Role of autonomic nervous system and hemodynamics in cardiovascular homeostasis after orthostatic stress

Autonomic control of hemodynamics is crucial to maintaining cardiovascular homeostasis after orthostasis. Our goal was to assess the effect of passive orthostatic stress on cardiovascular hemodynamics regulation by analysis of the autonomic nervous system (ANS) and gender influence on the variables. We used a passive 70 degrees tilt test as an orthostatic maneuver. For SNA measurements we used FFT for HRV and SBPV, temporal sequences and alpha index to calculate baroreceptor gain. Hemodynamic data was calculated non-invasively by modelflow. Orthostatic stress induces a rise in heart rate (+15%), total peripheral resistance (+28%), sympathetic tonus (LF_nu: +32%; LF_pas: +64%) and a reduction in cardiac output (-28%), systolic volume (-43%), and vagal activity both tonic (HF_nu:-115%) and reflex (BRG:-103%). Females had higher vagal activity regardless of position. We concluded that the autonomic nervous system plays a crucial role in cardiovascular hemodynamics control to maintain homeostasis after orthostatic stress.

Glucose-mediated Ca(2+) signalling in single clonal insulin-secreting cells: evidence for a mixed model of cellular activation

Using clonal insulin-secreting BRIN-BD11 cells, we have assessed whether the graded response of the whole cell population to glucose can be accounted for by a dose-dependent recruitment of individual cells, an amplification of the response of the recruited cells or both. Cytosolic free Ca(2+) concentration ([Ca(2+)](i)) is an established index of beta-cell function. We used fura-2 microfluorescence techniques to assess the [Ca(2+)](i) responsiveness of single BRIN-BD11 cells to glucose and other secretagogues. Glucose (1-16.7 mM) evoked oscillatory [Ca(2+)](i) rises in these cells resembling those found in parental rat pancreatic beta-cells. The percentage of glucose-responsive cells was 11% at 1 mM and increased to 40-70% at 3-16.7 mM glucose, as assessed by a single-stimulation protocol. This profile was unrelated to possible differences in the cell cycle, as inferred from experiments where the cultured cells were synchronized by a double thymidine block protocol. Individual cells exhibited variable sensitivities to glucose (threshold range: 1-5 mM) and a variable dose-dependent amplification of the [Ca(2+)](i) responses (EC(50) range: 2-10 mM), as assessed by a multiple-stimulation protocol. Glyceraldehyde and alpha-ketoisocaproic acid had glucose-like effects on [Ca(2+)](i). The data support a mixed model for the activation of insulin-secreting cells. Specifically, the graded secretory response of the whole cell population is likely to reflect both a recruitment of individual cells with different sensitivities to glucose and a dose-dependent amplification of the response of the recruited cells.

Objective assessment of the contribution of each diagnostic test and of the ordering sequence in jaundice caused by pancreatobiliary carcinoma

BACKGROUND

Computer-assisted diagnostic systems are not substantially more accurate than the clinician in the differential diagnosis of jaundice but may help in optimal selection and sequencing of tests. The present study aimed to assess with an electronic diagnostic tool the pattern of ordering tests and the diagnostic contribution and related financial cost of each test in jaundiced patients with pancreatobiliary carcinoma, in an effort to make the clinician's diagnostic behaviour more efficient and economical.

METHODS

Clinical and diagnostic test data were prospectively gathered from 356 jaundiced patients with pancreatobiliary carcinoma and entered in a Bayesian diagnostic programme. The test results were added to the existing diagnostic evidence, and the programme calculated the diagnostic contribution of each test.

RESULTS

A total of 1804 diagnostic tests were ordered. Quantitative assessment of the diagnostic contribution of each test showed that percutaneous transhepatic cholangiography and computed tomography were associated with the highest provision of information. The most cost-effective tests were ultrasonography and liver function tests.

CONCLUSIONS

It is possible to determine objectively the diagnostic contribution of each test in establishing the diagnosis of pancreatobiliary carcinoma. The observed physician behaviour in ordering the various diagnostic tests might be improved with regard to its efficacy and its cost-effectiveness profile.

Interaction between substance P and gastrin-releasing peptide on thyrotropin secretion by rat pituitary in vitro

The effect of substance P (SP) on thyrotropin (TSH) secretion is controversial. In this study we evaluated the effect of SP on TSH secretion by hemipituitaries of 3-month-old Wistar rats in vitro and its interaction with gastrin-releasing peptide (GRP) at equimolar concentrations (1 microM and 10 microM). TSH release was measured under basal conditions and 30 min after incubation in the absence or presence of SP, GRP or both peptides. Pituitary TSH content was also measured in the pituitary homogenate after incubation. SP at both concentrations caused a significant (P<0.05) increase in TSH secretion compared with all other groups, which was approximately 60% (1 microM) and 85% (10 microM) higher than that of the control group (23.3 +/- 3.0 ng/ml). GRP at the lower concentration did not produce a statistically significant change in TSH secretion, whereas at the concentration of 10 microM it produced a 50% reduction in TSH. GRP co-incubated with substance P completely blocked the stimulatory effect of SP at both concentrations. Pituitary TSH content decreased in the SP-treated group compared to controls (0.75 +/- 0.03 microg/hemipituitary) at the same proportion as the increase in TSH secretion, and this effect was also blocked when GRP and SP were co-incubated. In conclusion, in an in vitro system, SP increased TSH secretion acting directly at the pituitary level and this effect was blocked by GRP, suggesting that GRP is more potent than SP on TSH secretion, and that this inhibitory effect could be the predominant effect in vivo.

Modulation of glucose-induced insulin secretion by cytosolic redox state in clonal beta-cells

Nutrient stimulation of pancreatic beta-cells increases the cellular reduced pyridine nucleotide content, but the specific role of cytosolic redox state in glucose-induced insulin release (GIIR) remains undetermined. The role of cytosolic redox state has been assessed (as reflected by the lactate/pyruvate ratio) in nutrient- and non-nutrient-induced insulin release using a recently established glucose-sensitive clonal beta-cell line (BRIN-BD11). Long-term exposure to the NAD+ precursor vitamin nicotinic acid (NA, 100 microM) was used to promote a more oxidized state in the cytosol. Glucose (2-16 mM) evoked a dose-dependent rise in the cytosolic NADH/NAD+ ratio which was linearly related to the extent of GIIR. NA suppressed the glucose-induced rise in the NADH/NAD+ ratio and concomitantly reduced GIIR by 44%. It also inhibited, by 47%, the average glucose-induced rise in cytosolic free Ca2+ concentration ([Ca2+]i, assessed by fura-2 microfluorometry from single cells). The latter effect was not accounted for by a reduction in the activity of voltage-sensitive Ca2+ channels, inasmuch as both high K+- and tolbutamide-induced [Ca2+]i rises remained insensitive to NA exposure. NA did not affect insulin release evoked by any of the depolarizing agents, indicating that steps in the stimulus-secretion coupling cascade distal to Ca2+ influx are insensitive to changes in the cytosolic redox state. It is concluded that GIIR is partially controlled by the cytosolic redox state. Moreover, the impairment in GIIR, caused by a shift toward a more oxidized state in the cytosol, originates from an attenuated [Ca2+]i response. The latter is likely mediated by the influence of cytosolic redox state on specific metabolic pathways (NADH shuttle systems and/or the malonyl-CoA pathway), leading ultimately to enhancement of the activity of ATP-sensitive K+ channels.

Local drug delivery of argatroban from a polymeric-metallic composite stent reduces platelet deposition in a swine coronary model

Thrombus formation after intracoronary stent implantation provides a stimulus for neointimal hyperplasia and if excessive can result in stent thrombosis. We tested the hypothesis that local delivery of an antithrombin drug from a polymeric-metallic stent inhibits platelet thrombus formation. An uncoated metal slotted tube, a jellyroll slotted metal stent with an Argatroban-loaded polymeric sleeve, and a jellyroll slotted metal stent with a drug-leached polymeric sleeve were randomly deployed into the coronary arteries of eight juvenile farm swine. Platelet deposition in the stented segments was determined at 2 hr using autologous 111Indium oxime-labeled platelets. Platelet deposition was significantly less in the Argatroban-loaded stents compared to the Argatroban-leached stents (1.40 x 10(8) platelets/cm2 vs. 26.8 x 10(8) platelets/cm2; P=0.005). When corrected for differences in the metal surface area exposed to blood, platelet deposition was significantly lower in the Argatroban-loaded stent (1.74+/-1.95 x 10(8)/cm2) compared to the Argatroban-leached stent (33.5+/-39.1 x 10(8)/cm2; P=0.005) and the uncoated metal stent (36.2+/-73.3 x 10(8)/cm2; P=0.006). In this coronary stent thrombosis model Argatroban has local antithrombotic properties when delivered with a polymer-metallic stent. Improved polymeric designs may reduce risk of thrombus deposition at the site of stent implantation.

Regulation of Ca2+ influx by a protein kinase C activator in chromaffin cells: differential role of P/Q- and L-type Ca2+ channels

Phorbol esters reduce depolarization-evoked Ca2+ influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca2+ channels (VSCCs) are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that L- and P/Q-type Ca2+ channel subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term (10 min) incubations with phorbol 12-myristate 13-acetate (PMA) inhibited early high K+-evoked rises in cytosolic free Ca2+ concentration ([Ca2+]i) and the early component of the depolarization-evoked Mn2+ quenching of fura-2 fluorescence in a dose-dependent manner (IC50: 18 and 7 nM; maximal inhibitions: 45 and 48%, respectively). The protein kinase C inhibitor staurosporine (100 nM) reverted the inhibitory action of PMA. PMA (0.1-1 microM) inhibited the early and late phases of the ionomycin (2 microM)-evoked [Ca2+]i transients by 14-23%. Omega-agatoxin IVA, a blocker of P/Q-type Ca2+ channels, inhibited high K+-evoked [Ca2+]i rises in a dose-dependent fashion (IC50 = 50 nM). In contrast, 0.1 microM omega-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable (< 45%) component of early Ca2+ influx persisted in the combined presence of omega-agatoxin IVA (100 nM) and nitrendipine (1 microM). Simultaneous exposure to omega-agatoxin IVA and PMA inhibited both the early [Ca2+]i transients and Mn2+ quenching to a much greater extent than each drug separately. Inhibition of the [Ca2+]i transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over P/Q type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-P/Q type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells.

Progression of renal artery stenosis in patients undergoing cardiac catheterization

BACKGROUND

Renal artery stenosis is potentially correctable by either revascularization surgery or percutaneous methods. However, appropriate use of these techniques has been hampered by a lack of data on the natural history of this disease. This study assesses the prevalence, risk factors for progression, and effect on renal function of angiographically demonstrated renal artery disease in patients undergoing cardiac catheterization.

METHODS

The severity of renal artery stenosis was quantified in all patients who underwent abdominal aortography as part of a diagnostic cardiac catheterization study at Duke University Medical Center between January 1989 and February 1996.

RESULTS

There were 14,152 patients in the study (mean age 61+/-12 years, 62% male). Normal renal arteries were identified in 12,543 (88.7%) patients, insignificant disease (<50% stenosis) in 1 or more vessels in 726 patients (5.1 %), and significant stenosis in 883 patients (6.3%). Significant bilateral renal artery stenosis was present in 178 patients (1.3%). By multivariate logistic regression, elevated serum creatinine level, coronary artery disease, peripheral vascular disease, hypertension, cerebrovascular disease, older age, female sex, and family history of coronary artery disease were identified as independent predictors of significant renal arterial disease. Disease progression was assessed in 1189 patients. Mean time between cardiac catheterizations was 2.6+/-1.6 years. Significant disease progression occurred in 133 patients (11.1 %). Independent predictors of disease progression were female sex, age, coronary artery disease at baseline, and time between baseline and follow-up. At follow-up, serum creatinine level was significantly higher in patients who demonstrated > or =75% stenosis in 1 or more vessels (mean creatinine level 141+114 micromol/L compared with those with insignificant disease (mean creatinine level 97+/-44 micromol/L (P= .01).

CONCLUSIONS

Renal artery disease is frequently progressive in patients who undergo cardiac catheterization for investigation of coronary artery disease. Significant stenotic disease may develop over a short period despite evidence of normal renal arteries at prior catheterization.

Immobilization of luciferase from a firefly lantern extract on glass strips as an alternative strategy for luminescent detection of ATP

The bioluminescent reaction catalysed by firefly luciferase has become widely established as an outstanding analytical system for assay of ATP. When used in solution, luciferase is unstable and cannot be re-used, a problem that can be partially circumvented by immobilizing the enzyme on solid substrates. Transparent glass is especially advantageous over alternative immobilizing matrices, since it allows most of the emitted photons to be detected. We report a new method for luciferase immobilization on glass which does not require prior silanization and glutaraldehyde activation, thus saving preparation time and minimizing enzyme inactivation. Our method is based on the co-immobilization by adsorption of luciferase (from a firefly lantern extract) and poly-L-lysine (PL) on non-porous glass strips. Luciferase immobilized in this way exhibits minimal variations in intersample activity, high sensitivity for ATP detection (linear luminescence responses down to 50 nmol/L) and good stability (full activity for at least 60 days when stored at -80 degrees C). PL-mediated immobilization of luciferase on glass strips provides an attractive strategy for the design of specific ATP biosensors, with potential in industry, environmental screening, medicine and biological research.

Kissing stents in the aortic bifurcation

BACKGROUND

We report the first series of simultaneously delivered stents used to treat stenosis of the aortic bifurcation. Surgical treatment of aortoiliac occlusive disease carries up to a 3% mortality rate. Percutaneous balloon techniques to treat aortic bifurcation stenosis, although safer, are still associated with up to a 9% incidence of dissection, thrombosis, or significant residual stenosis. Kissing stent insertion should decrease the incidence of these complications.

METHODS

Twenty patients underwent kissing stent insertion. Suitable candidates included patients with symptoms of lower limb ischemia and significant atherosclerotic lesions in both ostial common iliac arteries (n = 15) or with extremely complex single ostial iliac stenoses (n = 5). Palmaz stents were delivered simultaneously to both limbs of the aortic bifurcation.

RESULTS

Kissing stent insertion was successfully performed in all 20 patients without acute complications. Mean percent stenosis decreased from 46.2%+/-24.8% to -6.8%+/-13.3% (P = .0001) in the right iliac artery, 42.3%+/-22.8% to -1.6% +/-18.1% (P = .0001) in the left iliac artery, and 19.1%+/-16.6% to 2.3%+/-16.4% (P= .0008) in the distal aorta. Intermittent claudication symptoms were improved in 18 (95%) of 19 patients with 12 (63%) of 19 patients becoming totally asymptomatic. The strongest predictor of clinical outcome after kissing stent insertion was the preprocedural extent of femoropopliteal disease: 8 (89%) of 9 patients with femoropopliteal narrowing <75% bilaterally became completely asymptomatic at follow-up compared with only 3 (30%) of 10 patients with more severe stenoses (P = .02).

CONCLUSIONS

We have demonstrated in 20 patients that stenoses of the aortic bifurcation can be treated effectively with kissing stents with few serious adverse events.

Local administration of L-703,081 using a composite polymeric stent reduces platelet deposition in canine coronary arteries

We compared the effect on platelet deposition of the glycoprotein IIb/IIIa receptor antagonist L-703,081, administered locally via a drug delivery stent, with that of a standard metal stent in a canine coronary model. There was a significant reduction in platelet deposition using the L-703,081-impregnated stent compared with the bare metal stent. This study demonstrates an alternative route of delivery of GPIIb/IIIa antagonists with potential advantages over systemic administration.

Control of pulsatile 5-HT/insulin secretion from single mouse pancreatic islets by intracellular calcium dynamics

1. Glucose-induced insulin release from single islets of Langerhans is pulsatile. We have investigated the correlation between changes in cytosolic free calcium concentration ([Ca2+]i) and oscillatory insulin secretion from single mouse islets, in particular examining the basis for differences in secretory responses to intermediate and high glucose concentrations. Insulin release was monitored in real time through the amperometric detection of the surrogate insulin marker 5-hydroxytryptamine (5-HT) via carbon fibre microelectrodes. The [Ca2+]i was simultaneously recorded by whole-islet fura-2 microfluorometry. 2. In 82 % of the experiments, exposure to 11 mM glucose evoked regular high-frequency (average, 3.4 min-1) synchronous oscillations in amperometric current and [Ca2+]i. In the remaining experiments (18 %), 11 mM glucose induced an oscillatory pattern consisting of high-frequency [Ca2+]i oscillations that were superimposed on low-frequency (average, 0.32 min-1) [Ca2+]i waves. Intermittent high-frequency [Ca2+]i oscillations gave rise to a similar pattern of pulsatile 5-HT release. 3. Raising the glucose concentration from 11 to 20 mM increased the duration of the steady-state [Ca2+]i oscillations without increasing their amplitude. In contrast, both the duration and amplitude of the associated 5-HT transients were increased by glucose stimulation. The amount of 5-HT released per secretion cycle was linearly related to the duration of the underlying [Ca2+]i oscillations in both 11 and 20 mM glucose. The slopes of the straight lines were identical, indicating that there is no significant difference between the ability of calcium oscillations to elicit 5-HT/insulin release in 11 and 20 mM glucose. 4. In situ 5-HT microamperometry has the potential to resolve the high-frequency oscillatory component of the second phase of glucose-induced insulin secretion. This component appears to reflect primarily the duration of the underlying [Ca2+]i oscillations, suggesting that glucose metabolism and/or access to glucose metabolites is not rate limiting to fast pulsatile insulin release.

[Changes of the complement system in myelodysplastic syndromes]

PURPOSE

To search complement system activity and detection of circulating immune complexes in a group of patients with myelodysplastic syndromes (MDS) classified as with relatively favourable and unfavourable prognosis.

PATIENTS AND METHODS

28 patients with MDS were examined, 12 with refractory anaemia and 4 with refractory ringed sideroblastic anaemia, both subgroups of relatively favourable prognosis; 9 patients with refractory anaemia with blastic excess and 3 with refractory anaemia with blastic excess in transformation, subgroups of unfavourable prognosis. We determined factor B, alternate and classical hemolytic activity, as well as C3 and C4 quantification of the complement system. Circulating immune complexes detection was performed by the C1q deviation test and polyethylene glycol 6000 precipitation method at 3.75% final concentration.

RESULTS

A significant decrease in C3 level, alternate haemolytic activity and factor B was demonstrated in the unfavourable prognosis subgroups in comparison with patients of favourable prognosis and normal controls.

CONCLUSIONS

Our results suggest the possibility of subclinical infections in patients with unfavourable prognosis.

TGF-beta isoforms in alcoholic liver disease

The increased deposition of extracellular matrix proteins in the liver is a key factor in the morbidity and mortality of alcoholic liver disease (ALD). This increased fibrosis may be due to a superabundance of profibrogenic factors such as transforming growth factor-beta (TGF-beta). The original peptide is now called TGF-beta 1, and two other isoforms have been recognized in humans (TGF-beta 2 and TGF-beta 3). It was the aim of the present study to determine the expression of the TGF-beta isoforms in different stages of ALD. Thirty patients with ALD had percutaneous liver biopsies performed for diagnostic purposes. They were grouped by clinical findings and by liver histology into four groups: I, steatosis; II, fibrosis; III, hepatitis; and IV, cirrhosis. An unused portion of each biopsy sample was used to evaluate the gene expression of TGF-beta 1, TGF-beta 2, and TGF-beta 3 by reverse transcription polymerase chain reaction (RT-PCR). The expression of all isoforms from patients was significantly greater than their expression in controls. No significant correlation was determined between TGF-beta isoform expression and liver function test results. When the different isoforms were grouped by histology, increased expression with more severe disease was found; however, differences existed among the isoforms. In ALD, all TGF-beta isoforms were increased and their expression was significantly greater in patients with more active and advanced disease. RT-PCR is an effective method for evaluating gene expression in clinical samples which often provide a limited amount of tissue.

[Subisthmic aortic coarctation. Apropos a rare case of arterial hypertension]

An uncommon case of high blood pressure secondary to subisthmic aortic coarctation and cured surgically by arterial revascularization is reported. It is probably a congenital coarctation, because the extensive clinical evaluation and condition of the patient ruled out an acquired disease of the aorta, confirmed by the morphologic operative findings and pathological examinations of the aortic wall. In this case, the clinical diagnosis was initially suggested due to an abdominal systolic-diastolic bruit, associated to hypertension and weak femoral pulses. Although doppler ultrasonography and nuclear magnetic resonance were useful diagnostic tools, a complete angiography should be performed because it is currently the most informative technique to evaluate the aorta and its collaterals, frequently involved in association with subisthmic aortic coarctation. Surgical exploration revealed a hypoplastic descending thoracic aorta and histological data confirmed the nonexistence of acquired pathology of the aortic wall. Surgical management was accomplished successfully and the patient was asymptomatic 41 months after the operation with normal blood pressure, no treatment, and a normal pattern of life. Based on this presentation, the authors describe the etiology, morphology and anatomic classification, clinical manifestations, diagnosis and management of subisthmic aortic coarctation.

Real time electrochemical detection of 5-HT/insulin secretion from single pancreatic islets: effect of glucose and K+ depolarization

We report a highly sensitive electrochemical approach suitable for the real time measurement of insulin release from single islets of Langerhans, the functional endocrine units in the pancreas. The method is based on the detection of the insulin surrogate 5-hydroxytryptamine (5-HT) by carbon fibre microelectrodes implanted in the islets. Based on the combination of this novel approach with the simultaneous microfluorometric recording of cytosolic free Ca2+ concentration ([Ca2+]i), we demonstrate that glucose-stimulated islets secrete 5-HT/insulin in a pulsatile fashion under physiological conditions, and that this activity is encoded by synchronous [Ca2+]i oscillations. The sensitivity to detect variations in minute amounts of secreted materials is partially conferred by the fact that the tracer is released into a relatively confined space (the intraislet interstitial space).

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

Protein kinase C activator inhibits voltage-sensitive Ca2+ channels and catecholamine secretion in adrenal chromaffin cells

We have investigated the effects of the phorbol ester 12-myristate 13-acetate (PMA) on depolarization-evoked Ca2+ influx and catecholamine secretion in bovine adrenal chromaffin cells. PMA (100 nM) strongly inhibited K(+)-evoked [Ca2+]i transients and Mn2+ quenching of fura-2 fluorescence. In contrast, 4 alpha-phorbol 12,13-didecanoate, a phorbol ester inactive on protein kinase C (PKC), had no effect. Maximal PMA-mediated inhibition occurred at 5-10 min incubations and were variable from cell to cell, ranging from 25 to 65% of controls. The [Ca2+]i transients evoked by the L-type Ca2+ channel activator Bay K 8644 were strongly inhibited by 100 nM PMA. PMA (0.1-10 microM) inhibited K(+)-evoked adrenaline and noradrenaline release by 23-44%. The data indicate that phorbol ester-mediated activation of PKC inhibits voltage-sensitive Ca2+ channels in chromaffin cells, leading to a prominent depression of depolarization-evoked catecholamine secretion.

Background Ca2+ influx mediated by a dihydropyridine- and voltage-insensitive channel in pancreatic beta-cells. Modulation by Ni2+, diphenylamine-2-carboxylate, and glucose metabolism

A stepwise increase in extracellular Ca2+ concentration ([Ca2+]o) can evoke insulin release from pancreatic islets in the absence of secretagogues. We have investigated the ionic mechanism underlying this secretory response by recording intracellular free Ca2+ concentration ([Ca2+]i) from single mouse islets of Langerhans using ratiometric fura-2 microfluorometry. In the presence of 11 mM glucose, the [Ca2+]i undergoes fast oscillations associated with bursting electrical activity. Nifedipine (10 microM) suppressed these oscillations and markedly lowered the [Ca2+]i. Raising the [Ca2+]o from 2.56 to 12.8 mM in the continued presence of 11 mM glucose and nifedipine evoked pronounced [Ca2+]i rises of variable amplitude and time course. This effect was dose-dependent (EC50 = 3.6 mM) and remained essentially unchanged in the absence of glucose or in the presence of 3 mM glucose and nifedipine, conditions where beta-cells are hyperpolarized by approximately -25 mV. Depleting the acetylcholine-mobilizable internal Ca2+ pools by repetitively challenging the islets with acetylcholine in the absence of Ca2+ actually potentiated the standard high Ca2+ responses. The latter were strongly reduced by millimolar concentrations of Ni2+ (70% reduction at 3 mM) and by diphenylamine-2-carboxylate (DPC; IC50 = 145 microM), a blocker of nonselective cation channels. The standard high Ca2+ responses were relatively insensitive to the glycolytic inhibitor mannoheptulose. It is proposed that the high Ca(2+)-evoked [Ca2+]i responses are primarily accounted for by Ca2+ influx through dihydropyridine- and voltage-insensitive, nonselective cation channels. These channels do not appear to be under the control of glucose metabolism. Although their function is unknown, they may be essential to supplying the beta-cells with Ca2+ in the absence of stimulatory levels of fuel secretagogues.

Differential modulation of pancreatic beta-cell bursting by intracellular pH in the presence and absence of a K-ATP channel blocker

The study of the influence of intracellular pH (pHi) changes on the mechanism underlying pancreatic beta-cell bursting has been hampered by concomitant effects on the activity of background ATP-dependent K+ (K-ATP) channels. beta-cells were made to burst in the absence of active K-ATP channels by raising external Ca2+ in the presence of 11 mM glucose and tolbutamide. An alkalinizing pHi shift (exposure to 20 mM NH4Cl) increased the burst active phase duration. Conversely, an acidifying shift (NH4Cl withdrawal) suppressed the electrical activity. This is the mirror image of the effects recorded in the absence of tolbutamide. Glibenclamide and quinine suppressed the alkalinization-evoked hyperpolarization. This study emphasizes the differential sensitivity of different beta-cell ion channels to pHi and the prevalent role of K-ATP channels as electrical transducers of cytoplasmic pH changes under regular physiological conditions.

Bursting electrical activity in pancreatic beta-cells: evidence that the channel underlying the burst is sensitive to Ca2+ influx through L-type Ca2+ channels

In glucose-stimulated pancreatic beta-cells, the membrane potential alternates between a hyperpolarized silent phase and a depolarized phase with Ca2+ action potentials. The molecular and ionic mechanisms underlying these bursts of electrical activity remain unknown. We have observed that 10.2-12.8 mM Ca2+, 1 microM Bay K 8644 and 2 mM tetraethylammonium (TEA) trigger bursts of electrical activity and oscillations of intracellular free Ca2+ concentration ([Ca2+]i) in the presence of 100 microM tolbutamide. The [Ca2+]i was monitored from single islets of Langerhans using fura-2 microfluorescence techniques. Both the high-Ca(2+)- and Bay-K-8644-evoked [Ca2+]i oscillations overshot the [Ca2+]i recorded in tolbutamide. Nifedipine (10-20 microM) caused an immediate membrane hyperpolarization, which was followed by a slow depolarization to a level close to the burst active phase potential. The latter depolarization was accompanied by suppression of spiking activity. Exposure to high Ca2+ in the presence of nifedipine caused a steady depolarization of approximately 8 mV. Ionomycin (10 microM) caused membrane hyperpolarization in the presence of 7.7 mM Ca2+, which was not abolished by nifedipine. Charybdotoxin (CTX, 40-80 nM), TEA (2 mM) and quinine (200 microM) did not suppress the high-Ca(2+)-evoked bursts. It is concluded that: (1) the channel underlying the burst is sensitive to [Ca2+]i rises mediated by Ca2+ influx through L-type Ca2+ channels, (2) both the ATP-dependent K+ channel and the CTX- and TEA-sensitive Ca(2+)-dependent K+ channel are highly unlikely to provide the pacemaker current underlying the burst.(ABSTRACT TRUNCATED AT 250 WORDS)

Neomycin blocks dihydropyridine-insensitive Ca2+ influx in bovine adrenal chromaffin cells

There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Although recent electrophysiological work indicates that the dihydropyridine-resistant pathway is partially mediated by omega-conotoxin-sensitive and -insensitive Ca2+ channels, the pharmacological sensitivity of the latter channels remains elusive. We have now found that combined incubations with nitrendipine (1 microM) and neomycin (0.5 mM) reduced high K+ (50 mM)-evoked intracellular Ca2+ concentration ([Ca2+]i) transients to a larger extent than each drug separately. [Ca2+]i was measured using the fluorescent intracellular Ca2+ indicator fura-2. Neomycin (0.05-2 mM) reduced high K(+)-evoked 45Ca2+ uptake in a dose-dependent manner (IC50 = 0.09 mM). In the presence of nitrendipine (1 microM), the minimal neomycin concentration necessary for total blockade of 45Ca2+ uptake was reduced to 0.3 mM. Moreover, in the absence of nitrendipine the 45Ca2+ uptake remaining in 0.3 mM neomycin (26% of maximum) was similar to the fractional inhibition by nitrendipine alone (29%). Neomycin (0.05-2 mM) inhibited the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) much more extensively at 2 mM than at 0.3 mM (percent inhibition = 59% and 15%, respectively). Neomycin (0.05-2 mM) blocked high K(+)-evoked noradrenaline and adrenaline release in a dose-dependent fashion (IC50 = 0.8-1.1 mM), the blockade efficiency being enhanced in the presence of 1 microM nitrendipine (IC50 = 0.17-0.19 mM). It is concluded that neomycin (< or = 0.3 mM) blocks preferentially the dihydropyridine-insensitive Ca2+ influx pathway of the chromaffin cell. Moreover, both the dihydropyridine-sensitive and the dihydropyridine-resistant, neomycin-sensitive Ca2+ influx pathways contribute strongly to depolarization-evoked catecholamine secretion.

High external Ca2+ levels trigger membrane potential oscillations in mouse pancreatic beta-cells during blockade of K(ATP) channels

Glucose depolarizes the pancreatic beta-cell and induces membrane potential oscillations, but the nature of the underlying oscillatory conductance remains unknown. We have now investigated the effects of the Ca2+ ionophore ionomycin and high external Ca2+ concentration ([Ca2+]o) on glucose-induced electrical activity and whole islet intracellular free Ca2+ concentration ([Ca2+]i), under conditions where the K(ATP) channel was blocked (100 microM tolbutamide or 4 microM glibenclamide). Raising [Ca2+]o to 10.2 or 12.8 mM, but not to 5.1 or 7.7 mM, turned continuous electrical activity into bursting activity. High [Ca2+]o (12.8 mM) regenerated a pattern of fast [Ca2+]i oscillations overshooting the levels recorded in tolbutamide. Ionomycin (10 microM) raised the [Ca2+]i and synergized with 5.1 mM Ca2+ to hyperpolarize the beta-cell membrane. The data indicate that a [Ca2+]i-sensitive and sulphonylurea-insensitive oscillatory conductance underlies the beta-cell bursting activity.