The influence of calcium on sodium efflux in squid axons

1. Previous work has shown that the sodium efflux from the axons of Loligo forbesi increases when external sodium is replaced by lithium.

2. The increase in efflux in lithium was unaffected by ouabain but was abolished by removal of external calcium; in these respects it differed from the potassium-dependent sodium efflux which was abolished by ouabain but not reduced by removal of external calcium.

3. Strontium but not magnesium could replace calcium in activating the ouabain-insensitive sodium efflux; lanthanum had an inhibitory effect.

4. Replacing all the external NaCl by choline chloride or dextrose gave a rise in Na efflux which was abolished by ouabain but not by removal of external calcium.

5. The rise in Na efflux resulting from partial replacement of NaCl by dextrose or choline chloride consisted of two components one of which was ouabain-insensitive and calcium-dependent and the other was inhibited by ouabain but calcium-insensitive.

6. The ouabain-insensitive component of the Na efflux was activated by low concentrations of Na, Li or K but inhibited by high concentrations of Na and to a lesser extent Li. The inhibiting effect of high Na was of the kind expected if these ions displace calcium from an external site.

7. The ouabain-insensitive component of the Na efflux was abolished by cyanide, had a Q₁₀ of 2·7; and was roughly proportional to [Na]_i². It was much more variable in magnitude than the ouabain-sensitive, potassium-dependent component of the sodium efflux.

8. The calcium influx increased five to fortyfold when external NaCl was replaced by LiCl or dextrose, the increase for Li being larger than the increase for dextrose.

9. The calcium influx from Na, Li or dextrose sea water was increased three to tenfold by increasing the internal Na about fourfold.

10. The experiments provide evidence for a coupling between an inward movement of calcium and an outward movement of sodium.

     

Chemosensitivity of cardiac muscle

1. Measurements of contractions, conduction velocity and intracellular potential were made on isolated rabbit atria under four sets of conditions: high bicarbonate/high CO₂ (HH), low bicarbonate/low CO₂ (LL), high bicarbonate/low CO₂ (HL) and low bicarbonate/high CO₂ (LH). The ratio high/low was the same for the bicarbonate and CO₂ concentrations, so that HH had the same pH as LL.

2. Acid solutions caused a fall of a few mV in the resting potential, but not in the overshoot. They reduced conduction velocity and rate of rise of the action potential. They depressed contractions, but prolonged the tail of the action potential.

3. Alkaline solutions caused the converse changes, but, with the exception of the effect on the duration of the action potential, the relation with pH was markedly alinear, in that a rise in pH had much less effect than an equivalent fall.

4. Statistical tests were devised to decide whether the observed changes were associated primarily with pH, P_CO2 or bicarbonate. By far the strongest association was with external pH. Changes in P_CO2, per se, had no significant effect.

     

Calcium and sodium ions as charge carriers in the action potential of an identified snail neurone.

1. The soma of cell A in Helix aspersa produced action potentials in sodium-free or calcium-free saline, but not in saline with neither sodium nor calcium. 2. The axon had a sodium-dependent action potential. 3. Tetrodotoxin (5 x 10(-6) M) had no effect on the overshoot except at low external divalent ion concentrations. 4. The action potential in sodium-free saline was blocked by cobalt. 5. The slope of action potential overshoot against sodium concentration in the presence of 10 mM calcium was 10.5 mV/tenfold change. That of overshoot against calcium concentration in the presence of 75 mM sodium was 22 mV/tenfold change. 6. In sodium-free saline the slope of overshoot versus calcium concentration was 27 mV/tenfold change. 7. It is concluded that calcium is an important charge carrier in the action potential of cell A.     

The dual effect of calcium on the action potential of the frog's heart

1. Using ventricle strips of the frog's heart stimulated at the low rate of about 1 shock/min intracellular action potentials were recorded under conditions of varying calcium concentrations.2. Overshoots of action potentials were increased by about 18.3 mV as a result of a 10-fold increase, within the range of 0.1-5 mM, of the calcium concentration.3. A similar effect was obtained by strontium, but magnesium was ineffective.4. The increase of the overshoot by high calcium was associated with an increased rate of rise of the potential during the later part of its ascending phase. The initial fast upstroke remained unaltered.5. Another effect, a depression of the overshoot, developed during periods of repetitive stimulation, at the rate of 20/min, and this was followed by a gradual recovery during subsequent periods of rest.6. The depression of the overshoot increased with increasing calcium concentrations reaching values of over 40 mV.7. High concentrations of strontium and low concentrations of sodium also induced depression of the overshoot, but high magnesium was ineffective.8. A tentative hypothesis has been proposed attributing these two effects: (a) to an entry of calcium through the excitable membrane thus contributing to the ionic inward current, and (b) to a resulting accumulation of calcium in some cellular store.     

Metabolism and the electrical activity of anoxic ventricular muscle

1. The action potential duration of anoxic guinea-pig ventricular muscle was related to ATP generated by glycolysis. In 50 mM glucose medium the action potential duration was maintained; in 5 mM glucose medium the action potential duration shortened, the glycolytic rate declined and the ATP content was reduced.2. The action potential amplitude was related to the metabolic state of the muscle but not to the intracellular sodium concentration.3. It is suggested that changes in the action potential duration and overshoot in anoxic muscle may be due to an influence of metabolism on the slow inward current.4. Anoxic muscle incubated for 8 hr in 5 mM glucose medium had an E(m) of -77.1 mV compared to -81.1 mV in fresh muscle. The calculated E(k) of anoxic muscle was -47.4 mV.5. The resting potential of anoxic muscle was separated into two components, one dependent on potassium distribution and the other on the activity of an electrogenic sodium pump.6. The electrogenic pump component was stimulated upon raising the glucose concentration of the medium or upon raising the external potassium concentration.7. The electrogenic pump component was inhibited by ouabain or by reduction of the temperature from 35 to 8 degrees C.     

Ionic mechanism of a two-component cholinergic inhibition in Aplysia neurones

1. A two-component inhibition, consisting of a rapid and slow i.p.s.p., has been observed in the medial cells of the pleural ganglion of Aplysia. Each i.p.s.p. has been shown to be mediated by a distinct cholinergic receptor. The ionic mechanisms of the two components of the inhibitory response (whether elicited synaptically or by ACh injection) are analysed in this paper.

2. The inversion potential (typically -60 mV) of the rapid i.p.s.p. and of the rapid response to ACh injection is selectively altered by an intracellular injection of chloride or by partial substitution of the external chloride by impermeant anions. The shift caused by this last procedure is similar to that predicted for the chloride equilibrium potential (E_Cl) by the Nernst equation.

3. The slow i.p.s.p. and the slow response to ACh injection (both of which invert around -80 mV) are insensitive to changes in either internal or external chloride concentrations; on the contrary, with alterations of the concentration of potassium in the external medium, the inversion potential of the slow responses is altered in a way similar to that expected for the potassium equilibrium potential (E_K).

4. It is concluded that the rapid i.p.s.p. and the corresponding ACh potential are due to a change in chloride permeability of the post-synaptic membrane, whereas the slow responses are due to a selective change in potassium permeability.

5. Additional data suggest that the fast, `chloride' channel is impermeable to sulphate and methylsulphate, but slightly permeable to propionate and isethionate. The slow, `potassium' channel is impermeable to caesium ions, whereas its permeability to rubidium ions is half that to potassium.

6. The potassium permeability of both the non-synaptic and synaptic membrane is markedly reduced by an intracellular injection of either tetraethylammonium (TEA) or caesium. These ions not only block the cholinergic potassium currents (whether inward or outward) but likewise block the potassium currents activated in the same cells by an iontophoretic injection of dopamine.

7. The potassium dependent synaptic potentials are also selectively affected by manipulations known to block the electrogenic sodium pump. In the presence of ouabain or in sea water in which sodium has been replaced by lithium, there is an apparent reduction of these potentials which was shown to be simply a reflexion of the movement of E_K towards a less polarized level. This shift in inversion potential was not seen for the potassium dependent response to ACh iontophoretic injection. These results are interpreted in terms of accumulation of potassium ions assumed to occur in the extracellular spaces of the neuropile, but not in the thoroughly dissected somatic region.

8. Cooling was shown to eliminate, selectively, the synaptic and ACh potential changes caused by an increase in potassium permeability.

     

The effect of sodium and calcium on the action potential of the smooth muscle of the guinea-pig taenia coli

1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea-pig taenia coli.2. Replacement of external NaCl with sucrose (leaving 10 mM-Na in the buffer) caused hyperpolarization and stopped spontaneous activity within 10 min. Spikes could, however, be evoked for 2-3 hr. The amplitude, the overshoot and rate of rise of the spike were increased.3. In 10 mM-[Na](o) the intracellular Na concentration was reduced from 35 to 24 mM, shifting the Na-equilibrium potential from +34 to -22 mV.4. Excess Ca (12.5 mM) caused hyperpolarization and increased membrane conductance. The amplitude and the rate of rise of the spike were increased, the threshold was raised and the latency of the spike evoked by threshold stimulation became shorter.5. The effect of reducing the external Ca concentration depended on the Na concentration present, being greater with higher external [Na](o). When the membrane was depolarized and spikes deteriorated in low Ca (0.2-0.5 mM) reduction of Na to 10 mM caused repolarization and recovery of the action potential.6. Mn (0.5-1.0 mM) blocked spontaneous spike discharge after 20 min. Higher concentrations (more than 2.0 mM) were required to block the evoked action potential.7. The results indicate that the smooth muscle spike in taenia is due to Ca-entry and that Na influences spike activity indirectly by competing with Ca in controlling the membrane potential.     

Action Potential Generation in Denervated Rat Skeletal Muscle

Redfern , P. and S. Thesleff , Action potential generation in denervated rat skeletal muscle. I. Quantitative aspects. Acta physiol. scand. 1971. 81. 557–564. Action potential generation was studied at various periods up to one week after denervation in individual muscle fibres of the extensor digitorum longus muscles of the adult rat. To allow a comparison of action potential generation at various stages of denervation, it was necessary to establish adequate conditions for spike generation. It was found that when fibres were locally polarized to a level of -90 to -100 mV, and the external calcium concentrations was increased to 4 mM, the peak rate of rise and the overshoot of the action potential were maximal. Between 30 and 40 hrs following section of the motor nerve, the mean maximal rate of rise of action potentials, recorded under the aforementioned conditions, was reduced by about one third, and remained at about this reduced level during the subsequent days. Two days after denervation the resting membrane potential was reduced from a mean of 82 mV in innervated muscle to a mean of 68 mV, and remained at about this level for the remaining 5 days studied. The electrical time constant and the input resistance of the muscle fibres gradually increased during the 7 days following denervation, the time constant by about 70 % and the input resistance by about 50 %. With anodal polarization in denervated muscle no significant correlation was found between the resting membrane potential and the maximal peak rate of rise of the spike. It was concluded that denervation produces a genuine reduction in the rate of rise of the action potential in muscle fibre.     

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.     

Fluid absorption with and without sodium in isolated perfused snake proximal tubules

Net fluid absorption (JV) was studied in isolated, perfused snake (Thamnophis spp.) proximal renal tubules. With standard (150 mmol/liter Na+) bicarbonate-buffered Ringer in perfusate and standard Ringer plus dextran (4 g/100 ml) in bath, JV was about 0.9 nl min-1 mm-1. Removing dextran from bath reduced JV by about 20 percent. When sodium in perfusate was replaced with choline JV approached zero. However, when sodium in bath as well as perfusate was replaced with choline, JV returned to control level. Results were the same when sodium was replaced with tetramethyl-ammonium, sodium chloride was replaced with sucrose or lactose, or chloride was replaced with methyl sulfate. In contrast, replacing sodium in perfusate or in both perfusate and bath with lithium did not reduce JV. Fluid absorption was always isosmotic. Replacing bicarbonate with phosphate or Tris in sodium-containing media had no effect on JV, but the presence of buffer in sodium-free or low-chloride media may have been important for JV. Reducing temperature 10 degrees C reduced JV by about 35 percent with either sodium chloride or sucrose in both perfusate and bath. The results indicate that isosmotic fluid absorption can occur when lithium is substituted for sodium or when some other substitution is made for sodium, chloride, or both in perfusate and bath simultaneously.     

Post-stimulus hyperpolarization and slow potassium conductance increase in Aplysia giant neurone

1. Intracellular records from Aplysia giant (R2) cell somata showed long lasting 4-10 mV hyperpolarizations after passage of outward current through a second intracellular electrode.

2. An increase in membrane slope conductance occurred simultaneously with the post-stimulus hyperpolarization (PSH).

3. Both the PSH and conductance-increase varied strongly with stimulus amplitude and duration.

4. Both the PSH and the conductance increase occurred in Ca-free medium containing tetrodotoxin, when action-potential production was completely blocked.

5. The PSH persisted in the presence of ouabain or DNP, with cooling, with removal of external K⁺, and in media where all the Na⁺ was replaced with Li⁺, suggesting that it was not due to the activity of an electrogenic pump.

6. A reversal potential for the PSH was demonstrated by application of maintained inward current following the end of an outward-directed stimulus.

7. The PSH reversal potential varied with [K]_o, but not with [Cl]_o or [Na]_o, suggesting that the PSH was mainly due to an increase in K conductance.

8. The PSH and the conductance increase were reduced strongly when all the Na⁺ was replaced with Tris, and only slightly when Na⁺ was replaced with sucrose.

     

Effects of anions and cations on the resting membrane potential of internally perfused barnacle muscle fibres

1. Single barnacle muscle fibres from Megabalanus psittacus (Darwin) were internally perfused with a number of K salt solutions (200 mM) which were made isotonic to the barnacle saline with sucrose.2. 200 mM-K acetate solution, in general, was found to be more effective than other solutions of K salts in generating and maintaining stable resting membrane potential of -56.0 +/- 0.7 mV (all potentials are referred to the external solutions as ground). The various K salts, on the basis of the magnitude of the resting potential they generated in the muscle fibres, followed the sequence, acetate > isethionate > aspartate > glutamate > fluoride > monohydrogen phosphate > succinate > citrate > sulphate > oxalate > iodobenzoate > ferrocyanide > chlorate > nitrate > chloride > thiocyanate > iodide > bromide > cyanide.3. The resting potential in muscle fibres perfused with solutions of acetate, aspartate and glutamate increased linearly with the logarithm of the K concentration (slope = 30.4 mV for K acetate and 27.4 for K aspartate and glutamate) when the ionic strength of the solutions was progressively increased from 50 to 650 mM. On the other hand, similar increase of ionic strength beyond 200 mM of solutions of K isethionate, fluoride, monohydrogen phosphate, succinate and citrate depolarized the muscle fibres.4. Perfusion of acetate solutions of other alkali metal ions gave low values for the resting potential and followed the sequence K > Na > Rb > Li > Cs. Also NH(4) and Tris ions gave low values for the resting potential which underwent oscillations associated with the twitching of the fibre and occasionally became positive in value (action potential).5. Addition of tetraethyl ammonium chloride (TEA-Cl), 20-100 mM, to K acetate solutions (200 mM) depolarized the fibre membrane and the consequent reduction of resting potential varied linearly with the logarithm of TEA concentration.6. Replacement of chloride ion by acetate or isethionate in the external solution did not change significantly the resting potential although the values were consistently lower by about 2 mV.7. Complete elimination of K in the external solution and reduction of its ionic strength using sucrose depolarized the muscle fibres by about 27 mV when Na was changed from 475 to 1 mM. Under these conditions, external solutions completely in acetate form gave resting potentials which were more positive than those observed in completely chloride solutions by 6-8 mV.8. Replacement of Na by Li, Tris, choline, tetramethyl or tetraethyl ammonium ion in the external solution made the values of the resting potential more positive (depolarization). Similarly increasing the concentration of K (or Cs or Rb in place of K) by correspondingly decreasing the concentration of Na in the outside solution depolarized the fibres and the resting potential became zero at a concentration of 280 mM (or 308 or 1500 mM for Rb or Cs, respectively) on extrapolation.     

Extracellular cations and the movement of choline across the erythrocyte membrane

1. The ability of human erythrocytes to accumulate choline is abolished when external Na is replaced by Cs, Rb, K or Li but is increased when the external cation is Mg or Ca.2. The unidirectional influx of choline is reduced when external Na is replaced by other monovalent cations but is not changed when Na is replaced by Mg or Ca.3. The unidirectional efflux of choline into a choline-free medium is increased when external Na is replaced by other monovalent cations and markedly reduced when Na is replaced by Mg or Ca. When the external medium contains 1 mM choline, changing the external cation has virtually no effect on the rate of choline efflux.4. When the extracellular concentrations of K and Na are similar to that found in the intracellular water, choline appears to become passively distributed across the cell membrane; when the extracellular K is then replaced by Cs a net efflux of choline against a concentration gradient results.5. It is concluded that the choline carrier may be described as a cation carrier with a high affinity for choline and affinities for Cs > Rb > K > Li > Na and that these monovalent cations can cross the membrane on the choline transport system.     

The effect of sodium ions on the light-induced86Rb release from the isolated crayfish retina

The effect of low external Na⁺ concentrations on the light-induced K⁺ release from crayfish photoreceptor cells was tested by labelling intracellular K⁺ with the isotope⁸⁶Rb. The amount of isotope released per light stimulus is roughly proportional to the external Na⁺ concentration if the osmolarity is kept constant by replacing Na⁺ with Tris, choline or sucrose. When sucrose is used to replace the depleted Na⁺ the light-induced K⁺ release is a linear function of the external Na⁺ concentration and is reduced by approx. 95% at an external Na⁺ concentration of 5 mmol/l. For choline and Tris substitutions the relationships are less clear but at Na⁺ concentrations 56 mmol/l it seems that in comparison with sucrose the light-induced K⁺ release is smaller in a Tris solution and larger in a choline solution. It is suggested that the light-induced K⁺ release is due mainly to an activation of voltage sensitive K⁺ channels.     

Manganese-dependent propagated action potentials and their depression by electrical stimulation in guinea-pig myocardium perfused by sodium-free media.

1. Propagated action potentials were recorded in right ventricular papillary muscles from guinea-pig heart while exposed to Na-free, Ca-free and Mg-free solutions containing Mn. 2. When Na was totally replaced by 95 mM-Mn the overshoot was about 45 mV while the resting potential was about -90mV. 3. The overshoot of action potentials was increased by about 20-30 mV per tenfold increase of Mn concentration over the range of 2-50 mM. 4. Similar increases of overshoots with increasing of Mn concentration also occurred in the presence of 0-6 mM-Ca. Increasing of Ca from 5 to 20 mM had little influence on the overshoot but shortened the duration of the Mn-dependent action potential in the presence of 5 mM-MN. 5 Mn-dependent action potentials were not depressed by 3 X 10(5) M tetrodotoxin but by La. 6. These results suggest that Mn passes through the slow inward current channel to generate the action potential seen under the Na-free condition. 7. The overshoot and duration of the Mn-dependent action potential decreased with stimulation. At stimulus frequencies (Hz) of 0-5, 0-2, 0-1, 0-017 and 0-0033 the overshoot of action potential in 5 mM-Mn Tyrode decreased by 0-5-1 mV per an action potential. This depression of the action potential is explained by assuming intracellular accumulation of Mn.     

Die Wirkung verschiedener Anionen auf den isolierten Ranvierschen Schnürring

Summary 1. The experiments were carried out on single circulated Ranvier nodes of Xenopus laevis and were concerned with the effect of replacing the NaCl in the Ringer's solution by NaNO₃, NaSCN, sodium acetylglycinate, sodium pyroglutamate or Na₂SO₄.2. Replacement of NaCl with NaNO₃ etc. produced a slight hyperpolarization of 2–4 mV. NaNO₃ and NaSCN decreased the resting membrane resistance by 15–25%.3. The height of the action potential and the maximum rate of rise were reduced by 10–50%. The reduction was most pronounced with NaSCN. The effect on the falling phase was less prominent; it consisted mainly in a reduction of the maximum rate of repolarization. The threshold potential decreased by 2–4 mV.4. Replacement of NaCl with NaNO₃ etc. shifted the curve relating action potential amplitude or maximum rate of rise to membrane potential 5–15 mV along the voltage axis in a direction such that the fibre was more inactivated at resting membrane potential. Anodal polarization hence restored the spike amplitude and the maximum rate of rise to its normal values.5. The depressing effect of NaNO₃ etc. on spike amplitude and maximum rate of rise was reduced to about half its normal size by a fourfold increase of [Ca]₀.6. The experimental findings could be explained by assuming adsorption of external anions at the outer edge of the excitable membrane.

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Mit 5 Textabbildungen

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Die Untersuchungen wurden mit Unterstützung der Deutschen Forschungs-gemeinschaft ausgeführt.     

Intracellular ionic activities in the EDL muscle of the mouse

The intracellular ionic concentrations of sodium, potassium and chloride in the mouse EDL muscle were measured by chemical analysis using inulin as the extracellular marker. Cellular concentrations of 157±8, 38±3, 44±5 mmol kg^–1 intracellular water were estimated for potassium, sodium and chloride respectively.The resting membrane potential was measured by a conventional microelectrode filled with 3 mol KCl and found to be –76±0.5 mV.Ion-selective microelectrodes were used to measure the intracellular ionic activities of potassium, sodium and chloride. The activities measured were 117±5, 16±2, 5±0.1 mmol l^–1 for potassium, sodium and chloride respectively.Apparent activity coefficients for the intracellular ions were calculated. The observed discrepancies between the extracellular activity coefficient and the calculated apparent intracellular activity coefficients for sodium and chloride might be explained in terms of the binding to cellular macromolecules and/or the compartmentalisation of these ions. Potassium appears uniformly distributed throughout the cellular water.Intracellular chloride activity was similar to that predicted by the Donnan distribution and it is concluded, therefore, that chloride is distributed at electrochemical equilibrium.     

Afterdepolarization mechanism in the in vitro, cesium-loaded, sympathetic preganglionic neuron of the cat

Intracellular recordings were performed in Cs-loaded sympathetic preganglionic neurons (SPNs) of the intermediolateral nucleus, identified by antidromic stimulation, in the slice of the T2 or T3 segment of the cat spinal cord. Loading the neurons with Cs resulted in broadening of the action potential, depression of the fast component of the afterhyperpolarization (AHP), and appearance of an afterdepolarization (ADP). A typical ADP in a Cs-loaded neuron had time to peak of 45-110 ms, half-decay time of 70-250 ms, and amplitude of 2-10 mV at membrane potentials between -60 and -70 mV and at a Ca and K concentration of 2.5 and 3.6 mM, respectively, in the superfusion medium. The ADP was associated with a decrease in neuron input resistance and increased in magnitude with hyperpolarization of the cell membrane. The relation between peak ADP amplitude and membrane potential was linear within the range of membrane potentials from -60 to -100 mV. The ADP was reversibly suppressed by the Ca-channel blocker cobalt (2 mM) or by low Ca Krebs solution (0.25 mM). Superfusion with BaCl2 (1.0 mM) or tetraethylammonium (TEA) (10-20 mM) caused an increase in amplitude of the ADP and an increase in action potential duration. Hyperpolarizing pulses, delivered during the course of the spike shoulder, resulted in a decrease of spike duration and ADP amplitude. The ADP was not affected by tetrodotoxin, at a dose blocking the Na-spike, and was enhanced, in association with an increase in action potential duration, when NaCl in the Krebs solution was replaced with choline chloride. Increasing intracellular Cl concentration or decreasing extracellular Cl concentration had no effect on the ADP. Changes in external K concentration from 3.6 to 10 or 0.36 mM increased and decreased, respectively, the amplitude of the ADP. In the absence of Cs, and ADP, with similar time course to that recorded in Cs-loaded SPNs, was recorded when CaCl2 was replaced by BaCl or NaCl was replaced by TEAC1. It is concluded that the SPN afterpotential includes a Ca-dependent inward current, in addition to the already described fast and slow outward K currents of the AHP.     

Effects of pindolol,sotalol and the optical isomers of propranolol on muscle action potentials and depolarization-secretion coupling in the rat

Muscle action potentials and miniature end-plate potential frequency were studied in different concentrations of pindolol, d-and l-propranolol and sotalol using intracellular microelectrode recording from rat phrenic nerve-diaphragm preparations, d-and l-propranolol at concentrations of 10 to 40 mg/1 and pindolol at 20 to 100 mg/1 slowed down the rate of rise, prolonged rise and half-decay times and reduced the overshoot of the muscle action potentials. Sotalol had similar effects but only at higher concentrations (100 to 200 mg/1). The drugs had no effect on the increase in the miniature end-plate potential frequency obtained with depolarization by increased extracellular potassium concentration.     

Water uptake by Bufo melanostictus, as affected by osmotic gradients, vasopressin and temperature

1. The rate of water uptake across the skin was studied in the live toad, Bufo melanostictus. When toads were kept in distilled water at 29° C the uptake of water amounted to 16·9 ± 1·3 μl./cm²/hr; when bathed in sucrose or urea solutions, the water uptake diminished with increasing osmotic pressure. There was no water uptake observed when toads were kept in 200 m-osmolar sucrose or urea.

2. Intramuscular injections of vasopressin increased the rate of water uptake from distilled water. There was a good relation between doses and responses over various time intervals. A dose of 4 m-u. vasopressin/g body wt. doubled the rate of water uptake over a period of 1 hr. The same dose of vasopressin doubled the rate of water uptake when the toads were kept in solutions of sucrose or urea of different osmolarity.

3. The rate of water uptake when the toads were bathed in sodium chloride solutions was consistently 8 μl./cm²/hr greater than when bathed in sucrose or urea solutions of equal osmolarity. There was no water uptake when the sodium chloride solution was 285 m-osmolar.

4. Vasopressin (4 m-u./g) injected intramuscularly doubled the rate of water uptake from sodium chloride solutions of different osmolarity.

5. With solutions of potassium chloride, sodium nitrate, and potassium nitrate, in concentrations up to 150 m-osmoles/l., the rate of water uptake was found to be the same as with solutions of sodium chloride of the same osmolarity. Similarly, it was doubled by injection of vasopressin (4m-u./g).

6. The effect of temperature on the rate of water uptake before and after injection of vasopressin was investigated in toads kept in distilled water, sucrose, or sodium chloride solutions. For temperatures between 20 and 37° C, vasopressin (4 m-u./g) reduced the activation energy involved in the process of water uptake by 4000 cal.

7. The results agree with the view that water uptake follows a diffusion process which is facilitated by vasopressin, possibly as a result of increasing the size or number of available pores.