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H Scherübl

Publications and source records attributed to H Scherübl.

At least 73 records · Page 4Linked to original sources
PubMed1993

Regulation of calcitonin secretion in vitro.

The concentration of extracellular calcium rightly regulates calcitonin secretion by calcium influx through dihydropyridine-sensitive voltage-dependent calcium channels; the result is an increase in intracellular calcium. There also exists a cAMP-dependent pathway of calcitonin release activated by glucagon or growth hormone releasing hormone. In thyroid C-cells, as in all cells, there is dual regulation of adenylate cyclase, mediated by inhibitory or stimulatory G proteins; glucagon stimulated cAMP production can be inhibited by somatostatin via pertussis toxin sensitive inhibitory G proteins. Somatostatin inhibits not only cAMP dependent but also calcium-dependent calcitonin secretion. Furthermore, somatostatin inhibits voltage dependent calcium channel currents thereby lowering cytosolic calcium. These actions also involve a pertussis toxin-sensitive inhibitory G protein but they occur independently of changes in the cytosolic cAMP concentration. Thus multiple interactions between second messenger systems at different cellular levels modulate calcitonin secretion.

Animals↗
PubMed1993

Major role of dihydropyridine-sensitive Ca2+ channels in Ca(2+)-induced calcitonin secretion.

Endocrine cells are known to possess multiple types of Ca2+ channels. In neurons, omega-conotoxin-sensitive N-type Ca2+ channels have been shown to play a dominant role in neurotransmitter release, but uncertainty remains about the types of Ca2+ channels involved in stimulus-secretion coupling in endocrine cells. We investigated the relative contribution of 1,4-dihydropyridine-sensitive and omega-conotoxin-sensitive Ca2+ channels to Ca(2+)-induced calcitonin release in parafollicular cells of the thyroid (C cells). In whole cell voltage-clamp experiments, both 1,4-dihydropyridine-sensitive and omega-conotoxin-sensitive Ca2+ channel currents were identified. The dihydropyridine isradipine (1 microM) but not omega-conotoxin (1 microM) inhibited the steady-state Ca2+ influx at physiological membrane potentials, the spontaneous electrical activity, and calcitonin secretion (at 2-h incubations). Moreover, suppression of the spontaneous electrical activity by the Na+ channel blocker tetrodotoxin did not affect calcitonin release. We conclude that 1,4-dihydropyridine-sensitive Ca2+ channels play a major role in Ca(2+)-dependent calcitonin release and that calcitonin secretion due to Ca2+ influx proceeds even in the absence of action potentials.

Animals↗
PubMed1993

Molecular mechanisms of somatostatin's inhibition of hormone release: participation of voltage-gated calcium channels and G-proteins.

Somatostatin is known to inhibit hormone release from various neuroendocrine cells. In order to understand the mechanisms underlying somatostatin's action we performed patch-clamp experiments in GH3 pituitary, rMTC 44-2 thyroid and BON carcinoid cells. Calcium-mediated hormone release depended on the intracellular calcium concentration and thus on the calcium influx through voltage-gated calcium channels. In addition to inhibiting the cAMP-dependent secretory pathway, somatostatin reduced the calcium inward currents and thereby hormone release. The inhibition of voltage-gated calcium channels by somatostatin was mediated by "signal transducing" Go proteins. Thus, somatostatin's actions on hormone release involve both cAMP and intracellular calcium as second messengers. Patch-clamp experiments of voltage-gated calcium channels allow functional studies on the coupling of somatostatin receptors to cellular effector systems.

Animals↗
PubMed1992

Different beta-subunits determine G-protein interaction with transmembrane receptors.

Regulatory GTP-binding proteins (G proteins) are membrane-attached heterotrimers (alpha, beta, gamma) that mediate cellular responses to a wide variety of extracellular stimuli. They undergo a cycle of guanine-nucleotide exchange and GTP hydrolysis, during which they dissociate into alpha-subunit and beta gamma complex. The roles of G-protein alpha-subunits in these processes and for the specificity of signal transduction are largely established; the beta- and gamma-subunits are essential for receptor-induced G-protein activation and seem to be less diverse and less specific. Although the complementary DNAs for several beta-subunits have been cloned, isolated subunits have only been studied as beta gamma complexes. Functional differences have been ascribed to the gamma-subunit on the basis of extensive sequence similarity among beta-subunits and apparent heterogeneity in gamma-subunit sequences. Beta gamma complexes can interact directly or indirectly with different effectors. They seem to be interchangeable in their interaction with pertussis toxin-sensitive alpha-subunits, so we tested this by microinjecting antisense oligonucleotides into nuclei of a rat pituitary cell line to suppress the synthesis of individual beta-subunits selectively. Here we show that two out of four subtypes of beta-subunits tested (beta 1 and beta 3) are selectively involved in the signal transduction cascades from muscarinic M4 (ref. 4) and somatostatin receptors, respectively, to voltage-dependent Ca2+ channels.

Animals↗
PubMed1992

Transient membrane hyperpolarizations due to spontaneous fluctuations of the cytosolic Ca2+ in osteoblast-like cells.

The recently developed nystatin modification of the patch clamp technique allows stable whole-cell recordings without affecting the intracellular Ca2+ buffering capacity and thereby may provide a means to indirectly monitor spontaneous changes in the intracellular Ca2+ concentrations. To test this hypothesis, we applied the nystatin method to the well-characterized ROS 17/2.8 osteoblast-like cell system, where rises of the intracellular Ca2+ are known to cause transient hyperpolarizations via activation of Ca2+ -dependent K+ channels. Additionally to minor fluctuations (10-20 mV) around a mean potential of -42.1 +/- 4.2 mV, we observed spontaneously occurring, transient hyperpolarizations to membrane potentials as negative as -80 mV. These transient hyperpolarizations were not eliminated by Ca2+ entry blockers but abolished by intracellular infusion of 10 mM EGTA. Thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, hyperpolarized the cells close to the K+ reversal potential. Moreover, voltage-clamp studies revealed an intermittendly activating Ca2+-dependent K+ conductance. These results strongly suggest that the nystatin method is particularly suitable to study Ca(2+)-dependent channels and thereby spontaneous changes in the intracellular Ca2+.

Animals↗
PubMed1992

Inhibitory effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells: involvement of pertussis toxin-sensitive G-proteins.

The effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells of the rat medullary thyroid carcinoma cell line rMTC 6-23 was investigated. Intracellular cAMP accumulation as well as calcitonin secretion could be dose-dependently stimulated by rat growth hormone releasing factor (rGRF). The long-acting somatostatin analogue octreotide inhibited rGRF-stimulated cAMP accumulation and calcitonin secretion dose dependently but failed to block 8-bromo-cAMP-stimulated calcitonin secretion. The inhibitory effect of octreotide on rGRF-induced calcitonin secretion was partially abolished by pretreating the cells with pertussis toxin. The octreotide effect was not due to changes in the degradation of cAMP, as it was similarly seen in the presence of isobutylmethylxanthine. Thus we conclude that pertussis toxin-sensitive G-proteins are involved in the cAMP-mediated regulation of calcitonin secretion in C-cells.

Animals↗
PubMed1992

Inhibition of Ca(2+)-induced calcitonin secretion by somatostatin: roles of voltage dependent Ca2+ channels and G-proteins.

Somatostatin has recently been applied therapeutically for hypercalcitonemia in patients with calcitonin-producing tumours. Using calcitonin-secreting cells (C-cells) of the medullary thyroid carcinoma cell line rMTC 44-2, we investigated the inhibitory action of somatostatin on calcitonin release, cytosolic Ca2+ and Ca2+ channel currents. The Ca(2+)-induced rises of the cytosolic Ca2+ and calcitonin secretion were greatly inhibited by somatostatin or its stable analogue octreotide. The effects of somatostatin were pertussis toxin-sensitive. Under voltage clamp conditions, C-cells exhibited slowly inactivating Ca2+ channel currents. Bath application of 100 nM somatostatin reversibly reduced the Ca2+ channel current by about 30%. The Ca2+ channel current and its inhibition by somatostatin were not affected by intracellularly applied cyclic AMP. Moreover, pretreating the cells with pertussis toxin had no effect on the control Ca2+ channel currents but greatly abolished its inhibition by somatostatin. The data show that somatostatin suppresses the Ca(2+)-stimulated calcitonin secretion by inhibiting voltage-dependent Ca2+ channel currents and by lowering cytosolic Ca2+. These actions of somatostatin involve pertussis toxin-sensitive G-proteins and occur independently of changes in the cyclic AMP concentration.

Animals↗
PubMed1991

Steady-state currents through voltage-dependent, dihydropyridine-sensitive Ca2+ channels in GH3 pituitary cells.

Ca2+ influx via voltage-dependent Ca2+ channels is known to be elicited during action potentials but possibly also occurs at the resting potential. The steady-state current through voltage-dependent Ca2+ channels and its role for the electrical activity was, therefore, investigated in pituitary GH3 cells. Applying the recently developed 'nystatin-modification' of the patch-clamp technique, most GH3 cells (18 out of 23 cells) fired spontaneous action potentials from a baseline membrane potential of 43.7 +/- 4.6 mV (mean +/- s.d., n = 23). The frequency of action potentials was stimulated about twofold by Bay K 8644 (100 nM), a Ca(2+)-channel stimulator, and action potentials were completely suppressed by the Ca(2+)-channel blocker PN 200-110 (100 nM). Voltage clamping GH3 cells at fixed potentials for several minutes and with 1 mM Ba2+ as divalent charge carrier, we observed steady-state Ca(2+)-channel currents that were dihydropyridine-sensitive and displayed a U-shaped current-voltage relation. The results strongly suggest that the observed long lasting, dihydropyridine-sensitive Ca(2+)-channel currents provide a steady-state conductivity for Ca2+ at the resting potential and are essential for the generation of action potentials in GH3 pituitary cells.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Electrophysiological properties of rat calcitonin-secreting cells.

The spontaneous electrical activity of calcitonin-secreting cells (C-cells) appears to play an important role in the coupling of fluctuations in the extracellular Ca2+ to changes in the intracellular Ca2+ concentration and thus for calcitonin secretion. Using the patch clamp technique, we have investigated the spontaneous electrical activity and the underlying ionic currents in C-cells of the rMTC 44-2 cell line. With 1.2 mM external Ca2+, the membrane potential was -46.1 +/- 1.7 mV (n = 58) and about 30% of the cells spontaneously fired action potentials. Rising the external Ca2+ to 1.8 mM caused the cells to depolarize to -42.1 +/- 2.1 mV (n = 56) and spontaneous electrical activity was seen in about 70% of cells. Under voltage clamp conditions, tetrodotoxin-sensitive voltage-dependent Na+ currents, outward-rectifying K+ currents and isradipine-, omega-conotoxin-sensitive as well as isradipine- and omega-conotoxin-insensitive Ca2+ currents were observed. These voltage-dependent currents appear to be the major ionic currents contributing to action potentials in C-cells and to participate in calcitonin secretion.

Action Potentials↗
PubMed1990

A slowly inactivating calcium current works as a calcium sensor in calcitonin-secreting cells.

Calcitonin (CT)-secreting cells (C-cells) are remarkably sensitive to changes in the extracellular Ca2+ concentration. In order to detect the mechanism by which C-cells monitor Ca2+, we compared a C-cell line responding to Ca2+ (rMTC cells) with another one known to have a defect in this Ca2+ signal transduction (TT cells). Rises of the Ca2+ concentration caused rMTC cells to depolarize and/or elicited spontaneous action potentials. Under voltage-clamp conditions, rMTC cells showed a slowly decaying Ca2+ inward current which was sensitive to dihydropyridines but not to Ni2+ at a low concentration. In contrast, the 'defective' TT cells neither depolarized nor fired action potentials with high Ca2+; they only exhibited an Ni2(+)-sensitive, transient Ca2+ current. The data strongly suggest that the slowly inactivating Ca2+ current is a prerequisite for Ca2(+)-sensitivity of C-cells and that fast inactivating channels are not sufficient to act as sensors of the extracellular Ca2+ concentration.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1990

Combination chemotherapy of advanced medullary and differentiated thyroid cancer. Phase II study.

A group of 20 patients with advanced medullary (MTC) or differentiated thyroid carcinoma (DTC) received a combination chemotherapy of doxorubicin (50 mg/m2), cisplatin (60 mg/m2) and vindesine (3 mg/m2). In the 18 (10 MTC, 8 DTC) evaluable patients only 1 partial remission (in a patient with MTC) and 3 minor responses (in 3 patients with DTC) were observed. These responses lasted for 15, 9, 13, and 22 months, respectively. Three MTC patients suffered from progressive disease and "no change" was seen in the other 11 patients. Toxicity, including 1 severe case of cardiomyopathy, was considerable. Thus, the combination chemotherapy of doxorubicin, cisplatin and vindesine has failed to prove superior to the commonly applied doxorubicin monotherapy in patients with advanced medullary or differentiated thyroid carcinoma.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed1990

[Combination therapy with adriamycin, cisplatin and vindesine in C cell carcinoma of the thyroid].

Ten patients with advanced medullary thyroid carcinoma were treated with the combination of adriamycin (50 mg/m2), cisplatin (60 mg/m2) and vindesine (3 mg/m2). Only one partial remission (PR) was seen. There were six "no changes" and three "progressive diseases (PD)". Comparing with data from the literature the polychemotherapy of adriamycin, cisplatin and vindesine seems not to be superior to the mostly employed adriamycin-monotherapy. The tumor-markers calcitonin (CT) and carcinoembryonic antigen (CEA) were pathologically elevated in any case. During chemotherapy they proved to be valuable parameters of the course of the disease. In the patient with PR the CT- and CEA-serum levels reflected the clinical remission. In the patients with PD, however, they rose continuously.

Adolescent↗
PubMed1990

Internalization of calcitonin receptors in primary rat kidney cell cultures.

Cultured rat kidney cells possess specific calcitonin receptors and a calcitonin-responsive adenylate cyclase. At 37 degrees C bound 125I-salmon calcitonin becomes increasingly resistant to acid washing. If 125I-salmon calcitonin is removed from the medium after binding, bound hormone decreases over the next 5 h. Monensin, which blocks lysosomal processing, inhibits the decrease of bound hormone. These data indicate that calcitonin receptors are internalized after binding of hormone in kidney cells. Cycloheximide prevents internalization, when it is administered 4 h before 125I-salmon calcitonin binding is studied. Pre-incubation of the cells with 10(-7) mol/l unlabelled salmon calcitonin decreases specific binding; recovery of binding needs 48 h to occur. The long time interval for recovery makes it unlikely that calcitonin receptors recycle. This is the first demonstration that normal rat kidney cells internalize calcitonin after binding. It might contribute to the loss of calcitonin bioactivity which is seen after continuous administration.

Adenosine Triphosphate↗
PubMed1989

Role of voltage-dependent calcium channels in secretion of calcitonin from human medullary thyroid carcinoma cells.

Extracellular calcium concentration is an important regulator of calcitonin secretion. We used primary cell cultures of human medullary thyroid carcinoma to study the role of voltage dependent calcium channels for stimulus secretion coupling. Increasing extracellular calcium concentration (1.6-5.0 mM) in the medium caused a dose dependent release of calcitonin. The calcium channel activator BAY K 8644, a dihydropyridine, stimulated calcitonin secretion in a dose dependent manner (10(-7)-10(-5) M). This effect was completely inhibited by equimolar concentrations of the calcium channel blocker nifedipine and abolished in the absence of extracellular calcium. Similarly, nifedipine suppressed the stimulatory action of extracellular calcium. The effects of calcium and BAY K 8644 with and without nifedipine suggest that calcium influx via voltage dependent calcium channels plays an important role in calcitonin secretion. The primary cell culture of human medullary thyroid carcinoma is a good model for the study of stimulus secretion coupling.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1989

[Hyperthyroidism caused by a hormone-producing follicular thyroid cancer].

A well differentiated carcinoma of the thyroid is usually less productive with respect to thyroid hormones than normal thyroid tissue. Very rarely, it happens though that a metastatic follicular carcinoma of the thyroid produces hyperthyroidism as the following example illustrates. In our patient with follicular thyroid carcinoma the radioiodine scan demonstrated an increased uptake of 123I in the right thyroid lobe but also in a lung metastasis, even before thyroidectomy. In contrast, the iodine uptake of the left - normal - thyroid lobe was suppressed. Following thyroidectomy the levels of the thyroid hormones were normal and only dropped into the hypothyroid range after the beginning of the therapy with radioiodine. Hence we conclude that the metastases of a thyroid carcinoma can attain the hormone secretion of a normal thyroid gland and take up more radioiodine than normal thyroid tissue.

Adenocarcinoma↗
PubMed1989

Reversible desensitization of calcitonin secretion by repetitive stimulation with calcium.

The extracellular ionized calcium concentration (Ca2+) is a main regulator of calcitonin (CT) release. Calcium-induced CT secretion differs for acute versus long-term alterations of Ca2+. Using the rat C cell line rMTC 6-23 we have investigated the effect of repetitive stimulation by Ca2+ on CT release. After a Ca-induced initial rise of CT secretion, repetitive Ca stimulation led to a decline of CT release to unstimulated levels (after about 4 h). Reversing the high Ca2+ concentration (2.0 mM) to basal (1.1 mM) for 2 h and then increasing Ca2+ again resulted in a restored stimulatory action of Ca2+ (about 100% increase above the control). In contrast, repetitive stimulation with the dihydropyridine Ca channel agonist Bay K-8644 showed an unchanged stimulatory effect, as observed for the cAMP analog 8-bromo-cAMP, too. The results indicate that the reversible desensitization of Ca-induced CT secretion might be due to a modification of the voltage-dependent Ca channels proximal to or at the site of Bay K-8644 action.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗