42K Efflux Research Articles

The kinetics of ouabain-sensitive ionic transport in the rabbit carotid artery.

1. Ouabain (0.1 mM)-sensitive 42K influx and 24Na efflux have been measured in rabbit carotid arteries under conditions of high cellular potassium, [K]i, as well as sodium, [Na]i. About 50% of the total fluxes are ouabain-sensitive (active) under conditions of high [K]i. 2. The extracellular space, determined by 60Co-EDTA, was relatively large in comparison to cellular water. The ionic concentrations in normal solution, estimated from isotope flux components, are: [Na]i = 24; [K]i = 169; [Cl]i = 68 mmol/l cell water. 3. The ouabain=sensitive 42K influx and 24Na efflux in high-K tissues were measured at varying external concentrations of potassium, [K]o, and normal concentrations of external sodium, [Na]o. Sigmoidal kinetics were observed and fitted to a co-operative interaction model. The maximal efflux of 24Na, 0.245 muequiv/g wet weight per minute, was about 1.4 times that for 42K influx. Half-maximal stimulation was achieved at [K]0.5o of 2.4 mM for Na, and 3.4 mM for K transport. The flux ratio of Na to K approximated 1.5. 4. Increased 42K efflux was found in the presence of ouabain and the passive influx of 42K was corrected for this effect. In the absence of this correction the ouabain-sensitive 42K influx would be reduced, and the Na/K flux ratio raised to about 2. 5. The [K]o-dependence of ouabain-sensitive fluxes was measured on Na-loaded tissues. 24Na efflux exhibited saturation kinetics with a maximum of 1.18 muequiv/g wet weight per minute and [K]0.5o = 3.1 mM. The 42K influx was two thirds the active Na efflux for [K]o less than or equal to 5 mM. At high [K]o, however, the influx greatly exceeded the predicted levels. Evidence is presented for a ouabain-sensitive membrane hyperpolarization being responsible for an additional influx of 42K. 6. The ouabain-sensitive 24Na efflux showed a sigmoidal dependence on [Na]i in the presence of [K]o = 10 mM and normal [Na]o. The maximal efflux was 0.88 muequiv/g weight per minute and [Na]0.5i = 49 mmol/l cell water, which is about twice the physiological operating point. 7. It is concluded that active Na and K transport in rabbit carotid artery follow sigmoidal kinetics and the flux ratio is about 1.5. Changes in [K]o and [Na]i over the physiological range can markedly affect transport, and may regulate vascular contraction by their action on electrogenic transport.

Epithelial potassium transport: tracer and electrophysiological studies in choroid plexus.

Transport of potassium by bullfrog choroid plexus was studied using tracers and ion-selective microelectrodes. 1) Tracers: We measured unidirectional uptakes of42K across each surface of the plexus, efflux of42K from loaded tissues, intracellular potassium pools, and [3H]-ouabain binding.42K uptake across the brush border membrane was composed of diffusional and saturable components. The saturable component exhibited kinetics of a two-site, model withKm's of 0.3mm and aVmax of 8 μmol cm−2 hr−1. Ouabain inhibited uptake across the brush border membrane with aKi of 1×10−7m. Ethacrynic acid, phloretin, amiloride, and low sodium concentrations inhibited uptake, whereas bicarbonate ions increased transport up to 100%. The rate of ouabain-sensitive uptake across the serosal surface was only 6% of that across the ventricular surface. The efflux of potassium across the brush border membrane could account for most of the efflux from the epithelium. Potassium was accumulated within the plexus to a concentration in excess of 100mm.42K in the extracellular compartments exchanged with 40–55% of the intracellular potassium. Ouabain bound to the brush border membrane with aKm of 8×10−7m, ak1 of 3.8×104 mol−1 min−1 and ak−1 of 3×10−2 min−1. Ouabain binding was blocked by cymarin and gitoxigin. 2) Electrodes: Under control conditions the intracellular electrical potential,Evc, was −45 mV and the apparent intracellular K+-concentration, Kc+, was 90mm. K+ ions appeared to be actively accumulated within the epithelium.Evc and Kc+ were followed under three experimental conditions: (i) treating the tissue with ouabain; (ii) varying the ventricular K+ concentration; and (iii) passing transmural currents. It is concluded that the permeability of the ventricular membrane to potassium (1–5 × 10−5 cm sec−1)_was much greater than the permeability of the serosal membrane and that the rate of K pumping into the epithelium was 0.35–0.55×10−9 mol cm−2 sec−1. 96% of the transmural current was paracellular, and the resistance of the serosal membrane was nine times greater than that of the ventricular membrane. Increasing Kc+ produced Nernstian changes inEvc andEKvc, but the cells only depolarized by 0.13 mV for each mV increase in the chemical potential of the ventricular solutions.

Changes in 42K efflux produced by alterations in transmembrane calcium movements in turtle cardiac pace-maker tissue.

1. 42K efflux has been measured from small strips of turtle sinus venosus which were electrically paced. Three different procedures for altering transmembrane calcium influx have been utilized to test whether changes in 42K efflux may be modulated by changes in intracellular calcium levels. 2. No significant changes in the 42K fractional escape rate (FER) were observed when external calcium was reduced to O mM or increased to 4 x normal (10 mM). In these experiments extracellular divalent cation concentration was held constant by adding or removing magnesium ions. 3. Application of 10 mM-Ba2+ also failed to alter 42K FER consistently. In red blood cells and snail neurones addition of barium ions has been shown to reduce significantly the calcium-mediated potassium current. 4. A tenfold increase in pacing rate (0.5-5 Hz) resulted in an augmented 42K FER, but repetition of this rate change in O mM-Ca2+ indicated that this increase in 42K FER was not strongly dependent on the amount of calcium entry. 5. Attempts to load the pace-maker cells with calcium by using the ionophore A23187 (10 micrograms ml . -1 of 2.0 x 10(-5) M) consistently resulted in very large increases in 42K FER. However, this effect (i) was blocked by atropine and (ii) was markedly reduced by pretreating the tissues with hemicholinium, indicating that A23187-induced release of acetylcholine from the endogenous nerve terminals was responsible for the observed increase in 42K FER. 6. In summary, three different experimental tests indicate that the majority of the 42K efflux is not tightly linked to transmembrane calcium movement in sinus venosus pace-maker tissue.

Are acetylcholine-induced increases in 42K efflux mediated by intracellular cyclic GMP in turtle cardiac pace-maker tissue?

1. 42K efflux has been measured from small strips of turtle sinus venosus tissue in order: (a) to characterize further the pharmacology of the acetylcholine response and (b) to test whether cyclic guanosine 3':5'-monophosphoric acid (cyclic GMP) is the intracellular mediator of this response. 2. The 42K wash-out curves show that the fractional escape rate (FER) of 42K efflux is nearly constant after 60-80 min, indicating that after this time period 42K FER is controlled by barrier-limited diffusion from a single intracellular compartment. 3. The threshold of the dose-response relationship for the acetylcholine-induced increase in 42K FER is about 10(-8) M and the Km is 2.75 x 10(-7) in non-eserinized preparations. 4. This acetylcholine response is completely blocked by atropine; but nicotinic blockers produce no detectable reduction of it. 5. Exogenous application of lipid-soluble analogues of cyclic GMP (dibutyryl or 8-bromo-cyclic GMP applied at 2-3 mM for 30 min) failed to mimic the acetylcholine-induced augmentation of 42K FER. 6. Experiments in which sodium nitroprusside (5 x 10(-4) M for 30 min) was applied in order to stimulate the guanylate cyclase and hence produce a large, maintained increase in intracellular cyclic GMP failed to show a significant increase in 42K FER. 7. When acetylcholine (10(-6)M) was applied in the presence of O[Ca2+]0 (in an attempt to inhibit the guanylate cyclase) there was no significant reduction in the acetylcholine-induced increases in 42K FER. 8. Hence, these three indirect tests indicate that the muscarinic acetylcholine-induced increase in 42K FER in cardiac pace-maker tissue is unlikely to be mediated entirely by changes in the levels of intracellular cyclic GMP.

Effects of hyperkalemia and glycoside on thallium exchange in rabbit ventricle.

The exchange characteristics of thallium-201 (201Tl) were studied using the isolated arterially perfused rabbit interventricular septum to gain insight into the mechanisms that govern its uptake and subsequent washout from myocardial tissue. The results were compared to potassium-42 (42K) exchange. Unlike 42K, the uptake and tissue washout exchange rates for 201Tl were not equal. The 201Tl washout exchange rate was slower than that for the uptake. Hyperkalemia increased the efflux of both 42K and 201Tl, but the increase in 201Tl efflux was not due to an increase in permeability, as was true for 42K, but to a hyperkalemia, induced increase in compartment size of exchangeable 201Tl. Acetylstrophanthidin (ACS) resulted in an increase in 201 Tl efflux similar to the increase in 42K efflux seen with ACS. However, ACS did not inhibit 201Tl influx, in contrast to the known inhibition of 42K influx. This indicated that influx of tracer amounts of 201Tl was not dependent on the sodium-potassium pump. A model of Tl exchange in rabbit ventricles is proposed.

ACTIONS OF VARIOUS MUSCARINIC AGONISTS ON MEMBRANE POTENTIAL, POTASSIUM EFFLUX, AND CONTRACTION OF LONGITUDINAL MUSCLE OF GUINEA‐PIG INTESTINE

1 Depolarizations were recorded intracellularly in smooth muscle from the taenia of the guinea-pig caecum in response to the iontophoretic application of acetylcholine, carbachol, oxotremorine-M, methylfurmethide, hexyl trimethylammonium and tetramethylammonium (TMA). 2 No differences between the iontophoretic responses to agonists stable to cholinesterase were detected. 3 The latency and time to peak of acetylcholine-induced depolarizations were less than those to stable agonists, and the response was briefer and less complex in shape. These differences were reduced, or disappeared, upon inhibition of cholinesterase. 4 The rate of loss of 42K and changes in length were measured in superfused strips weighing about 10 mg of separated longitudinal muscle of guinea-pig ileum. 5 Acetylcholine, carbachol, methylfurmethide, butyltrimethylammonium and TMA contracted the muscle and increased the rate of loss of 42K. 6 Contrary to previous reports, no evidence of a selective action of any of these agonists on 42K loss was detected. TMA appeared to be a partial agonist in evoking 42K loss, although it produced a maximum contraction. 7 The maximum 42K efflux produced by acetylcholine was about 40% of the maximum evoked by application of carbachol or methylfurmethide. If cholinesterase was inhibited, the 42K efflux evoked by maximally effective concentrations of acetylcholine was comparable to that evoked by a stable agonist. 8 These results are consistent with the idea that the muscarinic agonists used interact in an essentially similar way with muscarinic receptors to produce their effects on membrane potential, permeability, and tension.

The ionic relations of Porphyra purpurea(Roth) C.Ag. (Rhodophyta, Bangiales)

Abstract Radioisotope equilibration techniques have been used to determine the intracellular concentration of K+, Na+ and Cl−, together with the unidirectional ion fluxes across the plasmalemma of Porphyra purpurea. Influx and efflux of 42K+, 24Na+ and 36C1− are biphasic, the rapid, initial uptake and loss of tracer from individual thalli being attributable to desorption from extracellular regions. Cellular fluxes are slower and monophasic, cells discriminating in favour of K+ and Cl− and against Na+. A comparison between the equilibrium potential of individual ion species and the measured membrane potential demonstrates that there is an active component of K+ and Cl− influx and Na+ efflux. ‘Active’ uptake and ‘passive’ loss of K+ and Cl− are reduced when plants are kept in darkness, suggesting that a fraction of the transport of K+ and Cl− may be due to ‘exchange diffusion’ (K+/K+ and Cl−/Cl−antiport).

Characterization of the Concanavalin A-Induced Increase in Lymphocyte Cell Membrane Permeability by Furosemide

The effect of ConA on passive 42K + fluxes in human lymphocytes was analysed by treatment with furosemide and ouabain. Furosemide has no effect on net K+ transport and pump flux. The component of passive efflux of 42K + which depends on K +-K + exchange is inhibited by furosemide; its mechanism is discussed in detail. The 42K + uptake in lymphocytes consists of 1. an ouabain-sensitive pump influx, 2. an ouabain-insensitive leak influx and 3. a furosemide-sensitive K +-K +-exchange. Only the first two components of 42K + uptake are stimulated by ConA.

The influence of external potassium ions upon lens conductance characteristics investigated using a voltage clamp technique

An intracellular voltage clamp technique for the lens is described. Using this technique, it was possible to describe directly the conductance-voltage relationship of the lens over a wide range of lens potentials. The influence of potassium-rich solutions on lens conductance and 42K efflux rate was examined under conditions where the voltage clamp prevented changes in the lens potential. Under these conditions, potassium-rich solutions caused a reduction in lens conductance and a fall in the 42K efflux rate. These observations suggest that, at the resting potential, potassium-rich solutions influence conductance by decreasing lens potassium permeability. However, when the lens is not voltage clamped, the reduction of lens conductance is masked because the depolarization of lens potential increases the voltage-dependent conductance component, leading to an overall increase in the conductance of the lens.

The action of apamin on guinea-pig taenia caeci

Apamin (10 −7 M), a substance extracted from bee venom ( apis mellifica) causes stimulation of the taenia caeci as seen from an increase in spike activity. The inhibitory effect of ATP or adrenaline (Adr) was reflected by hyperpolarization of the muscle cell, cessation of spike activity and relaxation of the muscle. The 42K efflux and the membrane conductance were enhanced in the presence of these substances. Apamine converted the hyperpolarization caused by ATP or Adr into a transient depolarization which produced contraction of the muscle cells. The changes in membrane conductance and 42K efflux were diminished by the bee toxin. Furthermore, the potassium-dependent phase of the action potential was lengthened by apamine. Reduction of the extracellular chloride or sodium concentration, blockade of the nervous system by TTX (3 × 10 −7 M) or inhibition of spike activity by D600 (3 × 10 −6 M) did not affect the excitatory and blocking action of apamin. A high concentration of the calcium antagonist D600 (10 −4 M) or omission of extracellular calcium was needed to reduce the transient depolarization evoked by ATP or Adr in the presence of apamin. It is concluded that apamin prevents the opening of the ATP- and Adr-sensitive and voltage-dependent potassium channels in guinea-pig taenia caeci.

The effect of sodium removal on the contractile response of the guinea-pig taenia coli to carbachol.

1. The effects of Na-free solutions (using Li, Tris, sucrose or Mg as Na substitutes) on the contractile responses, membrane depolarization and 42K efflux produced by carbachol in the smooth muscle of the guinea-pig taenia coli have been investigated. The effect of these Na-free solutions on intracellular ion content of the muscle has also been studied. 2. Na removal induced a pattern of changes in the tone of the muscle characteristic of the substitute used, probably reflecting changes in transmembrane Ca fluxes involving Na. 3. Contractile responses to 10 sec application of 5 x 10(-5) M-carbachol were greatly reduced in Na-free solutions with all the Na substitutes used. This did not correlate with reduction in membrane depolarization or 42K efflux produced by the drug in the various Na-free medial used. 4. Intracellular Na seems important for maintaining the contractile response, since in Na-free solutions cellular Na levels and contractile responses were better maintained at 13 degrees than 34 degrees C and in tissues stimulated with carbachol every 10 min the final magnitude of the response was related to cellular Na content. If, however, the tissues was left unstimulated in Na-free Mg or sucrose solution a large response could still be obtained when cellular Na content was very low. A model is described which could account for these results in terms of an intracellular Ca store released by carbachol and requiring intracellular Na. 5. In tissues continuously exposed to 10(-4) M-carbachol Na removal, even for only 10 sec, produced rapid relaxation, probably secondary to changes in electrical properties of the membrane caused by removal of external Na.

Effects of sympathetic denervation or chronic reserpine on potassium ( 42K) and chloride ( 36Cl) efflux from guinea-pig vas deferens

Potassium ( 42K) or chloride ( 36Cl) efflux curves were determined in guinea-pig vas deferens from control, reserpine-treated animals (1 mg/kg/day i.p. for 5 days), and from animals whose vas deferens had been sympathetically denervated 1 week before. Steady-state 42K turnover in control tissues was 0.0052 ± 0.0002 min −1; neither reserpine treatment nor sympathetic denervation changed this parameter significantly. Control 36Cl turnover was 0.058 ± 0.002 min −1 and it was unaffected by either procedure. Methoxamine (3 × 10 −6 to 10 −4 M) induced dose-related increases in the fractional exchange of 42K and 36Cl. These were of greater magnitude after sympathetic denervation or reserpine treatment. Furtrethonium also produced dose-dependent increases in 42K efflux; its dose-response curve was shifted 2.6-fold to the left of the control curve by reserpine treatment. These results indicate that interruption of adrenergic transmission to the guinea-pig vas deferens is associated with increased changes in membrane permeability to Cl and possibly K in response to drug activation of α-adrenergic and cholinegic receptors. It is suggested that the supersensitivity phenomenon observed in the guinea-pig vas deferens after reserpine or sympathetic denervation is, in part, related to improved transduction of drug-receptor interaction into ionic permeability changes.

Potassium flux ratio in voltage-clamped squid giant axons.

The potassium flux ratio across the axolemma of internally perfused, voltage-clamped giant axons of Loligo pealei has been evaluated at various membrane potentials and internal potassium concentrations ([K]i). Four different methods were used: (a) independent measurement of one-way influx and efflux of 42K; (b) simultaneous measurement of net K current (IK) and 42K influx; (c) simultaneous measurement of IK and 42K efflux; and (d) measurement of potassium conductance and 42K influx at the potassium equilibrium potential. The reliability of each of these methods is discussed. The average value of the exponent n' in the Hodgkin-Keynes equation ranged from 1.5 at -4mV and 200 mM [K]i to 3.3 at -38 mV and 350 mM [K]i and appeared to be a function of membrane potential and possibly of [K]i. It is concluded that the potassium channel of squid giant axon is a multi-ion, single-file pore with three or more sites.

The action of isoprenaline on the smooth muscle of the guinea-pig taenia coli.

1. The beta-action of catecholamines on the smooth muscle of guinea-pig taenia coli was investigated by observing the effects of isoprenaline (2 X 10(-8)-7.2 X 10(-6) M) in the presence of an alpha-blocker, phentolamine (6.3 X 10(-6) M). Electrical and mechanical activity were recorded with the double sucrose gap method. Calcium and potassium fluxes were determined using the 45Ca and 42K isotopes. 2. Isoprenaline suppressed spontaneous spike generation, reduced the size of evoked phasic contractions, and caused a small hyperpolarization of the membrane without change in membrane resistance. These effects were abolished by a beta-blocker, propranolol (6.8 X 10(-6) M). 3. The hyperpolarization induced by isoprenaline was smaller in quiescent, nonstimulated muscle than in active, frequently stimulated preparations. It occurred with the same time course as the reduction in the size of evoked phasic contractions. Both effects were dose dependent and reached a maximum at 7.2 X 10(-7) M-isoprenaline. 4. Hyperpolarization by direct current application did not reduce the size of evoked phasic contractions until excitation threshold was reached. In the presence of isoprenaline, repolarization of the membrane to its original level by depolarizing current application did not restore the reduced phasic contractions to their original size. 5. The slopes of the current-voltage relation in the absence and presence of isoprenaline were parallel, confirming the absence of a change in membrane resistance. Isoprenaline also did not affect membrane resistance when applied in the modified ionic environments used. 6. In different external K concentrations (0.60-29.5 mM) the relationship between the size of the electrotonic potential and the magnitude of the isoprenaline-induced hyperpolarization was linear. A similar, direct relation was seen between isoprenaline hyperpolarization and membrane resistance when the latter was increased by lowering external chloride to 13.3 mM. 7. Excess Cao (7.5 mM) hyperpolarized the membrane and reduced membrane resistance, but the hyperpolarization by isoprenaline was larger than in normal solution, being inversely related to the membrane resistance. The hyperpolarization was directly related to the external Ca concentration, suggesting that the magnitude of the response to the beta-action might depend on the cytoplasmic Ca concentration. 8. In low external Na (18 mM-Nao) the beta-action was scarcely affected. Complete replacement of Na with choline increased membrane resistance, muscle tone and phasic contractions; in this condition the effects of isoprenaline were abolished. 9. When the Na pump was blocked by exposure to zero K, to ouabain, or to both simultaneously, isoprenaline remained highly effective. However, prolonged exposure to ouabain abolished the beta-action. 10. Isoprenaline (1.4 X 10(-6) M) increased 45Ca efflux by about 20%, while 45Ca influx was not changed, and 42K efflux remained constant. 11...

Hypoxic contraction of isolated canine coronary artery. Mediation by potassium-dependent exocytosis of norepinephrine.

Effects of hypoxia on arterial tone, efflux of potassium, and efflux of norepinephrine were monitored for isolated canine coronary arteries labeled with radioactive potassium (42K) and norepinephrine (3HNE). Hypoxia elicited transient relaxation and subsequent sustained contraction accompanied by marked increases in the effluxes of 42K and 3HNE. After sympathetic nerve injury with 6-hydroxydopamine or cold storage, arteries responded to hypoxia with sustained relaxation. Sustained relaxation occurred also after pretreatment with L-propranolol, but not with D-propranolol or phentolamine. Inhibition of hypoxic contraction by L-propranolol did not alter 42K or 3HNE efflux. Colchicine, an inhibitor of the exocytosis of NE, suppressed hypoxic 3HNE efflux and contraction, but not 42K efflux. Proadifen inhibited 42K and 3HNE efflux as well as contraction. During proadifen-inhibited 42K efflux, exogenous K+ augmented overflow of 3HNE, indicating that proadifen relaxed the hypoxic artery primarily by inhibiting K+-dependent exocytosis of NE. The ratio of NE to dopamine beta-hydroxylase activity was similar in effluents from oxygenated arteries exposed to elevated K+ concentrations and in effluents from hypoxic arteries. Thus, hypoxia evoked exocytotic release of norepinephrine which promoted contraction by a beta-adrenergic mechanism.

Papaverine decreases the efflux of 42K in guinea-pig atrial heart muscle.

The effects of papaverine on resting potential and efflux of42K were investigated in guinea-pig left atria. Papaverine significantly reduced the potassium efflux in beating preparations. In resting preparations, the efflux of potassium was only slightly affected. However, the resting potential was significantly reduced by papaverine by about 5 mV.

K efflux through inward rectifying K channels in voltage clamped Purkinje fibers.

The 42K efflux was measured in voltage clamped sheep Purkinje fibers. The voltage dependence of the K efflux can be described as N-shaped, showing a negative slope region. At potentials negative to -30mV, the K efflux is largely due to K flowing through a channel which rectifies in the inward direction and which is blocked by external application of 20mM Cs+. At potentials positive to -30mV an outward rectifier dominates the shape of the K efflux-voltage relationship. This component is insensitive to short external application of Cs+. Both components were also found when Na+ was replaced by tetramethylammonium. When the steady-state current-voltage relationship is compared with the K efflux one can conclude that the outward rectifying K flux largely determines the shape of this curve at positive membrane potentials, while the negative slope region of the K efflux correlates with the negative slope of the steady-state current-voltage relation. The K efflux is only slightly enhanced by stimulation of the preparation, corroborating the finding of inward-going rectification of the K channel. A clamp program repetitively activating the positive dynamic current e.g. by alternating the membrane potential between -70 and +10mV, increases the K efflux by about 50% as compared to the efflux measured in steady-state at this positive membrane potential. 4-Aminopyridine suppresses both this extra K efflux and the positive dynamic current. It is concluded that K ions contribute to the positive dynamic current.

Decrease of K efflux and influx by external Cs ions in cardiac Purkinje and muscle cells.

The effect of different external Cs concentrations on 42K efflux in nonstimulated cow Purkinje fibers and guinea-pig auricular preparations was studied in Tyrode solution containing varying amounts of external K (Ke). In some of these conditions also 42K influx and 137Cs influx were estimated. Resting membrane potentials in cow Purkinje fibers were measured by the standard microelectrode technique. Cs ions reduce K efflux and influx in a concentration dependent manner; the effect is due to an interaction of Cs ions with an external site of the membrane. The observed effects on K movement cannot be explained by the accompanying changes in membrane potential, but indicate an important reduction in K conductance. In the presence of 20 mM Cs, which removes most of the inward going rectification (Isenberg, 1976), a substantial K exchange persists and is sensitive to variations in external K concentration and to Cl substitution by acetylglycinate. A reduction of external Na ions does not modify the Cs effect. The results suggest that an important K movement occurs through the outward K rectifier. In the presence of Cs the preparation loses K in exchange for Cs; this Cs influx is substantially reduced by ouabain 10(-6) M and probably occurs through the active pump mechanism.

Na + and K + uptake and exchange by the amphibian oocyte during the first meiotic division

The uptake and efflux of 22Na and 42K were studied in denuded Rana pipiens oocytes following progesterone induction of the resumption of meiotic maturation. Coincident with the breakdown of the large nucleus, or germinal vesicle, there is a virtual disappearance of K + permeability of the oocyte plasma membrane. Only about 1–2% of the total [K +] i is exchanged by completion of nuclear breakdown (8–10 hr) and accounts for the finding that there is no detectable change in total [K +] i during the first meiotic division (20–24 hr). In the case of Na +, influx, exchange, and efflux kinetics were unchanged during the first meiotic division, with 20 and 35% of the total oocyte Na + exchanging by the completion of nuclear breakdown and first meiotic division, respectively. Removal of Na + from the incubation medium produced and earlier nuclear breakdown, whereas a K-free medium delayed breakdown. There was no effect of 10 μm/ml tetrodotoxin or 10 −5 M strophanthidin on the time course of nuclear breakdown. Thus one action of progesterone appears to be a selective turning off of “K channels” in the oocyte plasma membrane. The disappearance of K selectivity of the oocyte plasma membrane coincides with plasma membrane depolarization, as well as nuclear swelling and breakdown.

Relative roles of Ca 2+, Sr 2+, Ba 2+ and Mg 2+ in controlling lens permeability characteristics

The effects of barium, strontium and magnesium upon lens permeability characteristics were studied in the presence and absence of 2 mM calcium in the bathing medium. Permeability characteristics were determined by measuring lens potential, resistance and 42K efflux rates. Barium and strontium at equimolar concentrations to calcium were able to substitute for calcium in controlling lens sodium permeability. Magnesium was ineffective in this respect. Small changes in resistance and 42K efflux rates occurred when calcium was eliminated from bathing solution containing either 2 mM barium or strontium. These changes were interpreted to be the result of an increase in lens permeability to potassium. When 2 mM strontium was added to calcium-containing solution, there was no significant change in the electrical or flux parameters of the lens. However, the addition of 2 mM barium to calcium-containing solution resulted in a 54% increase in lens resistance and a 13 mV depolarization. These observations indicated a barium-induced decrease in lens permeability to potassium, and this was confirmed by an observed decrease in 42K efflux rate constant under similar experimental conditions. The rapid time course of all the observed changes implies that they are the result of changes in the permeability characteristics of membranes lying close to the surface of the lens.