Effects Of External Calcium Research Articles

Effect of External Calcium and other Divalent Cations on K+ Contractures in Denervated Slow Skeletal Muscle Fibers of the Frog

The influence of Ca2+ and other divalent cations on contractile responses of slow skeletal muscle fibers of the frog (Rana pipiens) under conditions of chronic denervation was investigated.Isometric tension was recorded from slow bundles of normal and denervated cruralis muscle in normal solution and in solutions with free calcium concentration solution or in solutions where other divalent cations (Sr2+, Ni2+, Co2+ or Mn2+) substituted for calcium. In the second week after nerve section, in Ca2+-free solutions, we observed that contractures (evoked from 40 to 80 mM-K+) of non-denervated muscles showed significantly higher tensions (p<0.05), than those from denervated bundles. Likewise, in solutions where calcium was substituted by all divalent cations tested, with exception of Mn2+, the denervated bundles displayed lower tension than non-denervated, also in the second week of denervation. In this case, the Ca2+ substitution by Sr2+ caused the higher decrease in tension, followed by Co2+ and Ni2+, which were different to non-denervated bundles, as the lowest tension was developed by Mn2+, followed by Co2+, and then Ni2+ and Sr2+. After the third week, we observed a recovery in tension. These results suggest that denervation altering the binding capacity to divalent cations of the voltage sensor.

Effects of external calcium on zinc modulation of AMPA receptors

The effects of calcium on zinc modulation of current responses from two similar homomeric glutamate receptors, GluR1 and GluR3, was examined using the Xenopus oocyte expression system. Previous experiments carried out using normal (calcium-containing) physiological solution showed that kainate-evoked current responses from GluR3, but not GluR1, were potentiated by low μM concentrations of zinc. Furthermore this potentiation was observed only over a narrow range of zinc concentrations. In contrast, the present experimens demonstrated that in calcium-free solution (magnesium replacement) zinc can potentiate both GluR3, as expected, but also GluR1. In addition, the potentiation occurs over a large range of zinc concentrations (> log units). Re-supply of calcium to the solution interferes with zinc potentiation in a concentration-dependent manner. When supernormal concentrations of calcium are present, zinc greatly inhibits currents at zinc concentrations that would cause potentiation in the absence of calcium, and that would cause smaller inhibition in normal solution. Because zinc is co-released along with neurotransmitter at many glutamatergic synapses in vertebrate brains, and synaptic calcium levels are subject to variable control, calcium regulation of zinc sensitivity may be physiologicall relevant.

Effects of external calcium on contractile responses in rat extensor digitorum longus muscles after sciatic nerve injury at birth

Effects of external calcium on contractile responses in rat extensor digitorum longus muscles after sciatic nerve injury at birth

Effects of external calcium on contractile responses in rat extensor digitorum longus muscles after sciatic nerve injury at birth

Two types of nerve lesions were performed at birth in rat extensor digitorum longus muscle: sciatic nerve transection (group A) and sciatic nerve crush allowing further reinnervation (group B). Contractile responses were then studied at different times after the denervation (7, 14, 30, and 60 days) and compared with control. Sixty days after the intervention, twitch and tetanic tensions remained dependent upon the extracellular Ca2+ concentration ([Ca]o) both in groups A and B. However, the depression of tensions following Ca2+ withdrawal was more important in group A. Sixty days after birth, in the presence of a Ca2+ channel blocker, Cd2+ (2 mmol L-1), a depression of the twitch tension was observed in group A (similarly to control 1-7 days postnatal muscles), whereas Cd2+ potentiated twitch tension in group B (similarly to control 14-60 days postnatal muscles). After glycerol treatment (detubulating procedure) performed in 60-day-old muscles, twitch tension was abolished in group B and control, whereas twitch tension was potentiated in group A. Thus, in developing muscles, neural control could be involved in the dependence of contractility toward [Ca]o. These results may be relevant for the understanding of the contractile properties of neuromuscular disorders with early onset.

Role of the sodium-calcium exchange mechanism and the effect of magnesium on sodium-free and high-potassium contractures in pregnant human myometrium

OBJECTIVE: Periodic uterine relaxation in labor relieves the fetus from hypoxia related to sustained contractions. A reduction in intracellular calcium is essential for relaxation. Therefore we aimed to clarify the extrusion mechanisms of intracellular calcium ions in pregnant human myometrium. STUDY DESIGN: Isometric contraction of small muscle bundles obtained from human myometrium at term was recorded. High-potassium and sodium-free solutions could induce stable contractures that revealed initial phasic contractions followed by tonic contractions. The effects of external calcium, external magnesium, nifedipine, and calcium adenosine triphosphatase inhibitors on both contractures were then examined. RESULTS: Because maximum tonic contraction was induced by 49 mmol/L potassium, this concentration was used. Both contractures depended on external calcium concentrations. Nifedipine, 10 −7 mol/L inhibited both initial phasic contractions and the tonic phase of the high-potassium contracture: however, it could not suppress the tonic phase of the sodium-free contracture. Calcium adenosine triphosphatase inhibitors, cyclopiazonic acid and lauryl sulfate, potentiated the tensions of tonic phases in both contractures; the effect of lauryl sulfate was stronger than that of cyclopiazonic acid. When external magnesium was gradually increased (from 0 to 9.6 mmol/L), both phasic contractions and the tonic phase of the high-potassium contracture transiently increased, followed by a reduction of tension, Whereas the tonic phase of the sodium-free contracture was markedly inhibited by magnesium in a dose-dependent manner. CONCLUSION: The presence of the sodium-calcium exchange mechanism and the specific inhibitory effect of magnesium were indicated in human myometrium during pregnancy. These mechanisms might prevent the long tonic contractions, to protect the fetus from hypoxia during pregnancy and parturition.

The effects of external calcium on the [formula omitted] induced short-term potentiation in the rat hippocampal slice

The effect of altered extracellular Ca concentration was studied on the N- methyl- d-aspartate (NMDA) induced short-term potentiation (STP) of the population excitatory post-synaptic potential recorded from the stratum radiatum of CA1 of the rat hippocampal slice. Perfusion of 130 μM of NMDA for 10 s in control media containing 2.0 mM extracellular Ca evoked an STP with a maximum amplitude of 46% and a duration of 16 min. Perfusion of media containing a reduced Ca concentration of 0.8 mM or 1.0 mM did not alter the amplitude or time course of the STP. However, raising the Ca concentration to 3.0 mM or 4.0 mM caused a significant reduction in the amplitude of the STP to 23% and 2% respectively. The abolition of the NMDA induced STP in 4 mM Ca could not have been produced by response saturation since an identical long-term potentiation (LTP) was produced in this high Ca media as in the control media. These studies show that the NMDA induced STP has a very different Ca dependency to LTP.

Effects of external calcium, magnesium, and temperature on spontaneous contractions of pregnant human myometrium.

Spontaneous isometric contractions of small isolated segments from the isthmic region of pregnant human myometrium were recorded to clarify the characteristics of and influences of experimental conditions on contractions. There is a spontaneous periodicity in membrane activity of human myometrium, and contractions evoked with a sustained electric stimulus were affected by spontaneous rhythmic contractions. The frequency of contractions increased markedly, but their duration decreased when temperature of the bathing fluid was increased from 26 degrees C to 39 degrees C. Both excess (7 mM) and low (0.5 mM) calcium levels suppressed the generation of spontaneous contraction. Frequency and half-duration gradually decreased when external magnesium was increased from 0 mM to 3.6 mM. These results indicate that the pattern of contraction depends on the frequency of and intervals between each action potential, and that spontaneous contractility-particularly its frequency-is extremely sensitive to external temperature and ionic conditions.

The effect of external calcium and pH on inositol trisphosphate-mediated calcium release from cerebellum microsomal fractions.

Binding of D-myo-inositol 1,4,5-trisphosphate (InsP3) to rat cerebellum membranes has previously been shown to be stimulated by alkaline pH and inhibited by low concentrations of Ca2+ [Worley, Baraban, Suppatopone, Wilson & Snyder (1987) J. Biol. Chem. 262, 12132-12136]. In the present study, Scatchard analysis of InsP3 binding to cerebellum microsomes indicates that the effects of Ca2+ and pH are exerted through changes in the apparent affinity of the receptor without effects on maximal binding. The influence of extravesicular Ca2+ and pH on InsP3-mediated 45Ca2+ release was investigated. Extravesicular Ca2+ inhibited InsP3-mediated Ca2+ release. The inhibitory effect of Ca2+ was most marked when a sub-optimal concentration of InsP3 was used. An increase in extravesicular pH produced a decrease in the concentration of InsP3 that yielded half-maximal Ca2+ release. Regulation of the affinity of the InsP3 receptor by Ca2+ and pH can qualitatively account for the observed effects of these factors on InsP3-mediated Ca2+ release. Feedback inhibition of InsP3 binding by Ca2+ could provide a mechanism to generate Ca2+ oscillations, particularly under hormonal conditions that produce sub-optimal elevations of InsP3 concentration.

Effects of external calcium on acetylcholine-evoked increases in membrane capacitance in rat pancreatic acinar cells.

Effects of external calcium on acetylcholine-induced increases in membrane capacitance and conductance were investigated with the patch-clamp technique in combination with the phase-sensitive detection method, in single dialysed pancreatic acinar cells of rats. Both increases depended on an increase in [Ca2+]i, and a high concentration of EGTA in the cell-dialysing solution made ACh ineffective. In acinar cells exposed to a bathing solution containing the normal concentration of Ca2+ (2.5 mM CaCl2), the increase in membrane capacitance was transient and synchronous with that in membrane conductance (current) in response to 0.2 microM acetylcholine. However, in a bathing solution without CaCl2 and with EGTA (0.2 mM), the increase in membrane capacitance was sustained after the membrane conductance recovered to the original level during the ACh-stimulation. The evidence suggests that external calcium facilitates either the resealing of the fusion- or fission-pores formed at the contact between the secretory granule and the luminal cell membrane, or the membrane retrieval (endocytosis) in Ca2+-dependent exocytosis.

The effects of external calcium on long-term potentiation in the rat hippocampal slice

Extracellular excitatory postsynaptic potentials (EPSPs) were recorded from the stratum radiatum of CA1 of the rat hippocampal slice. The effect of altering the external Ca concentration was studied on the amplitude of the low frequency evoked EPSP preceeding and following the production of long-term potentiation (LTP). The double logarithmic plot of the relationship between the EPSP amplitude and external Ca had a maximum slope of 2.1, implying Ca cooperativity in transmitter release. The production of LTP did not alter the slope. The amplitude of LTP was found to be highly dependent on the external Ca concentration, with LTP increasing from a 5% increase in the EPSP amplitude in 0.8 mM Ca, to a 65% increase in the EPSP amplitude in 2.0 mM Ca.

Effects of calcium and phosphate on the corpuscles of Stannius of the teleost fish, Oreochromis mossambicus

Removal of the corpuscles of Stannius (CS) in Oreochromis mossambicus leads to hypercalcemia and hypophosphatemia. The effects on CS size and ultrastructure of different calcium and phosphate concentrations of the ambient water and of the food were investigated. A six-fold increase of the calcium concentration of the water leads to a four-fold increase in CS volume; this is mainly caused by an increase in the size and number of the type-1 cells. The effect of external calcium is most probably mediated by the calcium concentration of the blood plasma. Plasma ionic calcium may be the relevant factor. Changes in the calcium concentration of the food had no effect on the CS. Similarly, hyperphosphatemia or hypophosphatemia induced by high phosphate concentrations of the water or the food, or by a phosphate-deficient diet, had no noticeable effect on the CS. The results support the hypothesis that the type-1 cells produce the hypocalcemic factor of the CS. There is no evidence for the production by the CS of an endocrine factor involved in the control of phosphate metabolism.

Effects of external calcium on horizontal cells in the superfused goldfish retina

Effects of external calcium on horizontal cells in the superfused goldfish retina

Effects of external calcium and calcium antagonist, diltiazem, on isolated segments of pregnant human myometrium.

AbstractSpontaneous electrical activity and contraction of isolated segments from the isth‐mic region of pregnant human myometrium were recorded by the single sucrose‐gap method, and effects of calcium and diltiazem on the muscle were studied.The segments, obtained at cesarean section, had plateau‐ or spike‐type action potentials. When external Ca2+ was increased from 2.5 mM (normal concentration in Krebs solution) to 7 mM, the durations of both types of action potentials gradually decreased and finally disappeared. The contractions were well synchronized with action potentials, and amplitude, frequency and half‐duration of the contraction were all decreased when extracellular Ca2+ was increased. On the other hand, at a lower external Ca2+ concentration (1.25 mM) the duration of the action potentials was reduced, as was the contraction force. Diltiazem (10‐8‐10‐5 g/ml) suppressed both types of action potentials, and the accompanying contractions were reduced. However, the frequency of contraction was increased by lower doses in some muscles. Diltiazem 10‐5g/ml completely abolished the spontaneous activity. High K+ in the muscles caused a phasic contraction followed by a tonic contracture. Diltiazem 10‐5 g/ml suppressed both responses.These results suggest that spontaneous action potentials and contractions are altered by changes in external Ca2+. High Ca2+ might increase potassium conductance and stabilize the membrane. Consequently action potential duration was shortened and finally eradicated. It is also suggested that diltiazem may antagonize Ca2+ mobilization at the membrane surface including the stabilizing site, hence affecting the frequency and suppressing the spontaneous contraction and high K+ contracture.

Effects of external calcium on potassium contractures in tonic muscle fibers of the frog (Rana pipiens).

K+ contractures of tonic bundles from cruralis muscle of the frog were studied with different K+ concentrations (10-120 mM). K+ contractures had an initial transient phase followed by a sustained tension. The amplitude of the sustained tension diminished with high K+ concentration (80-120 mM). However, in all cases, tension was maintained for several minutes. External Ca2+ reduction practically abolished the sustained phase of the K+ contractures. The initial phase was also reduced and tension spontaneously relaxed. The curve relating the peak tension with log [K+]o, showed that the threshold was not affected but the peak tension was reduced to about 70% in low-Ca2+ saline (0 Ca2+ + 3 mM-Mg2+) and 50% in Ca2+-free saline (1 mM-EGTA + 3 mM-Mg2+). The dependence of the sustained tension on external Ca2+ was further confirmed by Ca2+ withdrawal and re-establishment and/or by Ni2+ substitution for Ca2+ before or during K+ contractures. These results indicate that external Ca2+ had to be continuously present to maintain the tension during K+ contractures and that Ni2+ was not able to restore the normal temporal course of K+ contracture. The sustained phase was diminished by blocking agents of Ca2+ channels, such as nifedipine (1 microM) and diltiazem (1-10 microM). The present results can be explained by a direct control of the Ca2+ currents on K+ contracture or by specific interactions between external Ca2+ and Ca2+-binding sites in the membrane.

Effects of external calcium, calcium channel-blocking agents, and stimulation frequency on cycle length-dependent changes in canine cardiac action potential duration.

Effects of external calcium, calcium channel-blocking agents, and stimulation frequency on cycle length-dependent changes in canine cardiac action potential duration.

Dual effect of external calcium on the frequency of miniature synaptic potentials in frog sympathetic ganglion cells

The effect of calcium on spontaneous transmitter release and on the release induced by tetanic stimulation and by raising the external potassium concentration ([K]0) was studied in sympathetic ganglion cells of Rana esculenta. 1. In standard Ringer's solution the frequency of miniature excitatory postsynaptic potentials (mepsp) ranged from 0.05--2.0 s-1 (0.05 +/- 0.09 s-1, n = 37) at room temperature. 2. At a [K]0 of 2.5 mM mepsp frequency was approximately linearly related to the logarithm of the external calcium concentration (log [Ca]0) (0.1 mM less than or equal to [Ca]0 less than or equal to 20 mM). 3. Duration and amplitude of the potentiation of transmitter release after tetanic preganglionic stimulation increased depending on [Ca]0. 4. Mepsp frequency was strongly dependent on [K]0 between 10 and 20 mM; the frequency being increased to about 40 times control level at a [K]0 of 20 mM. 5. Raising [Ca]0 up to 1.8 mM in high K solutions resulted in an increase in mepsp frequency followed by a decrease at higher [Ca]0. 6. These results are consistent with the idea that the effect of calcium on mepsp frequency depends on: (a) the driving force for calcium entry, (b) the effect of Ca ions on the potential gradient within the nerve membrane.

Reduced external calcium or sodium stimulates calcium influx in Pelvetia eggs

The effect of external calcium and sodium ion concentrations on the calcium fluxes on the Pelvetia fastigiata De Toni egg was measured. Decreasing external [Ca(2+)] greatly increased the permeability of the eggs to Ca(2+); at 1 mM external Ca(2+) this permeability was 60 times as great as it was at the normal [Ca(2+)] of 10 mM. Lowering the external [Na(+)] also increased Ca(2+) influx; at 2 mM Na(+), the Ca(2+) influx was 2-3 times as great as it was at the normal [Na(+)] if choline was used as a Na(+) substitute. Lithium was less effective as a Na(+) substitute in increasing Ca(2+) influx. The extra Ca(2+) influx in low [Na(+)] seemed to be dependent on internal [Na(+)]. The Ca(2+) efflux increased transiently and then declined in low Na(+) media.

The effect of external calcium and lanthanum on platelet calcium content and on the release reaction

Calcium compartments in calf platelets were studied using a lanthanum washout procedure to distinguish between surface-bound calcium and intracellular calcium. The calcium content of calf platelets ranges from 20 to 60 nmol/10 9 platelets and is sensitive to the calcium concentration of the suspending medium. With 1 mM calcium in the medium, calcium uptake is rapid and reaches steady state within 1–2 min. Results obtained with the lanthanum procedure indicate that it is the surface compartment which is most affected by the extracellular calcium concentration. The surface compartment appears to be saturable and is highly exchangeable. Although the total calcium as well as the calcium content of the surface and internal compartments are variable, the ratio of calcium in either compartment to the total saturated calcium is quite constant. The data indicate that 68–85% of the platelet calcium is located internally. Thrombin produces an immediate release of platelet calcium and labeled serotonin and an increase in the 45Ca 2+ uptake of both the surface and internal compartments. The release reaction is not dependent upon exogenous calcium or an influx of exogenous calcium since it occurs even in the presence of ethyleneglycol-bis-(β-aminoethylether)-N,N′- tetraacetic acid. Lanthanum, however, inhibits the release reaction possibly by blocking surface calcium site and reducing the mobility of endogenous platelet calcium.

The effect of external calcium and magnesium ions on the response of denervated muscle to acetylcholine.

1. The effect of external Ca and Mg on the membrane depolarization and contracture of rat denervated muscle in response to acetylcholine, ACh, was studied. 2. Raising external Ca concentration reduced the rate of rise and the amplitude of the ACh contracture, and prolonged its time course. 3. Increasing external Ca reduced the membrane depolarization in response to ACh. The dose increment required to elicit depolarization in high Ca concentration increased with external Ca, and was greater for depolarization than for contracture. 4. External Mg was less effect than Ca in reducing ACh depolarization but was more effective in reducing contracture. In contrast to Ca, Mg did not alter the time course of relaxation. 5. It is concluded that external Ca has two opposing effects on the ACh contracture: one of stabilizing the membrane and the other of increasing intracellular Ca concentration. External Mg may interfere with Ca influx and hence reduce contractility.

Effect of external calcium and of temperature on contraction in snake muscle fibers.

The effect of external calcium and of temperature on the contractile responses has been studied in voltage clamped snake twitch muscle fibers. Increasing [Ca(++)](o) from 0.2 to 7.0 mM raised contractile threshold by 15-20 mV, the latter coinciding with the appearance of delayed rectification. The duration of contracture, the rates of rise and decay of tension depended on the level of depolarization and [Ca(++)](o). The minimum duration of repolarization necessary to restore the contractile response was much shorter in high [Ca(++)](o). When the bathing solution was cooled to 10 from 20 degrees C the time-course of contracture was markedly prolonged and the outward current was reduced without significant change in maximum tension. The threshold for contraction tended to be somewhat lower at the lower temperature. The contractile repriming was much slower at low temperature. However, reduction in temperature slowed the rate of recovery much less at low [Ca(++)](o) than at normal [Ca(++)](o).