Specific inhibition of long-lasting, L-type calcium channels by synthetic parathyroid hormone

Investigating the risk of metabolic and cardiovascular comorbidities among patients with parathyroid cancer: a nationwide representative cohort study in Taiwan

BACKGROUND

This study aimed to determine whether primary parathyroid cancer patients were associated with increased metabolic and cardiovascular comorbidities in comparison to the general population.

METHODS

We used the National Taiwan Cancer Registry Database to construct a cohort of patients with parathyroid cancer from January 1, 2004, to December 31, 2019. We compared the incidence of hypertension, diabetes mellitus, hyperlipidemia, atrial fibrillation, coronary heart disease, and heart failure with the general population matched based on a propensity score in a one-to-five fashion.

RESULTS

A total of 72 parathyroid cancer patients and 360 matched general population (mean age: 55 years; 59% women) were included, with different exclusive numbers for each metabolic and cardiovascular comorbidity cohort. The number of cases based on a total of 2347.7 person-years of observation included 53 deaths, 29 hypertension, 9 diabetes, 13 hyperlipidemia, 10 atrial fibrillation, 18 coronary artery disease, and 13 heart failure. According to multivariate analysis, parathyroid cancer remained significantly associated with diabetes [hazard ratio (HR): 9.28; 95% confidence interval (CI): 1.72-50.07], hyperlipidemia (HR: 5.86; 95% CI: 1.61-21.31), and heart failure (HR: 4.46; 95% CI: 1.18-16.84). Sub-distribution of competing mortality events and subgroup analysis showed robust evidence of metabolic and cardiovascular comorbidities. This national cohort study demonstrated that adult parathyroid cancer patients had a significantly higher incidence of diabetes mellitus, hyperlipidemia, and heart failure than the general population.

CONCLUSIONS

An increased risk of metabolic and cardiac comorbidities among parathyroid cancer patients required great caution.

Abnormal glucose metabolism in virus associated sepsis

Sepsis is identified as a potentially lethal organ impairment triggered by an inadequate host reaction to infection (Sepsis-3). Viral sepsis is a potentially deadly organ impairment state caused by the host's inappropriate reaction to a viral infection. However, when a viral infection occurs, the metabolism of the infected cell undergoes a variety of changes that cause the host to respond to the infection. But, until now, little has been known about the challenges faced by cellular metabolic alterations that occur during viral infection and how these changes modulate infection. This study concentrates on the alterations in glucose metabolism during viral sepsis and their impact on viral infection, with a view to exploring new potential therapeutic targets for viral sepsis.

Association of pulse wave velocity with single nucleotide polymorphisms related to parathyroid hormone

OBJECTIVE

Carotid-femoral pulse wave velocity (cfPWV) was associated with serum parathyroid hormone (PTH) in untreated Chinese. We investigated in the same cohort whether cfPWV, brachial-ankle (baPWV) and heart-brachial (hbPWV) pulse wave velocity (PWV) were associated with rs6127099 (CYP24A1) and rs4074995 (RGS14). A previously published genome-wide association study demonstrated that each additional copy of the T (rs6127099) or G (rs4074995) allele was associated with a 7% or 3% higher serum PTH, respectively.

METHODS

In 1601 untreated Chinese patients (mean age, 51.0 years; 51.9% women), we measured cfPWV by tonometry (SphygmoCor) and baPWV and hbPWV by combined oscillometry and plethysmography (VP-2000 PWV/ABI analyser), serum PTH by an immunoassay, and genotypes by the SNapShot method.

RESULTS

cfPWV, baPWV and hbPWV averaged 7.9, 14.6 and 5.5 m/s and serum PTH 65.7 pg/mL. Genotype frequencies were in Hardy-Weinberg equilibrium, amounting to 41.7% (AA), 44.9% (AT) and 13.4% (TT) for rs6127099 and to 70.7% (GG), 26.9% (GA) and 2.3% (AA) for rs4074995. With adjustments applied for sex, age, body mass index, heart rate and season, hbPWV was 0.05 m/s (p = .042) lower with each additional copy of the minor allele (T) of rs6127099. In similarly adjusted analyses of 157 normotensive participants younger than 50 years, cfPWV was 0.32 m/s (p = .004) higher per additional copy of the T allele. Sensitivity analyses additionally accounting for the total-to-HDL serum cholesterol ratio, plasma glucose, glomerular filtration rate and 24 h systolic blood pressure were consistent. No other association of PWV with the genetic variants reached significance.

CONCLUSIONS

With an increasing number of rs6127099 T alleles, arterial stiffness, as exemplified by PWV, was lower in all participants in a muscular artery (hbPWV), but higher in young normotensive participants in an elastic artery (cfPWV).

Cardiovascular manifestations of primary hyperparathyroidism: a narrative review

Data on cardiovascular disease in primary hyperparathyroidism (PHPT) are controversial; indeed, at present, cardiovascular involvement is not included among the criteria needed for parathyroidectomy. Aim of this narrative review is to analyze the available literature in an effort to better characterize cardiovascular involvement in PHPT. Due to physiological effects of both parathyroid hormone (PTH) and calcium on cardiomyocyte, cardiac conduction system, smooth vascular, endothelial and pancreatic beta cells, a number of data have been published regarding associations between symptomatic and mild PHPT with hypertension, arrhythmias, endothelial dysfunction (an early marker of atherosclerosis), glucose metabolism impairment and metabolic syndrome. However, the results, mainly derived from observational studies, are inconsistent. Furthermore, parathyroidectomy resulted in conflicting outcomes, which may be linked to several potential biases. In particular, differences in the methods utilized for excluding confounding co-existing cardiovascular risk factors together with differences in patient characteristics, with varying degrees of hypercalcemia, may have contributed to these discrepancies. The only meta-analysis carried out in PHPT patients, revealed a positive effect of parathyroidectomy on left ventricular mass index (a predictor of cardiovascular mortality) and more importantly, that the highest pre-operative PTH levels were associated with the greatest improvements. In normocalcemic PHPT, it has been demonstrated that cardiovascular risk factors are almost similar compared to hypercalcemic PHPT, thus strengthening the role of PTH in the cardiovascular involvement. Long-term longitudinal randomized trials are needed to determine the impact of parathyroidectomy on cardiovascular diseases and mortality in PHPT.

Hope for CKD-MBD Patients: New Diagnostic Approaches for Better Treatment of CKD-MBD

BACKGROUND

Chronic kidney disease-mineral and bone disorder (CKD-MBD) patients have a huge morbidity and mortality. Only relatively minor progress in therapeutic strategies has been made in the past decades. This is at least partially due to a lack of predictive diagnostic tools allowing personalized treatment of CKD-MBD patients.

SUMMARY

In this review we describe recent progress in the diagnosis of disturbances of calcium and phosphate metabolism in patients with CKD-MBD, measuring biological active nonoxidized parathyroid hormone as well as the overall likelihood of a patient to get calcified.

KEY MESSAGE

There is hope. The new tools have the potential of allowing personalized therapy for the treatment of CKD-MBD and hence improving outcome.

Growth hormone production and action in N1E-115 neuroblastoma cells

Neuroblastoma cells are undifferentiated cells derived from the neural crest and are commonly used as models for studying neural function. Mouse N1E-115 neuroblastoma cells are derived from cancerous tissue and provide a model for studying the oncogenesis of neural cells. As growth hormone (GH) has been implicated as an autocrine or paracrine involved in neural regulation and in the induction or progression of cancer, the possibility that N1E-115 cells are sites of GH production and GH action was assessed. Using RT-PCR, cultured N1E-115 cells were found to express the mouse GH and GH receptor (GHR) genes. Immunocytochemistry demonstrated that both of the translated proteins (GH and its receptor) were abundantly present in the cytoplasm of these cells and their co-localization was established by confocal cytochemistry. GH action in these cells was determined in cells cultured for 72 h in the presence or absence of 10(-6) M or 10(-9) M mouse GH, which induced neurite sprouting and increased axon growth. In summary, the expression of GH and its receptor in GH responsive tumor-derived N1E-115 neuroblastoma cells suggests they provide a useful experimental model to assess GH actions in neural function or neural oncogenesis.

N-terminal parathyroid hormone-related peptide hyperpolarizes endothelial cells and causes a reduction of the coronary resistance of the rat heart via endothelial hyperpolarization

Parathyroid hormone-related peptide (PTHrP) is known to be a strong vasorelaxant peptide. The mechanisms by which PTHrP reduces the coronary resistance of the rat heart have not been worked out but seem to be independent of the classical PTH/PTHrP receptor-mediated, cAMP-dependent effect. In this study we hypothesized that PTHrP reduces the coronary resistance of the rat heart via endothelial cell hyperpolarization. Isolated microvascular endothelial cells from rat heart were incubated with PTHrP(1-36), and changes in the membrane potential were recorded via DiBAC fluorescence. Cells exposed to PTHrP showed a hyperpolarization of approximately 7mV. In the isolated Langendorff preparation, PTHrP-dependent vasodilatation of l-nitro-arginine-exposed hearts was abolished under depolarizing conditions (high potassium). Denudation of the endothelial cell layer significantly impaired the vasodilatory effect of PTHrP. In the presence of H89 (a cAMP/protein kinase A pathway antagonist) and indomethacin (a cyclooxygenase inhibitor), PTHrP dilated the vessels. In conclusion, PTHrP exerted a nitric oxide-independent vasodilatory effect that depends on endothelial cell hyperpolarization.

Inhibitory effect of mitragynine, an analgesic alkaloid from Thai herbal medicine, on neurogenic contraction of the vas deferens

The effect of an indole-alkaloid mitragynine isolated from the Thai medicinal herb kratom (Mitragyna speciosa) on neurogenic contraction of smooth muscle was studied in guinea-pig vas deferens. Mitragynine inhibited the contraction of the vas deferens produced by electrical transmural stimulation. On the other hand, mitragynine failed to affect the responses to norepinephrine and ATP. Mitragynine did not reduce KCl-induced contraction in the presence of tetrodotoxin, prazosin and alpha,beta-methylene ATP. Mitragynine inhibited nicotine- or tyramine-induced contraction. By using the patch-clamp technique, mitragynine was found to block T- and L-type Ca2+ channel currents in N1E-115 neuroblastoma cells. In the Ca2+ measurement by a fluorescent dye method, mitragynine reduced KCl-induced Ca2+ influx in neuroblastoma cells. The present results suggest that mitragynine inhibits the vas deferens contraction elicited by nerve stimulation, probably through its blockade of neuronal Ca2+ channels.

Endogenous parathyroid hormone-related protein functions as a neuroprotective agent

Parathyroid hormone-related protein (PTHrP) was discovered a dozen years ago as a product of malignant tumors. It is now known that PTHrP is a paracrine factor with multiple biological functions. One such function is to relax smooth muscle by inhibiting calcium influx into the cell. In the central nervous system, PTHrP and its receptor are widely expressed in neurons in the cerebral cortex, hippocampus and cerebellum. The function of PTHrP in the CNS is not known. Previous work has shown that expression of the PTHrP gene is depolarization-dependent in cultured cerebellar granule cells and depends specifically on L-type voltage sensitive calcium channel (L-VSCC) Ca(2+) influx. PTHrP has also been found to be capable of protecting these cells against kainic acid-induced excitotoxicity. Here, we tested the idea that mice with a PTHrP-null CNS might display hypersensitivity to kainic acid excitotoxicity. We found that these mice were six-fold more sensitive than control littermate mice to kainic-acid-induced seizures as well as hippocampal c-Fos expression. PTHrP-null embryonic mixed cerebral cortical cultures were more sensitive to kainic acid than control cultures, and PTHrP addition was found to be protective against kainate toxicity in both PTHrP-null and control cultures. By whole-cell techniques, PTHrP was found to reduce L-VSCC Ca(2+) influx in cultured mouse neuroblastoma cells. We conclude that PTHrP functions as a component of a neuroprotective feedback loop that is structured around the L-type calcium channel. This loop appears to be operative in vivo as well as in vitro.

Characterization of voltage-gated calcium currents in freshly isolated smooth muscle cells from rat tail main artery

The aim of the present study was to characterize voltage-gated Ca2+ currents in smooth muscle cells freshly isolated from rat tail main artery in the presence of 5 mmol L(-1) external Ca2+. Calcium currents were identified on the basis of their voltage dependencies and sensitivity to nifedipine, Ni2+ and cinnarizine. In the majority of the cells studied, T- and L-type currents were observed, while the remaining cells showed predominantly L-type currents. In the latter group of cells, holding potential change from -50 to either -70 or -90 mV increased the corresponding inward current amplitude while its voltage activation threshold remained unchanged. The steady state inactivation of L-type Ca2+ channels showed half-maximal inactivation at -38 mV. A Ca2+-dependent inactivation was also evident. Nifedipine (3 micromol L(-1)) blocked L-type but not T-type Ca2+ currents. Ni2+ (50 micromol L(-1)) as well as cinnarizine (1 micromol L(-1)) suppressed the nifedipine-resistant, T-type component of the currents. At higher concentrations, both Ni2+ (0.3-1 mmol L(-1)) and cinnarizine (10 micromol L(-1)) blocked the net inward current. Replacement of Ca2+ with 10 mmol L(-)1 Ba2+ significantly increased the amplitude of L-type Ca2+ currents. These results demonstrate that smooth muscle cells freshly isolated from rat tail main artery may be divided into two populations, one expressing both L- and T-type and the other only L-type Ca2+ channels. Furthermore, this report shows that in arterial smooth muscle cells cinnarizine potently inhibited T-type currents at low concentrations (1 micromol L(-1)) but also blocked L-type Ca2+ currents at higher concentrations (10 micromol L(-1)).

Signaling pathway and chronotropic action of parathyroid hormone in isolated perfused rat heart

Parathyroid hormone (PTH) activates both adenylyl cyclase and phospholipase C via the PTH-1 receptor. We previously reported that PTH increased heart rate and that this effect was mediated via the pacemaker current (I f ). However, it has been reported that PTH exerts its chronotropic effect via an interaction with adrenergic receptors or via L-type calcium channels. Thus, the objective of the study was to elucidate the exact mechanism of the chronotropic effect of PTH. We tested whether its chronotropic effects could be abolished by inhibitors of the following systems in isolated perfused rat hearts: alpha-adrenergic (prazosin); beta-adrenergic (propranolol); angiotensin II (CV11974); endothelin-1 (TAK044); calcium channel (verapamil); adenylyl cyclase (miconazole); phospholipase C (U73122) or I f (CsCl). In addition, we measured the cyclic adenosine monophosphate level of the heart after PTH administration. Whereas prazosin, propranolol, CV11974, TAK044, verapamil, and U73122 did not inhibit the chronotropic effect of PTH, CsCl or miconazole suppressed it significantly. PTH increased the cyclic adenosine monophosphate level of the atrium but not the left ventricle. These results indicate that the chronotropic actions of PTH are mediated via selective activation of adenylyl cyclase to increase the I f current.

New members of the parathyroid hormone/parathyroid hormone receptor family: the parathyroid hormone 2 receptor and tuberoinfundibular peptide of 39 residues

The parathyroid hormone (PTH) family currently includes three peptides and three receptors. PTH regulates calcium homeostasis through bone and kidney PTH1 receptors. PTH-related peptide, probably also through PTH1 receptors, regulates skeletal, pancreatic, epidermal, and mammary gland differentiation and bladder and vascular smooth muscle relaxation and has a CNS role that is under investigation. Tuberoinfundibular peptide of 39 residues (TIP39) was recently purified from bovine hypothalamus based on selective PTH2 receptor activation. PTH2 receptor expression is greatest in the CNS, where it is concentrated in limbic, hypothalamic, and sensory areas, especially hypothalamic periventricular neurons, nerve terminals in the median eminence, superficial layers of the spinal cord dorsal horn, and the caudal part of the sensory trigeminal nucleus. It is also present in a number of endocrine cells. Thus TIP39 and PTH2 receptor-influenced functions may range from pituitary and pancreatic hormone release to pain perception. A third PTH-recognizing receptor has been found in zebrafish.

Parathyroid hormone-related protein protects against kainic acid excitotoxicity in rat cerebellar granule cells by regulating L-type channel calcium flux

The parathyroid hormone-related peptide (PTHrP) and PTH/PTHrP receptor genes are widely expressed in the CNS and both are highly expressed in the cerebellar granule cell. We have shown previously that PTHrP gene expression in granule cells is depolarization-dependent in vitro and is regulated specifically by Ca2+ influx via L-type voltage-sensitive calcium channels (L-VSCCs). Kainic acid induces long-latency excitotoxicity in granule cells via L-VSCC-mediated Ca2+ influx. Here, we show that PTHrP is just as effective as the L-VSCC blocker, nitrendipine (NTR), in preventing kainate excitotoxicity. A competitive inhibitor of PTHrP binding abrogates its neuroprotective effect. Both NTR and PTHrP decrease 45Ca2+ influx to the same degree. These findings suggest that PTHrP functions in an autocrine/paracrine neuroprotective feedback loop that can combat L-VSCC-mediated excitotoxcity.

Parathyroid hormone-related protein in rat penis: expression, localization, and effect on cavernosal pressure

Although PTH-related protein-(1-36) [PTHrP-(1-36)] is known to be expressed in smooth muscle and to exert potent myorelaxant effects, its tonic effects on cavernosal smooth muscle has not yet been explored. Using the RT-PCR technique, the present study establishes that PTHrP messenger RNA is present in microdissected corpus cavernosa in the rat. In immunohistochemical studies using affinity-purified antibodies to middle regions of PTHrP, immunostaining was localized throughout the penile structures, including vessels, cavernosal smooth muscle, and trabecular fibroblasts. Strong immunostaining for PTHrP was also detected in the dorsal nerve bundles. In anesthetized rats, intracavernosally injected boluses of increasing doses of PTHrP-(1-36) (0.3-30 pmol in 100 microl saline) had little effect on intracavernosal pressure. However, they markedly potentiated the dilatory response to papaverine (8-800 nmol), increasing the papaverine-induced intracavernous pressure by 2.5-fold, close to the mean arterial pressure. In conclusion, the cavernosal expression of PTHrP messenger RNA, the distribution of immunoreactive PTHrP throughout the structuro-functional components of the erectile apparatus and its strong potentiating action on papaverine-induced cavernosal relaxation, collectively suggest that PTHrP participates in the control of cavernosal tone.

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.

Influence of thyroid status on hepatic alpha 1-adrenoreceptor responsiveness

The present work aimed to elucidate the influence of thyroid functional status on the alpha 1-adrenoreceptor-induced activation of hepatic metabolic functions. The experiments were performed in either a nonrecirculating liver perfusion system featuring continuous monitoring of portal pressure, PO2, pCa, and pH, or isolated hepatocytes from euthyroid, hyperthyroid, and hypothyroid rats. Hypothyroidism decreased the alpha 1-adrenergic stimulation of respiration, glycogen breakdown, and gluconeogenesis. These effects were accompanied by a decreased intracellular Ca2+ mobilization corroborating that those processes are regulated by the Ca(2+)-dependent branch of the alpha 1-adrenoreceptor signaling pathway. Moreover, in hyperthyroid rats the alpha 1-adrenergic-induced increase in cytosolic Ca2+ was enhanced, and glucose synthesis or mobilization was not altered. The thyroid status influenced neither the alpha 1-adrenergic stimulation of vascular smooth muscle contraction nor the alpha 1-agonist-induced intracellular alkalinization and protein kinase C (PKC) activation. Thus the distinct impairment of the Ca(2+)-dependent branch of the alpha 1-adrenoreceptor signaling pathway by thyroid status provides a useful tool to investigate the role played by each signaling pathway, Ca2+ or PKC, in controlling hepatic functions.

Activity-dependent expression of parathyroid hormone-related protein (PTHrP) in rat cerebellar granule neurons. Requirement of PTHrP for the activity-dependent survival of granule neurons

To identify genes whose expression is neuronal activity-dependent, we used an mRNA differential display technique and discovered that parathyroid hormone-related protein (PTHrP) is expressed in an activity-dependent manner in primary cultures of rat cerebellar granule neurons. PTHrP mRNA was expressed as early as 1 h by the addition of KCl to a final concentration of 25 mM to the culture medium. This expression was induced by Ca2+ influx through voltage-dependent L-type Ca2+ channels and regulated at the transcriptional step. PTHrP mRNA was persistently expressed before and after the time of commitment of granule neurons to apoptosis when they are cultured in the presence of 25 mM KCl or both 150 microM N-methyl-D-aspartic acid and 15 mM KCl, both of which promote the survival of these neurons. PTHrP was rapidly secreted into the culture medium in a depolarization-dependent manner. Parathyroid hormone/PTHrP receptor mRNA was also expressed in the primary cultures, and its expression was up-regulated by KCl and/or N-methyl-D-aspartic acid. The addition of anti-PTHrP antiserum to the culture medium resulted in a reduction of the activity-dependent survival of the granule neurons. These results suggest that PTHrP is involved in an autocrine loop and required for the survival of granule neurons.

Parathyroid hormone/adenylate cyclase coupling in vascular smooth muscle cells

Parathyroid hormone (PTH) has been implicated in hypertension, but PTH infusion results in vasodilation. PTH activates adenylate cyclase in vascular smooth muscle, but little is known about the factors that regulate PTH receptor/adenylate cyclase coupling in vascular cells. To characterize hormone-receptor signaling, we measured cyclic AMP levels in rat arterial smooth muscle cells in culture exposed to PTH (bovine 1-34). PTH yielded time- and concentration-dependent increases in cyclic AMP levels. Compared with isoproterenol, PTH was more potent, with a threshold at 2 x 10(-9) versus 5 x 10(-8) mol/L and half maximal responses at 10(-8) versus 2.4 x 10(-7) mol/L. PTH-induced increases in cyclic AMP were independent of extracellular calcium, cyclooxygenase metabolites, phospholipase C, and protein kinase C because PTH-induced increases in cyclic AMP were not prevented by variations in extracellular calcium, indomethacin, angiotensin II, vasopressin, and protein kinase C activators or inhibitors. PTH/adenylate cyclase coupling was G protein-dependent because increases in cyclic AMP were prevented by preincubation with cholera toxin but not with pertussis toxin. Prolonged exposure to PTH resulted in time- and concentration-dependent homologous desensitization of cyclic AMP responses. Desensitization occurred proximal to G protein/adenylate cyclase because after prolonged PTH, responses to forskolin and cholera toxin remained intact. Desensitization was independent of protein kinase A and receptor sequestration because cyclic AMP responses remained after prolonged exposure to forskolin and pretreatment with phenylarsine oxide, colchicine, and cytochalasin D. We conclude that in vascular smooth muscle cells, PTH is coupled to adenylate cyclase through a cholera toxin-sensitive G protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone analogues inhibit calcium mobilization in cultured vascular cells

Parathyroid hormone and parathyroid hormone-related protein lower blood pressure and relax contracted arteries. Parathyroid hormone also attenuates angiotensin II-induced vasoconstriction. To determine the cellular mechanism or mechanisms by which parathyroid hormone analogues antagonize pressor effects, we examined the effect of these peptides on angiotensin II-induced calcium mobilization in fura 2-AM-loaded cultured rat vascular smooth muscle cells. Either 100 nmol/L parathyroid hormone or parathyroid hormone-related protein significantly reduced the amount of calcium mobilized by 100 nmol/L angiotensin II. The attenuating effect of these peptides was mimicked by 10 mmol/L forskolin and 10 mmol/L isobutylmethylxanthine and was not dependent on the presence of extracellular calcium. This effect of the parathyroid hormone analogues was reduced when cells were pretreated with 100 mmol/L 2',5'-dideoxyadenosine, an adenylate cyclase inhibitor. Combined inhibition of cyclic nucleotide-dependent protein kinases eliminated the inhibitory effect of parathyroid hormone, whereas protein kinase C inhibition had no effect. Parathyroid hormone analogues decreased the amount of calcium released by inositol 1,4,5-trisphosphate in digitonin-permeabilized vascular smooth muscle cells. This effect was inhibited by treatment with 2',5'-dideoxyadenosine. These results suggest that these peptides attenuate inositol 1,4,5-trisphosphate-sensitive calcium mobilized by angiotensin II via an adenylate cyclase-dependent mechanism. This may be a mechanism by which acute administration of parathyroid hormone or parathyroid hormone-related peptide antagonizes vasoconstriction.

Renin stimulating properties of parathyroid hormone-related peptide in the isolated perfused rat kidney

Previous studies showed that PTHrP exhibits renal vasodilating, arteriolar cAMP stimulating and receptor binding properties. The present experiments were designed to study whether PTHrP may influence renin secretion. Rat kidneys were isolated and single-pass perfused at constant flow and stabilized pressure. Exposures to PTHrP or PTH stimulated a dose-dependent renin release reaching similar Vmax. The affinity (0.1 nM) and threshold concentration (0.01 nM) for PTHrP were about 10 times lower than for PTH. Compared to 10 microM isoproterenol, the maximum renin responses to PTHrP were similar but of shorter duration. The PTHrP dose-response curve was not affected by 10 microM indomethacin. Administered simultaneously, PTHrP and PTH displayed no additive effects. PTHrP-induced renin release as well as the role of extracellular calcium were further studied in nonfiltering kidneys, which were perfused at a constant flow and stable pressure in a closed circuit. Basal renin release was inversely related with perfusate calcium and was depressed by the calcium ionophore BAY-K8644. PTHrP (100 nM) induced a 1.6-fold increase of basal renin release in normocalcic perfusate. Removing calcium abolished renin responses. PTHrP reversed the inhibiting effects of hypercalcic media or BAY-K8644 on basal renin release. The results support calcium-mediated renin stimulating properties for PTHrP, via PTH receptors, independently from baroreceptors, macula densa and prostaglandins.

Parathyroid hormone activation of stretch-activated cation channels in osteosarcoma cells (UMR-106.01)

Cell-attached patches of membrane of osteoblast-like cells UMR-106.01 respond to bath application of parathyroid hormone (PTH) with an increase in the average activity, as well as the single channel conductance, of a stretch-activated non-selective cation channel. Correlations with whole cell membrane potential and conductance changes are considered.

Modification by solvents of the action of nifedipine on calcium channel currents in neuroblastoma cells

The effect of nifedipine dissolved in different solvents on the two types of calcium channel currents in neuroblastoma cells was investigated using the whole cell version of the patch clamp technique. Nifedipine dissolved in dimethylsulfoxide (nifedipine/DMSO) decreased the transient calcium channel (T channel) current by 50% at a concentration of 10 microM. This inhibitory effect was concentration-dependent and reversible. In contrast, T channel currents were not inhibited by nifedipine at a similar concentration dissolved in acetone or ethanol. Further experiments were carried out with dried nifedipine/DMSO. Dried nifedipine/DMSO powder re-dissolved in acetone or ethanol at a concentration of 10 microM decreased the T channel current by 32% and 37%, respectively. In addition, within the concentration range of 10 nM to 100 microM nifedipine/DMSO inhibited the long-lasting calcium channel (L channel) current more effectively than did nifedipine dissolved in acetone. The concentration of solvent (DMSO, ethanol, acetone) in the bath was fixed at 0.3% to reach different final concentrations of nifedipine. Solvents alone at a final concentration of 0.3% did not show any effect on T or L channel currents. UV absorbance measurements indicated that the combination of nifedipine, solvent and bath solution did not result in precipitation of the dihydropyridine during the experimental protocol. It is concluded that when DMSO is used as the solvent, nifedipine is not only a more effective L channel antagonist but also a T channel antagonist in neuroblastoma cells.

Parathyroid hormone selectively inhibits L-type calcium channels in single vascular smooth muscle cells of the rat

1. The active synthetic N-terminal fragment of bovine parathyroid hormone, bPTH-(1-34) at a concentration of 1 microM, decreased the peak amplitude of the long-lasting (L-type) calcium channel current by 37% (n = 14, P less than 0.01) in rat tail artery smooth muscle cells. By contrast, this fragment of parathyroid hormone (PTH) (1 microM) had no effect on the transient (T-type) calcium channel current in the same cell preparation. 2. The inhibitory effect of bPTH-(1-34) on L-channel currents was reversible and could be antagonized by the L-channel agonist, Bay K 8644. In contrast, bPTH-(1-34) inhibited Bay K 8644-induced amplification of L-channel currents. 3. The inhibitory effect of bPTH-(1-34) on L-Channel currents was dose dependent with a threshold concentration of less than 10(-7), and voltage dependent with increased inhibition at more positive holding potentials. However, this effect of bPTH-(1-34) was not dependent on different pulse lengths or interpulse intervals. 4. The kinetics of deactivation of L-channel currents were not changed although the instantaneous amplitude of the L-channel tail current was reduced by bPTH-(1-34). 5. Application of bPTH-(1-34) antagonists (10(-6) M-bPTH-(3-34) and 10(-5) M-bPTH-(7-34] did not result in any significant change in the magnitude of L-channel currents (n = 15 and n = 7, respectively). 6. Pre-incubation of cells with bPTH-(3-34) for more than 15 min abolished the inhibitory effect of bPTH-(1-34) on L-channel currents. 7. The present study provides direct evidence to demonstrate the PTH, an endogenous circulating hormone, is a selective inhibitor of L-channel currents in vascular smooth muscle cells.

The effects of parathyroid hormone on L-type voltage-dependent calcium channel currents in vascular smooth muscle cells and ventricular myocytes are mediated by a cyclic AMP dependent mechanism

The present study demonstrated that L channel currents were decreased in smooth muscle cells, and increased in ventricular myocytes by both bovine parathyroid hormone, (bPTH-(1-34)), and dibutyryl cyclic AMP (db-cAMP), using the whole cell version of the patch clamp technique with Ba2+ as the charge carrier. The effects of bPTH-(1-34) and db-cAMP on L channel currents were additive but not synergistic. Furthermore, the effects of bPTH-(1-34) on L channel currents in these 2 cell preparations were abolished in the presence of a cAMP antagonist. These results suggest that the effects of bPTH-(1-34) on L channel currents in vascular smooth muscle cells and ventricular myocytes are mediated by a cAMP-dependent mechanism.