Cardiac L-type calcium channel alpha 1-subunit is increased by cyclic adenosine monophosphate: messenger RNA and protein expression in intact bone

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

The CaV1.2 L-type calcium channel regulates bone homeostasis in the middle and inner ear

Bone remodeling of the auditory ossicles and the otic capsule is highly restricted and tightly controlled by the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa-Β ligand (RANKL)/receptor activator of nuclear factor kappa-Β (RANK) system. In these bony structures, a pathological decrease in OPG expression stimulates osteoclast differentiation and excessive resorption followed by accrual of sclerotic bone, ultimately resulting in the development of otosclerosis, a leading cause of deafness in adults. Understanding the signaling pathways involved in maintaining OPG expression in the ear would shed light on the pathophysiology of otosclerosis and other ear bone-related diseases. We and others previously demonstrated that Ca2+ signaling through the L-type CaV1.2 Ca2+ channel positively regulates OPG expression and secretion in long bone osteoblasts and their precursor cells in vitro and in vivo. Whether CaV1.2 regulates OPG expression in ear bones has not been investigated. We drove expression of a gain-of-function CaV1.2 mutant channel (CaV1.2TS) using Col2a1-Cre, which we found to target osteochondral/osteoblast progenitors in the auditory ossicles and the otic capsule. Col2a1-Cre;CaV1.2TS mice displayed osteopetrosis of these bones shown by μCT 3D reconstruction, histological analysis, and lack of bone sculpting, findings similar to phenotypes seen in mice with an osteoclast defect. Consistent with those observations, we found that Col2a1-Cre;CaV1.2TS mutant mice showed reduced osteoclasts in the otic capsule, upregulated mRNA expression of Opg and Opg/Rankl ratio, and increased mRNA expression of osteoblast differentiation marker genes in the otic capsule, suggesting both an anti-catabolic and anabolic effect of CaV1.2TS mutant channel contributed to the observed morphological changes of the ear bones. Further, we found that Col2a1-Cre;CaV1.2TS mice experienced hearing loss and displayed defects of body balance in behavior tests, confirming that the CaV1.2-dependent Ca2+ influx affects bone structure in the ear and consequent hearing and vestibular functions. Together, these data support our hypothesis that Ca2+ influx through CaV1.2TS promotes OPG expression from osteoblasts, thereby affecting bone modeling/remodeling in the auditory ossicles and the otic capsule. These data provide insight into potential pathological mechanisms underlying perturbed OPG expression and otosclerosis.

Increased Ca2+ signaling through CaV1.2 promotes bone formation and prevents estrogen deficiency-induced bone loss

While the prevalence of osteoporosis is growing rapidly with population aging, therapeutic options remain limited. Here, we identify potentially novel roles for CaV1.2 L-type voltage-gated Ca2+ channels in osteogenesis and exploit a transgenic gain-of-function mutant CaV1.2 to stem bone loss in ovariectomized female mice. We show that endogenous CaV1.2 is expressed in developing bone within proliferating chondrocytes and osteoblasts. Using primary BM stromal cell (BMSC) cultures, we found that Ca2+ influx through CaV1.2 activates osteogenic transcriptional programs and promotes mineralization. We used Prx1-, Col2a1-, or Col1a1-Cre drivers to express an inactivation-deficient CaV1.2 mutant in chondrogenic and/or osteogenic precursors in vivo and found that the resulting increased Ca2+ influx markedly thickened bone not only by promoting osteogenesis, but also by inhibiting osteoclast activity through increased osteoprotegerin secretion from osteoblasts. Activating the CaV1.2 mutant in osteoblasts at the time of ovariectomy stemmed bone loss. Together, these data highlight roles for CaV1.2 in bone and demonstrate the potential dual anabolic and anticatabolic therapeutic actions of tissue-specific CaV1.2 activation in osteoblasts.

Voltage-dependent calcium channels in chondrocytes: roles in health and disease

Chondrocytes, the single cell type in adult articular cartilage, have conventionally been considered to be non-excitable cells. However, recent evidence suggests that their resting membrane potential (RMP) is less negative than that of excitable cells, and they are fully equipped with channels that control ion, water and osmolyte movement across the chondrocyte membrane. Amongst calcium-specific ion channels, members of the voltage-dependent calcium channel (VDCC) family are expressed in chondrocytes where they are functionally active. L-type VDCC inhibitors such as nifedipine and verapamil have contributed to our understanding of the roles of these ion channels in chondrogenesis, chondrocyte signalling and mechanotransduction. In this narrative review, we discuss published data indicating that VDCC function is vital for chondrocyte health, especially in regulating proliferation and maturation. We also highlight the fact that activation of VDCC function appears to accompany various inflammatory aspects of osteoarthritis (OA) and, based on in vitro data, the application of nifedipine and/or verapamil may be a promising approach for ameliorating OA severity. However, very few studies on clinical outcomes are available regarding the influence of calcium antagonists, which are used primarily for treating cardiovascular conditions in OA patients. This review is intended to stimulate further research on the chondrocyte 'channelome', contribute to the development of novel therapeutic strategies and facilitate the retargeting and repositioning of existing pharmacological agents currently used for other comorbidities for the treatment of OA.

Simulated microgravity inhibits L-type calcium channel currents partially by the up-regulation of miR-103 in MC3T3-E1 osteoblasts

L-type voltage-sensitive calcium channels (LTCCs), particularly Cav1.2 LTCCs, play fundamental roles in cellular responses to mechanical stimuli in osteoblasts. Numerous studies have shown that mechanical loading promotes bone formation, whereas the removal of this stimulus under microgravity conditions results in a reduction in bone mass. However, whether microgravity exerts an influence on LTCCs in osteoblasts and whether this influence is a possible mechanism underlying the observed bone loss remain unclear. In the present study, we demonstrated that simulated microgravity substantially inhibited LTCC currents and suppressed Cav1.2 at the protein level in MC3T3-E1 osteoblast-like cells. In addition, reduced Cav1.2 protein levels decreased LTCC currents in MC3T3-E1 cells. Moreover, simulated microgravity increased miR-103 expression. Cav1.2 expression and LTCC current densities both significantly increased in cells that were transfected with a miR-103 inhibitor under mechanical unloading conditions. These results suggest that simulated microgravity substantially inhibits LTCC currents in osteoblasts by suppressing Cav1.2 expression. Furthermore, the down-regulation of Cav1.2 expression and the inhibition of LTCCs caused by mechanical unloading in osteoblasts are partially due to miR-103 up-regulation. Our study provides a novel mechanism for microgravity-induced detrimental effects on osteoblasts, offering a new avenue to further investigate the bone loss induced by microgravity.

L-type calcium channels play a crucial role in the proliferation and osteogenic differentiation of bone marrow mesenchymal stem cells

L-type voltage-dependent Ca(2+) channels (VDCC(L)) play an important role in the maintenance of intracellular calcium homeostasis, and influence multiple cellular processes. They have been confirmed to contribute to the functional activities of osteoblasts. Recently, VDCC(L) expression was reported in mesenchymal stem cells (MSCs), but the role of VDCC(L) in MSCs is still undetermined. The aim of this study was to determine whether VDCC(L) may be regarded as a new regulator in the proliferation and osteogenic differentiation of rat MSC (rMSCs). In this study, we examined functional Ca(2+) currents (I(Ca)) and mRNA expression of VDCC(L) in rMSCs, and then suppressed VDCC(L) using nifedipine (Nif), a VDCC(L) blocker, to investigate its role in rMSCs. The proliferation and osteogenic differentiation of MSCs were analyzed by MTT, flow cytometry, alkaline phosphatase (ALP), Alizarin Red S staining, RT-PCR, and real-time PCR assays. We found that Nif exerts antiproliferative and apoptosis-inducing effects on rMSCs. ALP activity and mineralized nodules were significantly decreased after Nif treatment. Moreover, the mRNA levels of the osteogenic markers, osteocalcin (OCN), bone sialoprotein (BSP), and runt-related transcription factor 2 (Runx2), were also down-regulated. In addition, we transfected α1C-siRNA into the cells to further confirm the role of VDCC(L) in rMSCs, and a similar effect on osteogenesis was found. These results suggest that VDCC(L) plays a crucial role in the proliferation and osteogenic differentiation of rMSCs.

L-type calcium channel activity in osteoblast cells is regulated by the actin cytoskeleton independent of protein trafficking

Voltage-dependent L-type calcium channels (VDCC) play important roles in many cellular processes. The interaction of the actin cytoskeleton with the channel in nonexcitable cells is less well understood. We performed whole-cell patch-clamp surface biotinylation and calcium imaging on different osteoblast cells to determine channel kinetics, amplitude, surface abundance, and intracellular calcium, respectively. Patch-clamp studies showed that actin polymerization by phalloidin increased the peak current density of I (Ca), whereas actin depolymerization by cytochalasin D (CD) significantly decreased the current amplitude. This result is consistent with calcium imaging, which showed that CD significantly decreased Bay K8644-induced intracellular calcium increase. Surface biotinylation studies showed that CD is not able to affect the surface expression of the pore-forming subunit α(1C). Interestingly, application of CD caused a significantly negative shift in the steady-state inactivation kinetics of I (Ca). There were decreases in the voltage at half-maximal inactivation that changed in a dose-dependent manner. CD also reduced the effect of activated vitamin D(3) (1α,25-D3) on VDCC and intracellular calcium. We conclude that in osteoblasts the actin cytoskeleton affects α(1C) by altering the channel kinetic properties, instead of changing the surface expression, and it is able to regulate 1α,25-D3 signaling through VDCC. Our study provides a new insight into calcium regulation in osteoblasts, which are essential in many physiological functions of this cell.

Regulation of chondrogenesis and chondrocyte differentiation by stress

Chondrogenesis and endochondral ossification are the cartilage differentiation processes that lead to skeletal formation and growth in the developing vertebrate as well as skeletal repair in the adult. The exquisite regulation of these processes, both in normal development and in pathologic situations, is impacted by a number of different types of stress. These include normal stressors such as mechanical loading and hypoxia as well pathologic stressors such as injury and/or inflammation and environmental toxins. This article provides an overview of the processes of chondrogenesis and endochondral ossification and their control at the molecular level. A summary of the influence of the most well-understood normal and pathologic stressors on the differentiation program is also presented.

Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3)

The cardiac subtype of the L-type voltage-sensitive Ca(2+) channel (VSCC) Cav1.2 (alpha(1C)) is the primary voltage-sensitive channel responsible for Ca(2+) influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca(2+) signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a coupling factor for bone resorption, coordinately downregulates Cav1.2 (alpha(1C)) and uniquely upregulates T-type channel Cav3.2 (alpha(1H)). No other voltage-sensitive channel alpha-subunit of the 10 that were surveyed was upregulated by 1,25(OH)(2)D(3). The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)(2)D(3) also alters the Ca(2+) permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)(2)D(3)-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 (alpha(1H)) in terminally differentiated osteocytes as we recently reported.

Expression of voltage sensitive calcium channel (VSCC) L-type Cav1.2 (α1C) and T-type Cav3.2 (α1H) subunits during mouse bone development

Voltage-sensitive calcium channels (VSCCs) are key regulators of osteoblast plasma membrane Ca(2+) permeability and are under control of calcitropic hormones. Subtype specific antibodies were used to probe L-type Ca(v)1.2 (alpha(1C)) and T-type Ca(v)3.2 (alpha(1H)) subunit expression during mouse skeletal development. Commencing from E14.5 and continuing through skeletal maturity, immunoreactivity of Ca(v)1.2 (alpha(1C)) subunits was evident in regions of rapid long bone growth, including the perichondrium, periosteum, chondro-osseous junction and trabecular bones. Ca(v)3.2 (alpha(1H)) subunits appeared simultaneously and followed a similar distribution pattern. Both subunits were observed in osteoblasts and chondrocytes under high magnification. Interestingly, Ca(v)3.2 (alpha(1H)) subunits were present, but Ca(v)1.2 (alpha(1C)) subunits were absent from osteocytes. Western Blot and immunohistochemical assessment of in vitro cell culture models of osteogenesis and chondrogenesis confirmed the in vivo observations. We conclude that both L-type Ca(v)1.2 (alpha(1C)) and T-type Ca(v)3.2 (alpha(1H)) VSCCs are dynamically regulated in bones and cartilages during endochondral bone development.

Topical glyceryl trinitrate treatment of chronic noninsertional achilles tendinopathy. A randomized, double-blind, placebo-controlled trial

BACKGROUND

Noninsertional Achilles tendinopathy is a degenerative overuse disorder. No method has been universally successful in treating this condition. Topically applied nitric oxide has been shown, in animal models, to be effective for the treatment of fractures and cutaneous wounds through mechanisms that may include stimulation of collagen synthesis in fibroblasts. The goal of the present study was to determine if topical glyceryl trinitrate improves clinical outcome measures in patients with Achilles tendinopathy.

METHODS

A prospective, randomized, double-blind, placebo-controlled trial involving a total of sixty-five patients (eighty-four Achilles tendons) was performed to compare continuous application of topical glyceryl trinitrate (at a dosage of 1.25 mg per twenty-four hours) with rehabilitation alone for the treatment of noninsertional Achilles tendinopathy.

RESULTS

Compared with the control group, the glyceryl trinitrate group showed reduced pain with activity at twelve weeks (p = 0.02) and twenty-four weeks (p = 0.03), reduced night pain at twelve weeks (p = 0.04), reduced tenderness at twelve weeks (p = 0.02), decreased pain scores after the hop test at twenty-four weeks (p = 0.005), and increased ankle plantar flexor mean total work compared with the baseline level at twenty-four weeks (p = 0.04). Twenty-eight (78%) of thirty-six tendons in the glyceryl trinitrate group were asymptomatic with activities of daily living at six months, compared with twenty (49%) of forty-one tendons in the placebo group (p = 0.001, chi-square analysis). The mean effect size for all outcome measures was 0.14.

CONCLUSIONS

Topical glyceryl trinitrate significantly reduced pain with activity and at night, improved functional measures, and improved outcomes in patients with Achilles tendinopathy.

Successful transgene expression with serial doses of aerosolized rAAV2 vectors in rhesus macaques

Bronchoscopic microspraying of recombinant adeno-associated viral (rAAV) vectors targets high doses of vector directly to pulmonary epithelium. Single-dose endobronchial gene therapy trials have been accomplished in cystic fibrosis patients; however, repeated dosing strategies are likely essential for lifetime correction. These studies address whether serial redosing with rAAV2 vectors results in an antiserotypic response and, furthermore, whether it triggers an inflammatory response prohibitive to transgene expression. Serial redosing of 9 x 10(11) infectious units of aerosolized rAAV2 vectors to rhesus macaques resulted in successful gene transfer by quantitative PCR (1.43 x 10(9) copies/g tissue) and transgene expression. Additionally, confocal microscopy and immunohistochemical analysis demonstrated in situ expression localized to the pulmonary epithelium. Although serial redosing did induce a heightened anti-neutralizing antibody response in sera, gene transfer prevailed with resultant expression. This study is the first to demonstrate successful gene transfer subsequent to repeated aerosolized doses of rAAV2 in immunocompetent nonhuman primates without associated inflammatory responses prohibitive to transgene expression.

Smooth muscle uses another promoter to express primarily a form of human Cav1.2 L-type calcium channel different from the principal heart form

Several different first exons and amino termini have been reported for the cardiac Ca channel known as alpha(1C) or Ca(V)1.2. The aim of this study was to investigate whether the expression of this channel is regulated by different promoters in smooth muscle cells and in heart in humans. Ribonuclease protection assay (RPA) indicates that the longer first exon 1a is found in certain human smooth muscle-containing tissues, notably bladder and fetal aorta, but that it is not expressed to any significant degree in lung or intestine. On the other hand, all four smooth muscle-containing tissues examined strongly express transcripts containing exon 1b, first reported cloned from human fibroblast cells. In addition, primary cultures of human colonic myocytes and coronary artery smooth muscle cells express predominantly transcripts containing exon 1b. The promoter immediately upstream of exon 1b was cloned, and it displays functional promoter activity when luciferase-expressing constructs were transfected into three different cultured smooth muscle cells: primary human coronary artery smooth muscles cells, primary human colonocytes, and the fetal rat aorta-derived A7r5 cell line. These results indicate that expression in smooth muscle is primarily driven by a promoter different from that which drives expression in cardiac myocytes.

A new promoter for alpha1C subunit of human L-type cardiac calcium channel Ca(V)1.2

The cardiac Ca channel known as alpha1C or Ca(V)1.2 is shown to express a new longer first exon equivalent to that formerly reported in rabbit heart or rat aorta. Ribonuclease protection assay indicates that this exon is found in the majority of Ca(V)1.2 transcripts in human heart RNA. The presence of this exon also suggests that expression of this transcript is driven by a promoter immediately upstream of this exon and its 5' untranslated region. The putative promoter exhibits 69% homology to its rat counterpart and displays functional promoter activity when transfected into heart cells in culture in luciferase-expressing constructs.

Tubular and cellular localization of the cardiac L-type calcium channel in rat kidney

BACKGROUND

The mRNAs of several types of calcium channels have been identified in intact rat kidney, and L-type calcium channels cause changes in intracellular calcium in primary cultures of distal tubule cells. The aim of this study was to evaluate the tubular and cellular distribution of the alpha1C subunit of the L-type calcium channel in intact kidney.

METHODS

RT-PCR and Northern blot analysis were used to assess the regional abundance of the mRNA of this channel. Immunocytochemistry combined with confocal microscopy and surface biotinylation were applied to determine the tubular and cellular localization of the protein.

RESULTS

Northern blot and RT-PCR analysis indicated that the mRNA of the alpha1C subunit of the cardiac L-type calcium channel was present in whole rat kidney, kidney tubules and kidney cell lines. Western blot of lysates from whole kidney, kidney tubules or cell lines revealed bands of approximately 190 kD for the alpha1C subunit and approximately 60 kD for the beta3 subunit. Confocal immunohistochemistry indicated that the alpha1C subunit of this channel was co-expressed in cells of the distal tubule that express calbindin-D28K, but not in intercalated cells. The alpha1C subunit was also highly expressed in both outer and inner medullary collecting ducts. Serial confocal microscopic images or surface biotinylation experiments determined that the channel was predominantly on the basolateral membrane but had some distribution on the apical membrane.

CONCLUSIONS

The distribution and cellular localization of the alpha1C subunit of cardiac L-type calcium channel suggest it is probably involved in intracellular and membrane calcium signaling.

Altered cellular calcium regulatory systems in a rat model of cirrhotic cardiomyopathy

BACKGROUND & AIMS

Decreased cardiac contractility has been observed in cirrhosis, but the cause remains unclear. Because cardiomyocyte contraction depends on Ca2+ influx entering via L-type Ca2+ channels (I(Ca,L)s) to activate Ca2+ release from the sarcoplasmic reticulum, we postulated that the Ca2+ transients may be abnormal in cirrhotic cardiomyocytes. We aimed to investigate the status of the cellular Ca2+-regulatory system in a rat model of cirrhotic cardiomyopathy.

METHODS

Cirrhosis was induced by bile duct ligation. The I(Ca,L) protein expression was detected by Western blotting. Ca2+ currents were measured electrophysiologically. The intracellular Ca2+ system, which includes the ryanodine receptor 2 (RYR2), sarcoplasmic reticulum Ca2+-pump adenosine triphosphatase (SERCA2), and Ca2+-binding protein were quantitatively assayed by reverse-transcription polymerase chain reaction and Western blots and functionally by 3H-ryanodine binding and radiolabeled Ca2+ uptake.

RESULTS

I(Ca,L) protein expression was reduced in cirrhotic rats compared with controls, and the peak inward Ca2+ current was significantly less. At all membrane potentials examined, I(Ca,L)s current densities from cirrhotic animals were consistently lower, and the response to maximal isoproterenol stimulation was also significantly lower. Protein expression and messenger RNA transcription for RYR2, SERCA2, and calsequestrin were quantitatively unchanged, and 3H-ryanodine binding characteristics and Ca2+ uptake were also unaltered.

CONCLUSIONS

We conclude that the decreased cardiac contractility in cirrhotic cardiomyocytes is caused by dysfunction of the Ca2+-regulatory system. Plasma membrane I(Ca,L)s are quantitatively reduced and functionally depressed, whereas intracellular systems are intact.

C-terminal parathyroid hormone-related protein (PTHrP) (107-139) stimulates intracellular Ca(2+) through a receptor different from the type 1 PTH/PTHrP receptor in osteoblastic osteosarcoma UMR 106 cells

Studies were undertaken to determine whether PTH-related protein (PTHrP) (107-139) mobilizes [Ca(2+)](i) in osteoblastic osteosarcoma UMR 106 cells. PTHrP (107-139), in a manner similar to PTHrP (107-111), induced a rapid [Ca(2+)](i) response in these cells that was dose dependent (EC(50) of approximately 0.1 pM) and more efficient than that of PTHrP (1-36) (EC(50) of approximately 1 nM). This effect of PTHrP (107-139) was abrogated by micromolar doses of verapamil or nifedipine. However, it was unaffected by 10 microM U73122 (a phospholipase C inhibitor), 100 microg/ml heparin (an inositol 1,4,5-trisphosphate receptor inhibitor), or 400 ng/ml pertussis toxin (a G(i) inhibitor), which inhibited the [Ca(2+)](i) response to PTHrP (1-36), or by either 25 nM bisindolylmaleimide I (BIM), a protein kinase (PK) C inhibitor, or 1 microM phorbol-12-myristate-13-acetate preincubation (22 h). PTHrP (107-139) and PTHrP (1-36), at 100 nM, desensitized the [Ca(2+)](i) response to a second challenge with the same peptide, but not with the other peptide in these cells. PTHrP (7-34), a type 1 PTH/PTHrP receptor (PTH1R) antagonist, decreased the effect of PTHrP (1-36) on [Ca(2+)](i). In contrast, PTHrP (107-111), but neither PTHrP (109-138) nor PTHrP (7-34), abolished this effect of PTHrP (107-139). Both PTHrP (107-139) and PTHrP (1-36), added together at submaximal doses, induced a higher [Ca(2+)](i) response. Moreover, PTHrP (107-139) increased the efficacy of PTHrP (1-36) on [Ca(2+)](i), but decreased its induced increase in PKA activity in these cells. Verapamil or nifedipine (at 50 microM) or 25 nM BIM, but not 25 microM adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, a PKA inhibitor, abolished the PTHrP (107-139)-induced increase in interleukin 6 messenger RNA (assessed by RT, followed by PCR) in UMR 106 cells. This peptide also increased c-fos messenger RNA in these cells; an effect inhibited by BIM, but unaffected by either verapamil or EGTA. These findings support the existence of high-affinity receptors for PTHrP (107-139), associated with an induced Ca(2+) influx, different from the PTH1R in UMR 106 cells. The present results suggest that PTHrP could affect bone turnover by interacting with the PTH1R and other yet unknown receptors in bone cells through complex mechanisms.