normal and normal-V rats were 9.6 +/- 44 and 9.6 +/- 0.18 micrograms oil/10(7) PMNL/min, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast axonal transport is modulated by altering trans-axolemmal calcium flux

Factors involved in fast axonal transport (motor proteins, microtubules, organelles, etc.) have been identified but the molecular mechanism controlling transport is unknown. We used video enhanced microscopy to directly evaluate the effect of calcium on fast axonal transport (FAxT). FAxT alterations included rapid speed decreases (within minutes) in Ca2+ free buffer and rapid speed increases (within seconds) when axons were treated with parathyroid hormone, BAY K 8644, or K+ depolarization. The speed increases were blocked by dihydropyridine Ca2+ channel antagonists. Ryanodine (20 microM), known to block calcium release from subcellular stores, caused a decrease in the rate of retrograde FAxT. Calcium ionophore A23187 (at 1 and 20 micrograms/ml) caused increases in FAxT, an effect also noted only in retrograde moving organelle traffic. Hyper- or hypo-tonic solutions produced no alterations making axoplasmic viscosity changes an unlikely explanation for the speed changes. Reproducible alteration of FAxT by manipulation of Ca2+ levels provides evidence that Ca2+ modulates fast axonal transport. Retrograde transport appears more sensitive to changes in Ca2+ and differential effects on antero- and retro-FAxT mechanisms suggest directional specificity for some of these signals which may be based upon the organelle size. Endogenous substances (e.g. PTH) that trigger axonal Ca2+ changes may rapidly modulate the rate of material delivery in axons. The results are discussed within the context of a Ca2+/calmodulin-dependent modification of the cytoskeletal matrix.

Parathyroid hormone enhances calcium current in snail neurones--simulation of the effect by phorbol esters

Effects of parathyroid hormone substance (PTH) on the voltage-activated calcium current (ICa) were studied on intracellularly perfused neurones of the snail, Helix pomatia, under voltage-clamp conditions. Application of 0.1 nM PTH produced a marked potentiation of the current. The effect developed slowly (60-70 min) and remained after removal of PTH. Potentiation could be observed in most neurones, but varied considerably from cell to cell; in some neurones ICa was increased 2- to 3-fold. Addition of ethylenebis(oxonitrilo)tetraacetate (EGTA, 10 mM) to, or removal of adenosine 5'-triphosphate (ATP, 2 mM) from the intracellular perfusing solution resulted in a suppression or attenuation of the potentiating effect. The effect could be reproduced by the synthetic 1-34 amino acid fragment of PTH. Extracellularly applied protein kinase-C (PK-C) activator phorbol ester phorbol 12-myristate 13-acetate (PMA, 0.1-10 microM) produced a similar slow increase in ICa (up to 1.5- to 2-fold), while its inactive analogue (4 alpha-phorbol ester) had no effect on ICa. The effects of PTH and PMA were not additive. PK-C inhibitors [1-(5-isoquinoline-sulphonyl)-2-methylpiperazine hydrochloride] (H-7, 100 microM) and staurosporine (100 microM) as well as calcium channel antagonists Cd2+, verapamil, nifedipine and nimodipine depressed the effect of PTH. The chloride channel blocker 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS, 1 mM) did not affect the potentiating action of PTH. Activation of the adenylate cyclase system also potentiated ICa in some neurones, but this effect had a different time course and was additive to the effect of PTH.2=

Parathyroid hormone stimulates protein kinase C but not adenylate cyclase in mouse epidermal keratinocytes

Intact human parathyroid hormone, hPTH [1-84], and the hPTH [1-34] fragment stimulated membrane-associated protein kinase C (PKC) activity in immortalized (but still differentiation-competent) murine BALB/MK-2 skin keratinocytes. Unexpectedly, the hormone and its fragment did not stimulate adenylate cyclase. The failure of PTH to stimulate adenylate cyclase activity was not due to the lack of a functioning receptor-cyclase coupling mechanism because the cells were stimulated to synthesize cyclic adenosine monophosphate (cyclic AMP) by the beta-adrenergic drug isoproterenol. Thus, skin keratinocytes seem to have an unconventional PTH receptor that is coupled to a PKC-activating mechanism but not to adenylate cyclase. These observations suggest that normal and neoplastic skin keratinocytes respond to the PTH-related peptide that they make and secrete.

Impaired potassium-induced insulin secretion in chronic renal failure

Extrarenal disposal of potassium load is impaired in chronic renal failure (CRF). This has been attributed to excess PTH since extrarenal disposition of potassium is normal in CRF-PTX animals. Insulin augments potassium entry into cells and hyperkalemia stimulates insulin secretion. Since glucose-induced insulin secretion is impaired in CRF and normal in CRF-PTX, it is possible that K(+)-induced insulin secretion is also impaired in CRF due to excess PTH. Such a defect would contribute to the abnormality in extrarenal disposal of potassium in CRF. We examined K(+)-induced insulin secretion, cytosolic calcium ([Ca2+]i) and the changes in [Ca2+]i in response to 20 mM KCl of islets from normal, CRF, and CRF-PTX rats; and normal and CRF animals treated with verapamil (normal-V and CRF-V). K(+)-induced insulin secretion by islets isolated from CRF rats was significantly (P less than 0.01) lower than that from normal, CRF-PTX, CRF-V and normal-V rats. Basal level of [Ca2+]i in islets of CRF rats was significantly (P less than 0.01) higher than in islets of the other four groups of animals. The calcium signal (delta [Ca2+]i) and the delta [Ca2+]i/basal [Ca2+]i ratio in response to 20 mM KCl observed in islets from CRF rats were significantly lower than in the other four groups of animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of guanine nucleotides and parathyroid hormone on inositol 1,4,5-trisphosphate metabolism in canine renal cortical tubular cell membranes

Parathyroid hormone (PTH) and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma S) increase levels of the second messenger inositol 1,4,5-triphosphate (IP3) and other inositol phosphates (IP) in several membrane preparations of PTH-responsive cells. We present evidence here indicating that in a membrane preparation of canine renal cortical tubular cells bPTH-(1-84), bPTH-(1-34), [N-Leu8,18Tyr34]bPTH-(3-34)NH2, and the human PTH related peptide fragment hPTHrP-(1-34)NH2 all increase levels of inositol phosphate (IP) but [Tyr34]-bPTH-(7-34)NH2 and hPTHrP-(7-34)NH2 have no significant effects on IP accumulation. Increases in IPs are generally attributed to increased formation of IPs and appear to be mediated by a G protein. However, increased levels of IPs may also result from inhibition of the phosphatases are responsible for their metabolism. We investigated the effect of PTH and GTP-gamma S on the metabolism of IP3 in canine renal cortical tubular membranes. These membranes rapidly metabolize [3H]IP3 (47% at 15 s). Decreases in [3H]IP3 at all time points are accounted for quantitatively by increases in the sum of its breakdown products: [3H]IP2, [3H]IP1, and [3H]inositol. After 5 minutes of exposure to membranes, the vast majority of [3H]IP3 (84%) is converted to its terminal metabolite, [3H]inositol. GTP-gamma S (100 microM) inhibits the amount of [3H]IP3 metabolized in 15 s by 70% and reduces the amount of [3H]inositol ultimately formed in 5 minutes by 64%. ATP-gamma S, ATP, and 2,3-bisphosphoglycerate (100 microM) also inhibit [3H]IP3 hydrolysis in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

On the mechanism of impaired insulin secretion in chronic renal failure

It has been suggested that a sustained rise in resting levels of cytosolic calcium [Ca2+]i of pancreatic islets is responsible for impaired insulin secretion in chronic renal failure (CRF). Evidence for such an event is lacking and the mechanisms through which it may affect insulin secretion are not known. Studies were conducted in normal, CRF, and normocalcemic, parathyroidectomized (PTX) CRF rats to answer these questions. Resting levels of [Ca2+]i of islets from CRF rats were higher (P less than 0.01) than in control of CRF-PTX rats. [3H]2-deoxyglucose uptake and cAMP production by islets were not different in the three groups. Insulin content of, and glucose-induced insulin secretion by islets from CRF rats was lower (P less than 0.01) than in control and CRF-PTX rats. In contrast, glyceraldehyde-induced insulin release by CRF islets was normal. Basal ATP content, both glucose-stimulated ATP content and ATP/ADP ratio, net lactic acid output, Vmax of phosphofructokinase-1, and Ca2+ ATPase of islets from CRF rats were lower (P less than 0.02-less than 0.01) than in normal or CRF-PTX animals. Data show that: (a) Glucose but not glyceraldehyde-induced insulin secretion is impaired in CRF; (b) the impairment in glucose-induced insulin release in CRF is due to a defect in the metabolism of glucose; (c) this latter defect is due to reduced ATP content induced partly by high [Ca2+]i of islets; and (d) the high [Ca2+]i in islets of CRF rats is due to augmented PTH-induced calcium entry into cells and decreased calcium extrusion from the islets secondary to reduced activity of the Ca2+ ATPase.

Parathyroid hormone-related protein: biochemistry and molecular biology

This article critically reviews the current state of knowledge regarding the recently identified and cloned novel hormone parathyroid hormone-related protein (PTHrP). PTHrP is produced by tumors associated with the syndrome of humoral hypercalcemia of malignancy giving rise to the parathyroid hormone (PTH)-like symptoms characteristic of the syndrome. Areas that will be reviewed include identification, purification and cloning, localization, actions, and significance of PTHrP in cancers and normal physiology. The structure and regulation of the PTHrP gene that may be ancestrally related to the PTH gene will also be discussed. Studies in vivo and in vitro with synthetic and recombinant PTHrP sequences and antibodies developed against them have established that the PTH-like actions of PTHrP are mediated via the N-terminal sequences, which show some limited sequence homology with PTH. Evidence for PTH and non-PTH-like actions of PTHrP in normal physiology, which implicate a role for PTHrP in fetal and neonatal development, is also presented.

Inositol 1,4,5-trisphosphate may regulate rat brain Cai++ by inhibiting membrane bound Na(+)-Ca++ exchanger

The role of inositol 1,4,5-trisphosphate (1,4,5-IP3) in regulating cytosolic Ca++ by stimulating Ca++ release from intracellular organelles is well established. However, other modes of intracellular Ca++ regulation by 1,4,5-IP3 have not been determined. To determine if 1,4,5-IP3 may regulate cell cytosolic Ca++ by acting on plasma membrane bound Na(+)-Ca++ exchanger, we investigated Ca++ transport in synaptosomes using 45Ca++ as tracer. In the presence of either an inhibitor of voltage gated Na+ channels (tetrodotoxin) or the K+ ionophore (valinomycin), Ca++ uptake was significantly inhibited (P less than 0.05) by 1,4,5-IP3 in a concentration dependent manner, with half-maximal inhibition occurring at submicromolar concentrations between 10(-9) M and 10(-10) M 1,4,5-IP3. Similarly, Ca++ efflux by the exchanger was significantly inhibited 40% by 1,4,5-IP3. The inhibitory effect of 1,4,5-IP3 on the Na(+)-Ca++ exchanger was observed in the presence of Ca++ channel blockers, and in vesicles pretreated with caffeine to deplete the 1,4,5-IP3-sensitive stores of Ca++. These results suggest that during signal transduction in brain, 1,4,5-IP3 may increase cytosolic [Ca++] in part by inhibiting the Na(+)-Ca++ exchanger and thus, Ca++ efflux from cell.

Parathyroid hormone stimulates formation of inositol phosphates in a membrane preparation of canine renal cortical tubular cells

Recent studies have shown that, in addition to its well-known action to stimulate adenylate cyclase activity, parathyroid hormone (PTH) may stimulate the inositol phosphate second messenger system in its target tissues, bone and kidney. We have developed a membrane preparation of canine renal cortex to test this hypothesis. We also have examined the potential role of guanine nucleotides on the formation of inositol phosphates (IPs) in this tissue. Collagenase-dispersed tubules were labeled with [3H]inositol, and membranes containing labeled phospholipase C (PLC) substrates ([3H]phosphatidyl inositol, [3H]phosphatidylinositol monophosphate, and [3H]phosphatidylinositol bisphosphate) were prepared. bPTH-(1-34) (100 nM) rapidly increased levels of all measured [3H]IPs (IP1, IP2, and IP3) 1.6-1.7-fold within the first 30 s of stimulation. The half-maximal concentration for the response to bPTH-(1-34) was approximately 8 nM. GTP gamma S (100 microM), a nonhydrolyzable analog of GTP, also increased levels of the three [3H]IPs (1.8 to 2.8-fold). The half-maximal concentration for the response to GTP gamma S was approximately 30 microM. In the presence of GTP gamma S, bPTH-(1-34) increased levels of IPs by up to 2.7 times more than GTP gamma S alone. The results indicate that bPTH-(1-34) can stimulate the formation of inositol phosphates in the kidney and suggest that PTH may activate a receptor coupled to this effect through a guanine nucleotide regulatory protein.

Stimulation of inositol phosphate formation in ROS 17/2.8 cell membranes by guanine nucleotide, calcium, and parathyroid hormone

In addition to stimulation of cyclic AMP, parathyroid hormone (PTH) may influence cellular events by utilizing other pathways of hormone action, such as the generation of inositol phosphates (IPs). We sought to examine this potential action of PTH by assessing the formation of inositol phosphates in PTH-sensitive ROS 17/2.8 cells. The polyphosphoinositides were labeled by growing the cells with [3H]inositol following which cell homogenates were prepared. The nonhydrolyzable guanine nucleotide, GTP gamma S, and calcium ion, alone and together, stimulated all three IPs, IP1, IP2, and IP3. IP1 formation was linear over 30 minutes but IP2 and IP3 accumulated more rapidly peaking by 5 minutes for all agonist conditions. The proportion of total P as IP3 was enhanced when the cells were grown with retinoic acid (1 microM) or when the assay was conducted at pH 4.5. In addition, the lower pH was associated with much more enzyme activity. PTH agonists, bPTH-(1-84) and bPTH-(1-34), both caused a small but significant stimulation of IP3 formation. When bPTH-(1-84), and the analog bPTH-(3-34)amide, that inhibits PTH-mediated adenylate cyclase activity were present together, there was additive stimulation of IP3 formation compared with that with either agent alone. The results demonstrate that inositol phosphate formation can be stimulated directly in a membrane preparation of ROS cells by GTP gamma S, calcium ion, and PTH and that the enzyme mediating this activity, phospholipase C, is regulated by a guanine nucleotide binding protein.