Cytoplasmic pH is differently regulated in the monoblastic U-937 and erythroleukemic K-562 cell lines

Regulation of cytoplasmic pH (pHi) of the human monoblastic U-937 and erythroleukemic K-562 cell lines was investigated. The apparent resting pHi, as assessed by the fluorescent pH probe quenel, were 6.61 and 6.75 for the U-937 and K-562 cells, respectively. When extracellular Na+ was substituted by equimolar choline+, pHi decreased by about 0.2 units. The protein kinase C activating beta-form of the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 10(-10) and 10(-7) M) induced a dose-dependent alkalinization in both cell types of 0.03-0.12 units, whereas the alpha-form was inactive. The response was detectable after about 2 min and reached steady-state 10-15 min later. In the K-562 cells the alkalinization was mediated by Na+/H+ exchange as it was accompanied by stimulation of H+ extrusion and abolished by Na+ removal. The TPA response in the U-937 cells, however, was unaffected by Na+ removal, not accompanied by H+-efflux, and thus unrelated to Na+/H+ exchange. Since electron microscopy indicated development of multivesicular bodies with an acidic interior, the alkalinization can probably be accounted for by an intracellular mechanism. Ionomycin (10(-5) M) induced a rapid increase in the cytoplasmic Ca2+ concentration of both cell types and this response was accompanied by acidification followed by a Na+-dependent recovery. In the U-937, but not in the K-562, cells this recovery was followed by a net alkalinization. It is concluded that both cell types possess a Na+/H+ exchange of importance for pHi but that this mechanism is regulated differently in the U-937 and K-562 cells.

Modulation of the Ca2+-sensing function of parathyroid cells in vitro and in hyperparathyroidism

When raising the extracellular Ca2+ concentration stepwise from 0.5 to 3.0 mM, bovine parathyroid cells reacted with initial transient and sustained elevations of the cytoplasmic Ca2+ concentration (Ca2+i), as well as more than 50% inhibition of parathyroid hormone (PTH) release. Human parathyroid adenoma cells and bovine cells cultured for 1 day or exposed to a low concentration of a monoclonal antiparathyroid antibody exhibited right-shifted dependencies of PTH release and Ca2+i on extracellular Ca2+ and reduced Ca2+i transients. The protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) further right-shifted the dose response relationship for Ca2+ regulated Ca2+i of the adenoma cells, whereas the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) tended to normalize it, without affecting Ca2+i of normal bovine cells. In cells from an oxyphil adenoma and a parathyroid carcinoma as well as in bovine cells cultured 4 days or exposed to a high concentration of the antiparathyroid antibody, there were no Ca2+i transients, very small increases in steady-state Ca2+i and nonsuppressible PTH release. The results suggest that reduced availability of a putative Ca2+-receptor and increased protein kinase C activity may be important factors in the decreased Ca2+ sensitivity of abnormal parathyroid cells.

Cytoplasmic Ca2+ concentration of single normal human and bovine parathyroid cells measured by dual wavelength microfluorometry

Dual wavelength microfluorometry was utilized to measure the cytoplasmic calcium concentration (Cai2+) of single parathyroid cells loaded with the indicator fura-2. The method enabled the first registrations of Cai2+ of normal human parathyroid cells, available only in minute numbers. At 0.5 mM extracellular Ca2+, the Cai2+ levels were similar in normal human and bovine cells. Both cell types responded with an initial Cai2+ transient followed by a sustained increase when raising extracellular Ca2+ to 3.0 mM. The sustained effect exhibited a sigmoidal relation to extracellular Ca2+ in the 0.5-3.0 mM range. Although the increase was somewhat greater in the human cells, the half maximal responses were obtained at almost identical extracellular Ca2+ concentrations. Whereas K+ depolarization decreased Cai2+, the Ca2+ channel blocker D-600 had dual actions, raising Cai2+ at 0.5 mM Ca2+ and decreasing it at 3.0 mM Ca2+, and the effects were similar in the bovine and human cells. The present experimental approach verified the validity of utilizing bovine cells as controls in studies of human parathyroid tissue and it appears suitable for analysis of the role of different subpopulations of parathyroid cells in the abnormal parathyroid tissue of patients with hyperparathyroidism.

TPA induces cytoplasmic alkalinization in human monoblastic U-937 cells without activation of Na+/H+ exchange

The effect of the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on cytoplasmic pH (pHi) and H+ extrusion was studied in the human monoblastic cell line U-937. About 2 min after addition of TPA, pHi started to increase and reached a steady state 10-15 min later. The resulting alkalinization corresponded to 0.03 and 0.09 pH units at 10(-10) and 10(-7) M TPA, respectively. The TPA-induced increase in pHi was independent of the presence of extracellular Na+. Moreover, TPA did not affect the H+ extrusion from the U-937 cells. Together these observations indicate the presence of a novel mechanism for TPA-induced cytoplasmic alkalinization. This mechanism is independent of Na+/H+ exchange across the plasma membrane, but may involve organelle sequestration of H+.

Development of abnormal parathyroid cell function during monolayer culture and its relation to cellular hypertrophy and proliferation

Primary cultures of adult bovine parathyroid cells grown for up to 8 days were used to study the relationship between regulation of parathyroid hormone (PTH) release, cytoplasmic Ca2+ (Ca2+i) and cell growth. Cell counting indicated that proliferation was inversely proportional to the seeding density. The culture was associated with cell enlargement without apparent relation to the proliferation rate. Moreover, flow cytometric analysis of the DNA distribution excluded that the hypertrophy was due to a high proportion of the cells being in the S and G2 stages of the cell cycle. With progressive time in culture there was a gradual deterioration of the ability of increasing concentrations of extracellular Ca2+ to suppress PTH secretion and raise Ca2+i. This functional dedifferentiation developed to the same extent despite considerable differences in proliferation rates. The data show that parathyroid cells in monolayer culture develop functional and some of the morphological characteristics of parathyroid cells from patients with hyperparathyroidism and that the functional abnormality seems to be related to hypertrophy but not to proliferation of the cells.

Dimethyl sulfoxide increases cytoplasmic Ca2+ concentration and inhibits parathyroid hormone release in normal bovine and pathological human parathyroid cells

The acute effects of dimethyl sulfoxide (DMSO) on parathyroid hormone (PTH) release and the cytoplasmic Ca2+ concentration (Ca2+i) were studied in dispersed bovine cells and cells isolated from human parathyroid adenomas. At extracellular Ca2+ concentrations in the 0.5-3.0 mM range, but not at less than 25 nM, addition of 2% DMSO caused a rapid rise of Ca2+i. This effect corresponded to an inhibition of PTH release and there was a strong negative correlation between Ca2+i and secretion. The actions of DMSO on Ca2+i and PTH release were less pronounced in the pathological human cells. The data are consistent with a DMSO effect on the Ca2+-sensor function of the parathyroid cell, possibly mediated by an altered plasma membrane fluidity.

Bimodal regulation of secretion by cytoplasmic Ca2+ as demonstrated by the parathyroid

Bovine parathyroid cells were used to study parathyroid hormone (PTH) release and the cytoplasmic Ca2+ concentration (Cai2+). When the extracellular Ca2+ concentration was decreased from 3.0 to 0.5 mM, perifused cells reacted with rapid stimulation of PTH release. However, a further reduction of extracellular Ca2+ to less than 10 nM resulted in prompt inhibition. Both effects were readily reversible. Using the intracellular Ca2+ indicator quin-2 also as a buffer for calcium it was possible to control Cai2+ within the 20-600 nM range. PTH release was found to increase with Cai2+ up to 200 nM but was gradually suppressed above this concentration.

Functional dedifferentiation of parathyroid cells results both from a defective regulation and action of cytoplasmic Ca2+

Monolayer culture of bovine parathyroid cells for 24 hours resulted in a right-shift of the dose-effect relationships for Ca2+-inhibition of parathyroid hormone (PTH) release and the dependence of the cytoplasmic Ca2+ concentration (Cai2+) on extracellular Ca2+ as well as in a less suppressible hormone release. After 4 days of culture, hormone secretion was almost non-suppressible and Cai2+ increased poorly in response to a rise in extracellular Ca2+. Ionomycin, a Ca2+ ionophore, raised Cai2+, but there was only a small inhibition of PTH release and the correlation between Cai2+ and secretion was weak. A deteriorated Cai2+ regulation and a decreased inhibitory action of cytoplasmic Ca2+ on PTH release were also found in cells from human parathyroid adenomas. Functional dedifferentiation of the parathyroid cell thus results from both defective regulation and action of cytoplasmic Ca2+.

Dual effects of a new hypocalcemic agent, WR-2721, on cytoplasmic Ca2+ and parathyroid hormone release of dispersed parathyroid cells from patients with hyperparathyroidism

The effects of the new hypocalcemic agent, WR-2721, on calcium-regulated parathyroid hormone (PTH) release, cytoplasmic Ca2+ (Ca2+i) and membrane potential were measured in dispersed parathyroid cells from patients with hyperparathyrodisim (HPT). The drug had no effects in the absence of extracellular Ca2+ but acted synergistically with Ca2+ in the 0.5-1.5 mM range by depolarizing the cells, increasing Ca2+i and inhibiting PTH release. Although the depolarizing effect of 3.0 mM Ca2+ was unaffected by WR-2721 the drug antagonized the effect of Ca2+ by decreasing Ca2+i and stimulating PTH release. Whereas the inhibitory actions of WR-2721 on PTH release may result from the activation of the mechanism for Ca2+ gating in the parathyroid cell plasma membrane, the stimulatory effect probably reflects increased intracellular Ca2+ sequestration. The drug is considered potentially important for the treatment of HPT.

Calcium-activated calcium permeability in parathyroid cells

The Ca2+ receptor mechanism of the parathyroid cell was studied using La3+ as a probe. La3+ was found to bind to the cell surface without further penetration. Measurements of 45Ca fluxes and the cytoplasmic Ca2+ concentration (Ca2+i) revealed a stimulatory component in the action of La3+ on Ca2+ permeability resulting in a rise in Ca2+i. These effects mimicked those obtained when raising the extracellular Ca2+ concentration from 0.5 to 3.0 mM, but the actions of La3+ and Ca2+ were not additive. The results suggest the existence of a novel Ca2+ permeability physiologically activated by binding of Ca2+ to an external receptor.

Effects of metabolic inhibitors on the distribution of 45-Ca in subcellular fractions from pancreatic islets

It is well established that intracellular calcium buffering is of vital importance for the regulation of insulin release. The metabolic inhibitors, cyanide and N-ethylmaleimide, both decreased the 45-Ca content of a mitochondrial fraction isolated from glucose-stimulated pancreatic islets. We suggest that the main mechanism behind this decrease is an impaired uptake of the isotope, since cyanide had no effect on the 45-Ca wash-out from the mitochondrial fraction of preloaded islets.

Clonidine promotes the accumulation of 45Ca in pancreatic beta-cell organelles

Glucose-stimulated insulin release from pancreatic islets of ob/ob-mice was inhibited by 10(-9) M of the alpha 2-adrenergic agonist clonidine. This inhibitory effect was abolished by 10(-7) M of the antagonist yohimbine. Loading the islets with 45Ca during the clonidine exposure followed by isolation of subcellular fractions under conditions known to minimize the 45Ca redistribution resulted in increased accumulation of the isotope in the mitochondrial and microsomal fractions. It is suggested that clonidine inhibits glucose-stimulated insulin release by increasing the organelle sequestration of Ca2+.