The control by internal calcium of membrane permeability to sodium and potassium

A calcium ionophore simulating the action of epinephrine on the alpha-adrenergic receptor

Calcium acting through the ionophore A-23187 caused massive K(+) release from rat parotid slices. The ionophore thus simulates the action of epinephrine on the alpha-adrenergic receptor in this system. The alpha-adrenergic and cholinergic receptors are, however, not involved in the action of the ionophore since phetolamine and atropine had no effect on the ionophore-induced K(+) release. Amylase secretion, which is induced by epinephrine through the beta-adrenergic receptor but does not require Ca(++) in the medium, was only slightly increased by the ionophore in the presence of Ca(++). The K(+) released by Ca(++) through the ionophore was regained by the cells upon chelation of Ca(++) with ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid. Similar to the action of epinephrine on the alpha-adrenergic receptor, the K(+) release induced by the ionophore is specific for Ca(++), which cannot be substituted by 2.5 mM Sr(++), Mg(++), or Ba(++). The results suggest that epinephrine, like the ionophore, introduces Ca(++) into the cell to mediate K(+) release. It is proposed that the ionophore A-23187 may be an effective probe to test whether other Ca(++)-requiring hormones and neurotransmitters act in a similar fashion to produce their diverse physiological responses.

Dependence on calcium concentration and stoichiometry of the calcium pump in human red cells

1. In resealed human red cells loaded with Ca-EGTA buffer solutions it was found that the intracellular free Ca(2+) concentration for half saturation of the Ca transport system (which pumps Ca out of the cell) is equal to or smaller than 4 x 10(-6)M and thus closely agrees with the dissociation constant of the Ca + Mg activated membrane ATPase.2. The maximal rate of Ca transport from resealed cells to medium was found to be 0.148 +/- 0.009 mumole/ml. cells.min at 28 degrees C.3. The rate of Ca transport was unaffected by a variation of the extracellular Ca(2+) concentration from 3.10(-7) to 5.10(-3)M.4. Evidence is presented making it probable that the stoichiometric relation between Ca transported and ATP hydrolysed is 1:1 rather than 2:1.5. As the Ca transport is quite rapid even at half saturation and the passive leak for Ca negligible in intact cells it can be predicted that the steady-state cellular Ca(2+) concentration must be low, most probably less than 10(-6) mumole/ml. cells. Transport from cells containing 5.10(-7) mumole/ml. into blood plasma is thermodynamically compatible with the normal plasma Ca(2+) concentration and the normal cellular ATP, ADP and P(i) content.6. Treatment with the mercurial PCMBS in the cold for 15 hr allows to load red cells with 1 mumole Ca/ml. cells without destroying their ability to transport Ca after removal of the mercurial.7. It is shown that at high cellular Ca concentrations (0.1-3 mumole/ml. cells) about 50% of the total is free Ca(2+) on account of binding mainly to dialysable cell constituents.

Dog red blood cells. Adjustment of salt and water content in vitro

Dog red blood cells (RBC) lack a ouabain-sensitive sodium pump, and yet they are capable of volume regulation in vivo. The present study was designed to find in vitro conditions under which dog RBC could transport sodium outward, against an electrochemical gradient. Cells were first loaded with sodium chloride and water by preincubation in hypertonic saline. They were then incubated at 37 degrees C in media containing physiologic concentrations of sodium, potassium, chloride, bicarbonate, glucose, and calcium. The cells returned to a normal salt and water content in 16-20 h. Without calcium in the medium the cells continued slowly to accumulate sodium. Removal of glucose caused rapid swelling and lysis, whether or not calcium was present. The net efflux of sodium showed a close relationship to medium calcium over a concentration range from 0 to 5 mM. Extrusion of salt and water was also demonstrated in fresh RBC (no hypertonic preincubation) when calcium levels in the media were sufficiently raised. The ion and water movements in these experiments were not influenced by ouabain or by removal of extracellular potassium. Magnesium could not substitute for calcium. It is concluded that dog RBC have an energy-dependent mechanism for extruding sodium chloride which requires external calcium and is quite distinct from the sodium-potassium exchange pump.

The influx of calcium ions into human erythrocytes during cold storage

1. When human erythrocytes, suspended in iso-osmotic sucrose containing CaCl(2), are stored at 3 degrees C, Ca(2+) influx into the cells occurs. Simultaneously, efflux of K(+), Na(+), Cl(-) and water takes place and cell volume diminishes. 2. The extent of Ca(2+) influx increases with duration of cold storage and with increasing concentration of Ca(2+) in the suspending medium. 3. Erythrocytes that have been thus loaded with Ca(2+) exhibit Ca(2+) efflux against a concentration gradient when subsequently incubated at 37 degrees C. 4. Ca(2+) influx likewise occurs when the sucrose of the medium is replaced by iso-osmotic solutions of other non-ionized compounds. 5. Replacement of sucrose by iso-osmotic KCl or NaCl greatly diminishes the rate of Ca(2+) influx during cold storage; however, in iso-osmotic choline chloride, Ca(2+) influx is as rapid as in sucrose. 6. Preincubation of erythrocytes in iso-osmotic sucrose at 37 degrees C causes rapid efflux of K(+) and Na(+) and renders the cell membranes highly permeable to Ca(2+) during subsequent cold storage. 7. Preincubation of erythrocytes in iso-osmotic NaCl at 37 degrees C with trypsin or neuraminidase is without effect on the permeability of the membrane towards Ca(2+). 8. The experimental results lead to the conclusion that the main prerequisite for Ca(2+) influx into erythrocytes is the partial depletion of the cells of their univalent cations.

Control of synthesis and release of radioactive acetylcholine in brain slices from the rat. Effects of neurotropic drugs

1. Studies of the synthesis and release of radioactive acetylcholine in rat brain-cortex slices incubated in Locke-bicarbonate-[U-(14)C]glucose media, containing paraoxon as cholinesterase inhibitor, revealed the following phenomena: (a) dependence of K(+)-or protoveratrine-stimulated acetylcholine synthesis and release on the presence of Na(+) and Ca(2+) in the incubation medium, (b) enhanced release of radioactive acetylcholine by substances that promote depolarization at the nerve cell membrane (e.g. high K(+), ouabain, protoveratrine, sodium l-glutamate, high concentration of acetylcholine), (c) failure of acetylcholine synthesis to keep pace with acetylcholine release under certain conditions (e.g. the presence of ouabain or lack of Na(+)). 2. Stimulation by K(+) of radioactive acetylcholine synthesis was directly proportional to the external concentration of Na(+), but some synthesis and release of radioactive acetylcholine occurred in the absence of Na(+) as well as in the absence of Ca(2+). 3. The Na(+) dependence of K(+)-stimulated acetylcholine synthesis was partly due to suppression of choline transport, as addition of small concentrations of choline partly neutralized the effect of Na(+) lack, and partly due to the suppression of the activity of the Na(+) pump. 4. Protoveratrine caused a greatly increased release of radioactive acetylcholine without stimulating total radioactive acetylcholine synthesis. Protoveratrine was ineffective in the absence of Ca(2+) from the incubation medium. It completely blocked K(+) stimulation of acetylcholine synthesis and release. 5. Tetrodotoxin abolished the effects of protoveratrine on acetylcholine release. It had blocking effects (partial or complete) on the action of high K(+), sodium l-glutamate and lack of Ca(2+) on acetylcholine synthesis and release. 6. Unlabelled exogenous acetylcholine did not diminish the content of labelled tissue acetylcholine, derived from labelled glucose, suggesting that no exchange with vesicular acetylcholine took place. In the presence of 4mm-KCl it caused some increase in the release of labelled acetylcholine. 7. The barbiturates (Amytal, pentothal), whilst having no significant effects on labelled acetylcholine synthesis in unstimulated brain except at high concentration (1mm), diminished or abolished (at 0.25 or 0.5mm) the enhanced release of acetylcholine, due to high K(+) or lack of Ca(2+). The fall in tissue content of acetylcholine, due to lack of Ca(2+), was diminished or abolished by pentothal (0.25 or 0.5mm) or Amytal (0.25mm).

Injections of calcium ions into spinal motoneurones

1. In cats under Dial anaesthesia, Ca(2+) was injected inside lumbosacral motoneurones, by passing currents between CaCl(2)- and KCl-containing barrels of compound micropipettes.2. There was a reduction in excitability and a fall in membrane resistance, both rapid in onset and quickly reversible.3. The minimum effective injection current was approximately 10 nA, and the effect reached a maximum with currents of approximately 30 nA. The mean slope of resistance change against injection current was -1.7%/nA (S.E. 0.35).4. The most common change in membrane potential was a hyperpolarization; but in nearly half the cases, there was no clear change or a small depolarization. A reversal level for the effect of Ca(2+) could be measured in five cells: on the average, it was 10 mV more negative than the resting potential.5. Observations on i.p.s.p.s showed that Ca(2+) probably does not alter g(Cl): it was concluded that the fall in membrane resistance caused by intracellular Ca(2+) is mainly due to an increase in g(K).6. These results confirm previous suggestions that a steep transmembrane gradient of Ca(2+) is essential for the maintenance of a low membrane conductivity, and that a rise in internal free Ca(2+) - whether due to influx or release from internal stores - may play an important role in regulating neuronal activity.

The effects of indomethacin on calcium, sodium, potassium and magnesium fluxes in various tissues of the guinea-pig

1. Isolated tissues of the guinea-pig were bathed with Krebs solution at 37 degrees C and subjected to 100 ms pulses of electrical stimulation for 30 min at a frequency of 0.1 or 1.0 Hz. The tissues were then dried, ashed, and the ash analysed for calcium, sodium, potassium and magnesium.2. Gastric smooth muscle, cardiac and skeletal muscles and brain all showed a gain of sodium and calcium and a loss of potassium in response to electrical stimulation, but there was no significant change in the magnesium content of any of these tissues.3. Indomethacin (0.5 mM) reduced the calcium content of unstimulated gastric smooth muscle and reduced the gain of calcium and sodium in response to electrical stimulation, but slightly increased the net loss of potassium in response to electrical stimulation.4. Gastric smooth muscle which had gained calcium as a result of electrical stimulation, gradually lost it again when stimulation ceased. Indomethacin (0.5 mM) hastened the net loss of calcium from previously stimulated muscle.5. Indomethacin (0.5 mM) failed to alter the calcium, sodium, potassium and magnesium contents of unstimulated cardiac muscle, skeletal muscle and brain. In these tissues indomethacin (0.5 mM) also failed to prevent the changes in the content of these minerals which occurred in response to electrical stimulation.

Potassium permeability in -thalassemia minor red blood cells

The intracellular content of K(+) in thalassemia minor red blood cells is markedly reduced after incubation in autologous serum for 24 hr at 37 degrees C. There is no compensatory increase in intracellular Na(+) concentration of the cell thus reduced. This change is due to an acquired increase in selective permeability of the membrane to K(+). This phenomenon follows the depletion of energy sources in the thalassemia minor cells but does not follow comparable depletion in normal cells. The loss of osmotically active intracellular contents probably accounts for the increased resistance of incubated thalassemia minor red blood cells to osmotic lysis.

Role of calcium and adenosine-3':5'-cyclic monophosphate in controlling fly salivary gland secretion

The action of 5-hydroxytryptamine (5-HT) on an insect salivary gland was associated with a rise in adenosine-3':5'-cyclic monophosphate (cAMP) concentration and an increase in calcium uptake. An increase in secretion induced either by 5-HT or exogenous cAMP required extracellular calcium. Both 5-HT and exogenous cAMP increased (45)Ca efflux from previously labeled glands, but only 5-HT caused an increase in calcium uptake. The transepithelial potential in this tissue became more negative after addition of 5-HT, but more positive after addition of cAMP. 5-HT and cAMP induced a more negative potential when calcium was removed from the medium. It was concluded that both calcium and cAMP serve as intracellular messengers when 5-HT acts upon fly salivary gland.

Nature of the calcium dependent potassium leak induced by (+)-propranolol, and its possible relevance to the drug's antiarrhythmic effect

1. Manninen (1970) recently reported that in the presence of calcium ions, (+)-propranolol causes a massive loss of potassium ions from human red cells, together with a marked inward ;counter-transport' of potassium ions from the medium.2. This paper shows, theoretically, that counter-transport of the kind he observed may be accounted for by the setting up of a diffusion potential across the cell membrane and does not imply that, under the influence of propranolol, potassium ions cross the membrane by some process other than free diffusion.3. The physiological and pharmacological significance of this interpretation of Manninen's findings is briefly discussed.

The influence of the chloride gradient across red cell membranes on sodium and potassium movements

1. A study has been made to see whether active and passive movements of sodium and potassium in human red blood cells are influenced by changing the chloride gradient and hence the potential difference across the cell membrane.2. Chloride distribution was measured between red cells and isotonic solutions with a range of concentrations of chloride and non-penetrating anions (EDTA, citrate, gluconate). The cell chloride concentration was greater than that outside with low external chloride, suggesting that the sign of the membrane potential was reversed. The chloride ratio (internal/external) was approximately equal to the inverse of the hydrogen ion ratio at normal and low external chloride, and inversely proportional to external pH. These results show that chloride is passively distributed, making it valid to calculate the membrane potential from the chloride ratio.3. Ouabain-sensitive (pump) potassium influx and sodium efflux were decreased by not more than 20 and 40% respectively on reversing the chloride gradient, corresponding to a change in membrane potential from -9 to +30 mV. In contrast, passive (ouabain-insensitive) movements were reversibly altered - potassium influx was decreased about 60% and potassium efflux was increased some tenfold. Sodium influx was unaffected by the nature of the anion and depended only on the external sodium concentration, whereas ouabain-insensitive sodium efflux was increased about threefold. When external sodium was replaced by potassium there was a decrease in ouabain-insensitive sodium efflux with normal chloride, but an increase in low-chloride medium.4. Net movements of sodium and potassium were roughly in accord with the unidirectional fluxes.5. The results suggest that reversing the chloride gradient and, therefore, the sign of the membrane potential, had little effect on the sodium pump, but caused a marked increase in passive outward movements of both sodium and potassium ions.