Effect Of Extracellular Calcium Research Articles

Effects of extracellular calcium and of calcium antagonists on the contractile responses of isolated human pial and mesenteric arteries.

In isolated human pial arteries (diameter 0.4-0.5 mm), contractions were produced by potassium, noradrenaline, serotonin, and prostaglandin F2 alpha. For comparison, experiments were also performed on human mesenteric arteries. Threshold concentration for potassium-induced contraction in pial arteries was about 10 mM; in mesenteric arteries it was 3-5 mM higher. In pial arteries the calcium antagonists nifedipine and nimodipine caused an almost complete relaxation of contractions induced by potassium at drug concentrations relaxing prostaglandin F2 alpha-contracted vessels to only about 60%. Both nifedipine and nimodipine effectively inhibited contraction elicited by noradrenaline and serotonin in pial arteries. Nifedipine had a higher potency for relaxing cerebral than mesenteric arteries contracted by potassium (p less than 0.001). No such difference was demonstrated for nimodipine. In pial arteries pretreated in a calcium-free medium for 30 min, potassium depolarisation elicited contractions reaching a maximum amplitude of about 40% of that evoked in normal Krebs solution. Both nifedipine and nimodipine effectively inhibited contractions induced by calcium in pial arteries pretreated in a calcium-free medium and depolarised by potassium. The results suggest that potassium, amines, and prostaglandin F2 alpha activate isolated pial and mesenteric arteries by different calcium-dependent mechanisms and confirm the potent relaxant effects of nifedipine and nimodipine in these vessels.

Calcium and calcium slow channel antagonists on cyclic nucleotide levels in the isolated rat heart

An investigation has been carried out into the effects of extracellular calcium on cyclic nucleotide levels in rat ventricle. Increasing extracellular calcium, over the range which directly affected contractility, produced a concentration related increase in the level of cyclic AMP. Positive correlations were found between extracellular calcium and cyclic AMP; and between cyclic AMP and isometrically developed tension. No reciprocal relationships were found for cyclic GMP over the same calcium concentration range. Increasing extracellular calcium above the maximal inotropic concentration did not produce a further increase in cyclic AMP, although increases in cyclic GMP were noted. Calcium slow channel antagonists, nifedipine and manganese both depressed developed tension at the concentrations used, but this effect was not associated with any significant change in cyclic nucleotide levels. Verapamil depressed developed tension and cyclic AMP levels but the effect on cyclic AMP was not concentration related. No calcium slow channel antagonist significantly affected cyclic GMP levels. In electrically paced hearts, increasing rate of stimulation depressed developed tension but had no significant effect on cyclic AMP.

Growth and differentiation of mouse epidermal cells in culture: effects of extracellular calcium.

The pattern of proliferation and differentiation in cultured mouse epidermal cells in markedly altered by modifying the ionic calcium concentration in the culture medium. When medium calcium is lowered from 1.44 mM to 0.05-0.1 mM, keratinocytes proliferate rapidly with a high growth fraction, do not stratify, but continue to synthesize keratin. The cells grow as a monolayer for several months and can be subcultured in low Ca++ medium. Ultrastructural examination of cells cultured under low Ca++ conditions reveals widened intercellular spaces with an absence of desmosomes. Microvilli are numerous, and tonofilaments and cellular organelles are organized perinuclearly. Epidermal cells growing as a monolayer in low Ca++ can be identified to terminally differentiate by adding calcium to the level normally found in the culture medium. Contact between cells occurs rapidly and desmosomes form within 2 hours. The cells stratify in 1-2 days and terminally differentiate in 3-4 days. After Ca++ addition, DNA synthesis decreases after a lag of 5-10 hours and is totally inhibited within 36 hours. In contrast, RNA and protein synthesis continue at 40-50% of the control level at Day 3, a time when many cells are detaching from the culture dish. Keratin synthesis is unaffected by the Ca++ switch. Manipulation of epidermal proliferation and differentiation by altering extracellular calcium levels should enhance the usefulness of epidermal cell cultures in the study of differentiation and carcinogenesis.

Depression by calcium of spontaneous transmitter release at the mammalian neuromuscular junction

The effect of extracellular calcium on the frequency of miniature end-plate potentials (MEPPs) at the rat neuromuscular junction was studied using 5 different concentrations of K + (2.5, 5, 10, 15 and 20 mM). At low and normal levels of K + (2.5 and 5 mM), incremental increase of Ca 2+ concentration (0.1, 0.5, 1, 2, 3, 4, 7 and 10 mM) produced a slight monotonic increase in MEPP frequency. In 10 or 15 mM-K +, the frequency of MEPPs increased monotonically as Ca 2+ concentration was raised from 0.1 to 4 mM, but was unaltered or declinedslightly with further increase of Ca 2+ concentration. In 20 mM-K +, the frequency reached its maximum at 1 mM-Ca 2+ and progressively decreased as Ca 2+ concentration was increased to 10 mM. Thus, the relationship between Ca 2+ concentration and MEPP frequency was monotonic at the lower levels of K + but became distinctly non-monoonic with increased K +. This non-monotonic effect of Ca 2+ is consistent with the hypothesis that there are two opposing effects of increasing Ca 2+ concentration on transmitter release: (i) an increased Nernst equilibrium potential for Ca 2+ which causes an increase in Ca 2+ influx and a consequent rise in MEPP frequency and (ii) increased electrostatic screening of negative surface charges near Ca 2+ channels which produces an effective hyperpolarization across the channels and a consequent decrease in Ca 2+ influx.

Effect of extracellular calcium on amylase release from dispersed pancreatic acini.

In dispersed acini prepared from guinea pig pancreas, removing extracellular calcium did not alter the basal rate of amylase release but reduced the stimulation of enzyme release caused by cholecystokinin, carbachol, secretin, and vasoactive intestinal peptide as well as that caused by derivatives of cyclic nucleotides. In acini incubated in a calcium-free, EGTA-containing medium the increase in amylase release caused by each secretagogue tested did not change during the initial 10 min of incubation, decreased by approximately 65% during the subsequent 40 min, and remained constant thereafter. Removing extracellular calcium did not alter the maximally effective concentrations of cholecystokinin or vasoactive intestinal peptide but abolished the decrease in stimulated enzyme secretion seen with supramaximal concentrations of cholecystokinin. Incubating pancreatic acini with cholecystokinin or carbachol plus secretin or vasoactive intestinal peptide caused potentiation of amylase release, and removing extracellular calcium reduced the stimulation of enzyme release caused by the two secretagogues in combination but did not alter their potentiating interactions.

Effects of calcium and chelating agents on the ability of various agonists to increase cyclic GMP in pancreatic acinar cells

In dispersed acinar cells from guinea pig pancreas we found that chelating extracellular calcium with EDTA did not alter cellular cyclic GMP but caused a 50% reduction in the increase in cyclic GMP caused by the synthetic C-terminal octapeptide of porcine cholecystokinin (cholecystokinin octapeptide). This effect was maximal within 2 min and preincubating the cells with EDTA for as long as 30 min caused no further reduction in the action of cholecystokinin octapeptide. In acinar cells preincubated without calcium, adding calcium caused a time dependent increase in the action of cholecystokinin octapeptide and this increase was maximal after 10 min of incubation. An effect of extracellular calcium on the action of cholecystokinin octapeptide could be detected with 0.5 mM calcium and was maximal with 2.0 mM calcium. Magnesium alone or with calcium did not alter the action of cholecystokinin octapeptide. Extracellular calcium did not alter the time course or the configuration of the dose vs. response curve for the action of cholecystokinin octapeptide on cellular cyclic GMP. Low concentrations of EGTA (0.1 mM) decreased the effect of cholecystokinin octapeptide on cellular cyclic GMP to the same extent as did EDTA or preincubating acinar cells without calcium. Increasing EGTA above 0.1 mM caused progressive augmentation of the action of cholecystokinin octapeptide on cellular cyclic GMP and this augmentation did not require extracellular calcium or magnesium. Results similar to those obtained with cholecystokinin octapeptide were also obtained with bombesin, carbamylcholine, litorin and eledoisin. In contrast, the action of sodium nitroprusside on cyclic GMP in pancreatic acinar cells was not altered by adding EDTA or EGTA. These results indicate that the ability of extracellular calcium to influence the action of cholecystokinin octapeptide and other agents on cyclic GMP results from changes in cellular calcium and not from effects of extracellular calcium per se. The action of low concentrations of EGTA on the increase in cyclic GMP caused by various agents reflects the ability of EGTA to chelate extracellular calcium. The actions of high concentrations of EGTA were independent of extracellular calcium or magnesium and appear to reflect a direct action of EGTA on pancreatic acinar cells.

Effects of extracellular calcium on potassium and noradrenaline induced contractions in the aorta of spontaneously hypertensive rats--increased sensitivity to nifedipine.

Isolated preparations of the thoracic aorta from spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR) were contracted by noradrenaline (NA) 1.8 X 10(-5)M, and potassium (K+) 127 mM, after 30 min. pretreatment in a Ca++-free medium. In both SHR and NWR aortae, the contractions were markedly reduced; no significant differences were found between the two types of vessels in Ca++-free medium. On addition of Ca++, the contractions were restored to a significantly greater extent in the NWR than in the SHR aortae. In the presence of nifedipine 7.2 X 10(-9)M, the response to Ca++ was significantly more reduced in the SHR than in the NWR preparations. Relaxation of NA and K+ contracted preparations was induced by wash-out of the contractile agents, by addition of nifedipine 2.9 X 10(-8)M, and by introduction of a Ca++-free medium. After wash-out of NA, relaxation was slower in SHR than in NWR vessels. Relaxation induced by nifedipine and Ca++-free medium was more complete in SHR than in NWR preparations. After wash-out of K+, relaxation was more rapid in NWR than in SHR aortae. Nifedipine and Ca++-free medium induced relaxation was more complete in SHR than in NWR preparations. The results suggest that in the SHR aortae contraction induced by NA and K+ is more dependent on extracellular Ca++ than is the response in the NWR preparations; the SHR vessels are also more sensitive to the relaxing effects of nifedipine.

Effects of extracellular calcium on the myocardial changes due to calcium overload: L.E. Alto, P.K. Singal and N.S. Dhalla. Department of Physiology, University of Manitoba, Faculty of Medicine, Winnipeg, Canada

Effects of extracellular calcium on the myocardial changes due to calcium overload: L.E. Alto, P.K. Singal and N.S. Dhalla. Department of Physiology, University of Manitoba, Faculty of Medicine, Winnipeg, Canada

The effect of extracellular calcium and several drugs on the premature action potential and postextrasystolic potentiation

The effect of extracellular calcium and several drugs on the premature action potential and postextrasystolic potentiation

Effects of extracellular calcium and sodium on depolarization-induced automaticity in guinea pig papillary muscle.

Regenerative discharge of action potentials is induced in mammalian papillary muscles by passage of small depolarizing currents. In this paper, the effects of various extracellular calcium and sodium concentrations and of tetrodotoxin on this phenomenon were studied in guinea pig papillary muscles in a sucrose gap chamber. Phase 4 diastolic depolarization was found to be associated with an increase in membrane resistance. The slope of phase 4 depolarization was decreased by reductions in extracellular calcium or sodium concentration. The range of maximum diastolic potentials and the thresholds from which regenerative potentials arose were reduced, especially at the positive limit of potentials, by a reduction in either ion. It was concluded that both calcium and sodium influence diastolic depolarization and participate in the regenerative action potentials of depolarization-induced ventricular automaticity.

The effect of extracellualr calcium on thermal excitability of the sensory units in the tooth of the cat.

AbstractIntradental sensory nerve impulse activity was measured from dentinal cavities in canine teeth of anesthetized cats. No spontaneous activity was usually recorded in cavities filled with isotonic saline. Heat stimulation of 15–20oC failed to give any impulse activity, while other stimuli, such as air blasts produced bursts of impulses. Lowering extracellular calcium ion concentration. by local application of sodium citrate, sodium oxalate or EDTA, induced impulse activity. Under these conditions, heat produced a rapid increase in discharge activity. which lasted as long as the stimulus was applied. Increased extracellular calcium concentration abolished this response to heat. The present findings show that a change in the extracellular calcium ion concentration modulates the excitability of intradental sensory units, and may thus also modulate the resulting experience of pain.

Calcium-Independent Intracellular Conversion of Proparathyroid Hormone to Parathyroid Hormone

Effects of extracellular calcium (0.5 to 5.0 mM) on the rate of intracellular conversion of proparathyroid hormone to parathyroid hormone were examined. Slices of bovine parathyroid glands were incubated in vitro with radioactive amino acids, and the amounts of radioactive PTH and proPTH in tissue extracts were assayed by electrophoresis on polyacrylamide gels. No changes in the ratio of proPTH to PTH were observed in media containing high concentrations of calcium (2.5 mM), which suppress both the secretion of parathyroid hormone by 80% and specific biosynthesis of prohormone and hormone by 15%. The results indicate that calcium does not regulate the bio-synthesis of parathyroid hormone by affecting its conversion rate from the prohormone.

The effect of extracellular calcium and magnesium on the proliferation of murine lymphoblasts.

AbstractWe have shown that the murine lymphoblast, L5178Y, requires extracellular magnesium or calcium for proliferation in suspension culture. Although both cations produce biphasic effects on growth, magnesium is the more potent since it: (1) stimulates proliferation at lower concentrations; (2) supports optimal proliferation over a wider concentration range; (3) maintains higher cell densities at stationary phase; and (4) produces less inhibition at high concentration.At suboptimal concentrations, calcium facilitates the effect of magnesium but at optimal concentration, no facilitation is evident. A concentration of 3.2 mM calcium or magnesium inhibits growth, while the same concentration composed of equimolar calcium and magnesium does not inhibit proliferation.Extracellular calcium and magnesium may influence proliferation by effecting changes in intracellular calcium and magnesium since: (1) reduction in extracellular concentrations sufficient to produce decreased growth rate is associated with decreased cell calcium and magnesium; (2) cell magnesium and growth rate are related such that an exponential decrease in relative doubling time occurs with decrease in cellular magnesium.Cells cultured at 37°C in magnesium and calcium‐deprived medium will proliferate at a normal rate if either or both cations are replaced within six to 10 hours; however, with longer deprivation the doubling time in restored medium is progressively lengthened.

The effect o f calcium on contraction and conductance thresholds in frog skeletal muscle.

1. The effect of extracellular calcium and magnesium on the contraction threshold and on the thresholds for an increase in sodium and potassium conductance with depolarization was studied in voltage-clamped frog muscle fibres.2. A larger depolarization was required to reach each of the three thresholds when the concentration of divalent cation was increased.3. The contraction and potassium conductance thresholds appeared to shift in parallel with alterations in calcium over the concentration range 0.2-10.0 mM and in magnesium over the concentration range 5.4-90.0 mM. The shift amounted to about 4 mV for a threefold change in concentration of divalent cation.4. The sodium conductance threshold was much more sensitive to alterations in divalent cation concentration than was either the contraction or the potassium conductance threshold.

Influence of calcium and magnesium ions on myocardial surface and intracellular electrogram and contractile force of a dog

Changes in the intracellular potential has been explained by the fluxes of the cations, especially sodium and potassium ions, through the cell membrane. Calcium and magnesium ions also affect the intracellular potential. There have remained still many problems about effects of extracellular calcium and magnesium ions on the intracellular electrogram (ICEG). On the other hand, it is well-known that calcium ion has markedly inotropic action and plays an important role in excitation-contraction coupling. On the contrary, effect of magnesium ion on myocardial contractility seems to be a little. In the present experiments, the author investigated changes in ICEG, surface electrogram (SEG), and contractility of dog's heart muscle in situ subjected to the coronary injections of calcium and magnesium ions of the various concentrations. Methods Sixteen mongrel dogs weighing 8 to 15 kg were used. After anesthetized, the chest was opened, followed by venous injection of 5000 units of heparin. Then a major branch of the anterior descending coronary artery was exposed. In eight dogs, a carotid artery was connected to the major branch of the coronary artery through a polyethylene catheter with a three-way stopcock, and ICEG was taken simultaneously with unipolar SEG from a point on the epicardium supplied by this coronary artery. These methods were similar to those reported by PRINZMETAL, TOYOSHIMA and their colleagues. In the other dogs, a major branch of the anterior descending coronary artery was perfused by arterial blood from a femoral artery with a catheter. Thereafter, 'myocardial tension tracing (MTT), SEG, and left ventricular pressure curve (LVP) were recorded. The load transducer divised by HISADA et al. was used for recording MTT. LVP was obtained by a cardiac catheter inserted through a carotid artery into the left ventricle. The exposed epicardium was irrigated with Tyrode's solution through a polyethylene tube. The temperature of the solution was 35 to 37°C but remained constant during any experiment for the purpose of keeping a constant temperature on the cardiac surface.

The effect of extracellular calcium on actions of noradrenaline, �-blockers and quinidine in isolated guinea-pig auricles

Isometrische Kontraktionen von elektrisch konstant gereizten linken Meerschweinchenherzohren wurden unter Einwirkung von Noradrenalin, MHIP, Pronethalol und Chinidin bei extracellularen Calciumkonzentrationen ([Ca] e ) zwischen 0,45 und 7,2 mM untersucht.