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At least 163 records · Page 9Linked to original sources
PubMed1992

Calcium mass transfer with dialysate containing 1.25 and 1.75 mmol/L calcium in peritoneal dialysis patients.

Studies with 1.75 mmol/L calcium dialysate have shown that patients gain calcium from dialysate. Thus, hypercalcemia, especially when calcium compounds are used for phosphate control, is a commonly seen complication. Dialysate with 1.25 mmol/L calcium has been available since 1989. Little is known about calcium mass transfer (CMT) with dialysate of this calcium concentration. CMT was measured in 20 stable adult peritoneal dialysis patients. Each CMT study consisted of a 2-L continuous ambulatory peritoneal dialysis (CAPD) exchange with a dwell time of 4 hours. CMT studies were performed using 1.25 and 1.75 mmol/L calcium dialysate with 1.5, 2.5, and 4.25 g/dL dextrose concentrations. CMT with 1.25 mmol/L calcium dialysate was compared to that with 1.75 mmol/L for each dextrose concentration. With a dextrose concentration of 1.5 g/dL, the mean CMT for 1.25 mmol/L calcium dialysate was -0.1 +/- 0.3 mmol versus 0.6 +/- 0.3 mmol for 1.75 mmol/L calcium dialysate (P < 0.0001). A dextrose concentration of 2.5 g/dL resulted in a mean CMT of -0.4 +/- 0.2 mmol for 1.25 mmol/L calcium versus 0.45 +/- 0.25 mmol for 1.75 mmol/L calcium (P < 0.0001). Using a dextrose concentration of 4.25 g/dL, the mean CMT was -0.7 +/- 0.25 mmol for 1.25 mmol/L calcium versus -0.05 +/- 0.35 mmol for 1.75 mmol/L calcium (P < 0.0001). Mean serum ionized calcium (SiCa) was between 1.15 and 1.20 mmol/L for all study groups. CMT inversely correlated with SiCa for each type of dialysate used. CMT was dependent on the concentrations of calcium and dextrose in the dialysate and the SiCa level at the time of the exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1986

Iron-containing metallocenes as active site directed inhibitors of the proteinase that cleaves the NH2-terminal propeptides from type I procollagen.

Derivatives of ferrocene (dicyclopentadienyliron) (Fc) were examined as active site directed inhibitors of type I procollagen N-proteinase, the enzyme that cleaves the NH2-terminal propeptides from type I procollagen. The compounds were shown here to be reversible, competitive inhibitors of the enzyme. The effectiveness of the Fc inhibitors varied with modification of the cyclopentadienyl (cp) rings. The monocarboxylic acid (I) and the 1,1'-dicarboxylic acid (II) derivatives of Fc inhibited 50% of the enzymic activity (I50) at concentrations of 1.0 and 0.5 mM, respectively. The Ki values were 0.3 mM for both I and II. Derivatization of the carbonyl alpha to the cp ring of compound I (FcCOCH2CH2COOH, III) increased the inhibitory activity (I50 = 0.100 mM; Ki = 0.065 mM). Removal of the carbonyl alpha to the cp ring of III did not improve inhibitory activity: FcCH2CH2COOH, I50 = 2 mM; FcCH = CHCOOH, I50 = 1.5 mM. The active inhibitory species apparently contained iron in the 3+ valence state since two ferrocenium derivatives were very effective inhibitors: ferrocenium tetrachloroferrate, IV (I50 = 0.030 mM; Ki = 0.004 mM), and carboxyferrocenium hexafluorophosphate, V (I50 less than 0.1 mM; Ki less than 0.05 mM). In addition, reduction of III with ascorbic acid abolished its inhibitory activity. Compounds I and III stabilized the enzyme to heat denaturation in the absence of exogenous calcium; compound IV did not stabilize the enzyme. Further observations indicated that Fc derivatives were specific inhibitors of procollagen N-proteinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Ca(2+)-dependent block and potentiation of L-type calcium current in guinea-pig ventricular myocytes.

1. The caged calcium compound nitr-5 has been used to investigate the response of the L-type calcium current (ICa) of guinea-pig ventricular cells to a rapid increase in the free intracellular calcium concentration ([Ca2+]i). 2. When 2 mM nitr-5 or 3 mM DM-nitrophen was loaded into cells via a patch pipette and photolysed during the decay phase of ICa, a partial block of the current developed within 75 ms. The block was reduced by increasing the pre-flash [Ca2+]i and enhanced by adding high concentrations of Ca2+ chelators to the pipette-filling solution. 3. The photolysis-induced block was not suppressed in the presence of isoprenaline, suggesting a direct action of Ca2+ on the channels rather than a mechanism involving channel phosphorylation. 4. The most prominent effect of nitr-5 photolysis was a slow potentiation of ICa. When ICa was activated at frequencies between 0.05 and 0.7 Hz with various levels of pre-flash [Ca2+]i, peak ICa was approximately doubled in amplitude following photolysis. 5. At a stimulation frequency of 0.05 Hz, when nitr-5 was the only chelator present in the pipette, the time course of the potentiation was fitted by a single exponential with a time constant (tau P) of 2.7 min. When 1 mM CaCl2 was added to the pipette-filling solution, the time course of the potentiation was slowed (tau P = 6 min), although its amplitude was unchanged. With 12 mM BAPTA (a calcium chelator) added instead of CaCl2, the response was accelerated (tau P = 1.7 min). 6. Equimolar substitution of extracellular Ca2+ with Ba2+ significantly suppressed the flash-induced potentiation. The time course of the potentiation of the barium current, IBa (tau P = 1.9 min) was similar to that of ICa with BAPTA in the pipette. Potentiation of IBa was largely blocked in Ca(2+)-depleted cells when CaCl2 was omitted from the pipette. 7. When ICa was activated at frequencies of > or = 0.1 Hz, with 1 mM CaCl2 added to the nitr-5 (2 mM) in the pipette, the onset of the flash-induced potentiation was best fitted by two exponentials; one was similar to the single component seen at 0.05 Hz and the other was approximately one order of magnitude faster. The contribution of the faster component was positively correlated to the stimulation frequency. 8. The flash-induced potentiation of ICa was suppressed in the presence of a supramaximal concentration of the beta-adrenergic agonist isoprenaline.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Calcium released by photolysis of DM-nitrophen triggers transmitter release at the crayfish neuromuscular junction.

1. Spontaneous and evoked transmitter release at the crayfish neuromuscular junction were potentiated in response to photolytic release of calcium from the 'caged' calcium compound DM-nitrophen, which had previously been injected into presynaptic terminals. 2. The amount of calcium released from DM-nitrophen photolysis depends on the concentration of DM-nitrophen, its photoproducts, Ca2+, Mg2+, H+, ATP and the cell's native buffer. Since none of these are known in the crayfish terminal, the study was conducted in a qualitative fashion. 3. Photolytic release of calcium from DM-nitrophen increased excitatory junctional potentials (EJPs) by a range of 2-31 times over control values and the miniature excitatory junctional potential (MEJP) frequency increased from resting values of 1-10 quanta/s to 3000-11,000 quanta/s. 4. Extracellular calcium was not required for the light-evoked asynchronous release of transmitter. Calcium-bound DM-nitrophen previously pressure injected into crayfish presynaptic terminals increased the MEJP frequency from resting values of 1-8 quanta/s to 800-10,000 quanta/s during photolysis in a calcium-free cobalt Ringer solution. 5. Iontophoresis of calcium-free DM-nitrophen into presynaptic terminals released transmitter upon photolysis, but only in a calcium-containing Ringer solution. This suggests that DM-nitrophen is capable of binding calcium once injected into terminals, but this is dependent on the presence of external calcium. 6. Photolysis of DM-nitrophen at lower light intensities produced a slower rate of transmitter release. 7. Brief light exposures, i.e. those which photolysed 5-20% of the DM-nitrophen, resulted in a rapid decay of postsynaptic responses on extinguishing the light, due to rebinding of photolytically released calcium to unphotolysed DM-nitrophen. Longer light exposures which completely photolysed DM-nitrophen, leaving only the low affinity photoproducts, produced a slow decay of transmitter release after the light pulse, presumably due to the active extrusion of calcium from the presynaptic terminals. 8. During photolysis of DM-nitrophen, the time courses of changes in EJP amplitude and MEJP frequency were different, indicating that the two measures of transmitter release were not linearly related. 9. MEJP frequency and EJP amplitudes during DM-nitrophen photolysis were fitted to a 'non-linear summation model' in which photolytically released calcium sums with calcium entering during an action potential to evoke transmitter release with a calcium co-operativity of five.

Acetates↗