Regulation of intracellular sodium concentrations in rat diaphragm muscle

Cationic environment and secretin secretion in canine duodenal mucosa in vitro

We examined the effects of the divalent cation calcium (Ca2+) and the monovalent cations potassium (K+) and sodium (Na+) and different modalities that affect the fluxes of these cations on immunoreactive secretin (IRS) secretion from canine duodenal mucosa in vitro. In the absence of extracellular Ca2+, the basal IRS secretion was inhibited by 29%. Increased uptake of Ca2+ by passive diffusion and facilitated uptake of Ca2+ by ionophore A23187 had no effect on basal IRS secretion. However, Ca2+ channel blocking agents, LaCl3 and verapamil, inhibited basal IRS secretion by 18 and 33% respectively. High concentrations of extracellular K+ caused a dose-dependent but delayed stimulation of IRS secretion. K+-stimulated IRS secretion was not neurally mediated. Similarly, 9-aminoacridine, which blocks K+ exchange, mimicked the effects of K+. Valinomycin (10(-6) M) inhibited both K+-stimulated and 9-aminoacridine-stimulated IRS secretion. In the absence of extracellular Na+, there was a delayed inhibition of both basal and K+-stimulated IRS secretion. These results suggest that changes in cationic environment are associated with alterations in the secretion of IRS. High extracellular K+ concentration is conducive, and the absence of extracellular Na+ and Ca2+ is inhibitory to IRS secretion.

Post-tetanic hyperpolarization evoked by depolarizing pulses in crayfish stretch receptor neurones in tetrodotoxin

A post-tetanic hyperpolarization (p.t.h.) that is quantitatively identical to that evoked by a train of action potentials in stretch receptor neurones of crayfish Procambarus clarki and Pacifastacus leniculus is evoked when the normal Na+ influx is blocked with tetrodotoxin (TTX) and a train of depolarizing pulses is used to simulate a train of action potentials. The p.t.h. evoked by depolarizing pulses in the presence of TTX is attributable to an electrogenic Na-K pump, because it (a) is abolished by strophanthidin, (b) is abolished by removal of external K+, (c) depends in magnitude on internal Na+ concentration, (d) is not associated with a change in membrane conductance and (e) does not exhibit a reversal potential. When each action potential in the stimulus train is followed by a hyperpolarizing pulse, generation of the p.t.h. is prevented even though the action potentials are unchanged. The time constant for build-up of the p.t.h. is longer than the time constant of decay. Increasing the magnitude of depolarizing pulses increases the magnitude of the p.t.h. response in the presence of TTX and also increases the time constant for its build-up. The suppression of the p.t.h. occurring in low external Na+ appears to represent a response to a change in internal Na+ concentration, characterized by a time constant much longer than the decay of the p.t.h. The activity of the pump appears to be regulated by two mechanisms: a Na+-sensitive mechanism with a time constant of the order of a minute and an apparently voltage-sensitive mechanism with a time constant of about 5 s. The hypothesis is proposed that changes in the transmembrane electric field influence the enzymatic systems of the pump and disrupt the steady-state distribution of conformation states. The decay of the p.t.h. represents a relaxation back to the resting distribution.

Energy coupling to ATP synthesis by the proton-translocating ATPase

This review summarizes recent work on energy coupling to ATP synthesis by the reversible, proton-translocating ATPase to mitochondria, chloroplasts, and bacteria. In the first sections, this enzyme is distinguished from other ATP-linked ion transport systems, and progress in the biochemical analysis is discussed. There is at present a reasonably consistent idea of the overall structure of the enzyme, and one can begin to assign specific functional roles to individual subunits of the complex. The latter half of the review deals with mechanisms of energy coupling, about which there is clear divergence of opinion. An "indirect coupling" model would allow for the possibility that H+ translocation transmits energy for ATP synthesis by driving the enzyme through a sequence of conformational states, so that H+ translocated need not take part in the chemistry of ATP synthesis. By contrast, a "direct coupling" mechanism would specify that H+ translocated must participate in the chemical reaction by combining with oxygen must participate in the chemical reaction by combining with oxygen from phosphate during the synthetic step. Such discussion is preceded by an outlined of the "proton well," since this idea forms the basis of one direct coupling model. In addition, it is suggested that the idea of a proton (ion) well may be of more general significance to the analysis of ion-coupled transport, because it includes the postulate that mechanistically significant ion binding can occur within the profile of the electric field. A proton (ion) well can be derived from both kinetic and equilibrium treatments, and from mechanistic considerations in fields as distinct as biochemistry and neurophysiology. As a result, it illustrates how further advances in formulating mechanisms of energy coupling might profit by a merger of technique and perspective from areas that have as a common goal an understanding of how large proteins catalyze movements of small molecules across a membrane.

Electrophysiological effects of hypertonic sucrose solutions on canine cardiac Purkinje fibers

Microelectrode techniques were used to study the electrophysiological effects of hypertonic solutions on canine cardiac Purkinje fibers. Tyrode's solution was rendered hyperosmotic by the addition of sucrose to produce 320, 350, 400, and 500 mOsm solutions. Hyperosmotic solutions produced sucrose concentration-dependent hyperpolarization, increased the current required to effect all-or-none depolarization, slowed transmission, and prolonged action potential duration. The action potential prolongation was reversed or prevented by lidocaine (10-5 M). Treatment of potassium-depolarized Purkinje fibers with hypertonic solutions produced hyperpolarization, increased Vmax phase 0, decreased the current required to evoke all-or-none depolarization, increased action potential duration, and increased transmission speed. The potent electrophysiological actions of hypertonic sucrose solutions probably derive from their osmotic effects rather than from selective actions at specific membrane sites or hormone receptors.

The role of intracellular buffers in acid-base disturbances: mathematical modelling

Shifts of Na+, K+, Cl- and HCO3- between the cells and extracellular fluid of nephrectomized mammals in acute response to hypercapnia and to HCl or KCl infusion are simulated in steady state models involving just intracellular buffering and a simply defined interdependence of ionic gradients. Such models integrate diverse kinds of data and suggest new interpretations. In respiratory acidosis K+, HCO3- and water leave some cells and move both to where chemical buffering is least (ECF and other cells) and to cells that regulate pH particularly well by active transport. Buffering by erythrocytes is important, but the effects of distinguishing erythrocytes from other cells in a model is mainly just to emphasize Cl- movements. Effects of departures from the mammalian norm of body composition are explored.

Influence of potassium ions and osmolality on the resting membrane potential of rabbit ventricular papillary muscle with estimation of the activity and the activity coefficient of internal potassium

Resting membrane potentials of rabbit right ventricular papillary muscles were measured in modified Tyrode's solutions that were isotonic (1.0 X T), hypertonic (1.58 X T), or hypotonic (0.76 X T) at eight different concentrations of external potassium, [K]o, ranging from 0.78 to 100 mM. The amount of hyperpolarization produced by exposure to the hypertonic solutions was relatively constant with an average of 4.6 mv at all levels of [K]o except 0.78 and 1.56 mM. This potential change is much less than the 10.6 mv which would be predicted if the papillary muscles behaved as nearly perfect osmometers and the activity coefficient of intracellular potassium (K) remained constant. The amount of depolarization produced by exposure to the 0.76 X T hypotonic solutions averaged 6.8 mv at all levels of [K]o except 0.78 mM; this value is close to the predicted value of 6.9 mv. Variations in the activity and the activity coefficient of intracellular K were introduced to explain these discrepancies. We estimated that the activity of intracellular K in 1.58 X T hypertonic solution was increased 15-23% compared with that in isotonic solution. This change is much less than the 58% that would be predicted if the papillary muscle behaved as a nearly perfect osmometer. We also estimated that the activity of intracellular K in 0.76 X T hypotonic solution fell 22-26%, which is very close to the predicted value of 24%. The activity coefficient of intracellular K appeared to fall in 1.58 X T hypertonic solution to about 0.76 of its value in isotonic solution. This decline in the estimated activity coefficient of intracellular K could not be attributed to an increase in Coulombic attractive and repulsive forces as predicted by the Debye-Hückel equation for a univalent electrolyte.

The independence of electrogenic sodium transport and membrane potential in a molluscan neurone

1. The current-voltage relations of the Anisodoris giant neurone (G cell) were studied in the presence and absence of Na pump activity.2. Inhibition of the electrogenic Na pump with ouabain had no effect on either the presence at warm temperatures (10-15 degrees C), or absence at cold temperatures (0-5 degrees C), of inward-going rectification.3. Abolition of inward-going rectification in the warm, by replacement of external K with Rb, did not affect the electrogenic Na pump.4. The current generated by the electrogenic pump was essentially constant between the membrane potentials of - 30 and - 100 mV.5. The potential produced by the electrogenic pump can be predicted by a modification of the constant field equation.6. It is estimated that the energy required to extrude Na was between 3160 and 3700 cal/g-atom, and that uncoupled Na efflux during pump activity was typically between 0.2 and 4.0 p-mole/cm(2).sec.

Effects of ouabain and diphenylhydantoin on transmembrane potentials, intracellular electrolytes, and cell pH of rat muscle and liver in vivo

1. The effects of ouabain and diphenylhydantoin (DPH) to inhibit and stimulate, respectively, the Na(+)-K(+) pump were used to correlate transmembrane resting potentials (RP), ionic gradients, and cell pH (DMO method) in rat muscle and liver in vivo.2. Ouabain effects included a rise in K(+) and fall in Na(+) concentration in plasma, a rise in intracellular Na(+) and Cl(-) and a fall in K(+) concentration and pH(i) in muscle, and a rise in intracellular K(+) concentration in liver.3. Measured muscle RP was decreased from -90 to -65 mV by ouabain with the RP predictable from the Goldman equation for Na(+) and K(+) with P(Na)/P(K) = 0.01.4. Measured hepatic RP was increased from -44 to -48 mV by ouabain, whereas the Goldman equation predicts the potential should decrease. A change in permeability of some ion or activation of an electrogenic pump component is necessary to explain this result.5. DPH produced no significant effect on muscle electrolytes or RP and failed to reverse the effect of ouabain at the time measured and in the doses used.6. DPH produced a slight rise in hepatic cell K(+) and a rise from -42 to -47 mV in hepatic RP. This hyperpolarization also cannot be explained without invoking a permeability change or activation of an electrogenic pump. In all cases intracellular Cl(-) in both muscle and liver changed in the direction expected from the change in the RP. Muscle Cl(-) appears passively distributed if a constant amount of extra or bound Cl(-) is first subtracted from each group. Hepatic intracellular Cl(-) is always less than expected on the basis of passive distribution, although errors in determination do not allow elimination of the possibility that Cl(-) distribution is determined only by the RP.8. Cell pH and RP data were used to calculate H(+) gradients. DPH had no effect on cell pH and only slightly increased the H(+) gradient in liver. Ouabain produced a slight fall in muscle cell pH but reduced the H(+) gradient by half. In liver only the H(+) gradient was increased slightly. The data support the concept of a loose coupling between active H(+) and Na(+)-K(+) transport.

Effects of nephrectomy and KC1 on transmembrane potentials, intracellular electrolytes, and cell pH of rat muscle and liver in vivo

1. The ability of 24 hr nephrectomy and KCl to raise plasma K(+) concentration was used to correlate transmembrane resting potential (RP), ionic gradients, and cell pH (DMO method) in rat muscle and liver in vitro.2. Effects of 24 hr nephrectomy on electrolytes included a rise in plasma K(+) and fall in Na(+), with a rise in intracellular K(+) and fall in intracellular Na(+) in both liver and muscle. Intracellular Cl(-) concentration rose in muscle and decreased in liver.3. Measured muscle RP was decreased from -91 to -77 mV by 24 hr nephrectomy, with the RP predictable from the Goldman equation for Na(+) and K(+) with P(Na)/P(K) = 0.01 and Cl(-) behaving as if passively distributed.4. Measured hepatic RP was increased from -43 to -48 mV by 24 hr nephrectomy, with a change in ionic permeability or activation of an electrogenic pump necessary to explain the results.5. Plasma acid-base changes consisted of metabolic acidosis with partial respiratory compensation. Cell pH rose slightly in both liver and muscle; the H(+) gradient remained constant in muscle but increased slightly in liver.6. KCl was injected into intact rats while the RPs were continuously measured in muscle or liver. Muscle RP was found to decrease and hepatic RP to increase along a similar time course.7. Infusion of KCl into the portal vein led to an increase in the hepatic RP for values of hepatic venous K(+) of 15-25 mM, whereas infusion sufficient to increase the hepatic venous K(+) concentration to 30-40 mM produced no change or a slight decrease in hepatic RP.8. The rat muscle RP can be adequately described by the Goldman equation for Na(+) and K(+), whereas the hepatic RP may well have both diffusion and electrogenic components which respond differently to an increase in plasma K(+) concentration.