Estimation of the physical state of potassium in frog bladder cells by ion exchanger microelectrode

A function that relates protein synthetic rates to potassium activity in vivo

A newly developed experimental system allows the controlled alteration of intracellular K+ activity (aK) and the measurement of amino acid incorporation rates in a single cell, the Xenopus oocyte. We found that as aK is increased by microinjecting a K+ salt, [3H]leucine incorporation (R) varies over a 100-fold range, first stimulated and then inhibited as it passes through four response regions (A-D). In region A (aK approximately 60-100 mM), R is at a nongrowth or maintenance level and is stimulated weakly by increasing aK. In region B (aK approximately 100-130 mM), R is stimulated intensely by increasing aK, roughly tripling with every 10 mM increase. In region C (aK approximately 130-160 mM), R is inhibited intensely by increasing aK. Finally, in region D (aK greater than 160 mM), R is inhibited weakly as aK increases. Collectively, the four response regions constitute the oocyte's R/aK response function. The function provides a comprehensive description of how K+ activity influences the rate of protein synthesis in an intact cell. In the subsequent discussion, we compared the oocyte response function with the K+ response determined in cell-free translational systems. While in vivo and in vitro functions are similar, differences exist that may be important in a cellular control system. We then considered the relevance of the oocyte R/aK response function to "normal" processes in the oocyte and in somatic cells, i.e., those in which aK is varied by physiological changes in the plasma membrane. We concluded that the intensely stimulatory region B is importantly involved in hormonal action and other growth-activating processes and that the entire R/aK response function may play a role in control of protein synthesis during the cell cycle.

Chloride secretion by canine tracheal epithelium: IV. Basolateral membrane K permeability parallels secretion rate

We evaluated the K conductance properties of the basolateral membranes of the surface cells of canine tracheal epithelium using microelectrode techniques. Studies were conducted under basal conditions (indomethacin, 10(-6) M, mucosal solution) and after stimulation of electrogenic Cl secretion with epinephrine (10(-6) M, serosal solution). Elevated serosal solution [K] depolarized the electrical potential differences across the apical (psi a) and basolateral (psi b) membranes in both the presence and absence of epinephrine. Serosal barium (0.5 mM) also depolarized psi a and psi b and selectively increased basolateral membrane resistance threefold. We also used K-selective microelectrodes to determine cell K activity (acK) and the driving force for K transport across the limiting membranes under basal and stimulated conditions. Stimulation of Cl secretion was not associated with significant changes in psi b or acK so that the driving force for K exit from cell to serosal solution (ca. 20 mV) was not altered. There was close agreement between the basolateral membrane electromotive force (Eb) determined from prior studies (M.J. Welsh, P.L. Smith and R.A. Frizzell, J. Membrane Biol. 71:209-218, 1983) and the chemical potential difference for K across this barrier (EbK) in the presence and absence of epinephrine. These findings support the notion that the basolateral membrane is characterized by a high conductance to K under both secreting and nonsecreting conditions and indicate that the decrease in basolateral membrane resistance that accompanies stimulation of Cl secretion results from an increase in its K conductance. This obviates changes in acK that would otherwise accompany increased Na/K pump activity and, by hyperpolarizing psi a, establishes the electrical driving force for Cl secretion across the apical membrane.

Intracellular ion activities in frog skin in relation to external sodium and effects of amiloride and/or ouabain

Intracellular activities of sodium, potassium and chloride ions, aiNa, aiK, and aiCl were measured with ion-selective single-, double- and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, psi ac and psi cs and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, delta psi ac/delta psi cs were measured as criteria of satisfactory membrane penetration by the micro-electrodes. Under control conditions, aiNa was 12.3 +/- 0.8 mM, aiK was 70.3 +/- 22 mM and aiCl was 20.3 +/- 1.6 mM with psi ac averaging -38.0 +/- 3.2 mV. When 10(-4) M-amiloride was added to the apical bathing fluid aiNa fell within 10 min to 1.18 +/- 0.1 mM and aiCl to 5.2 +/- 0.9 mM, while aiK increased to 86.2 +/- 3.8 mM as measured from the basolateral border of isolated epithelia. The sodium transport pool of the skin was measured from the fall in aiNa in the presence of amiloride and could be expressed as 33 X 10(-9) mol cm-2 of epithelium. The mean rate of fall of aiNa under these conditions corresponded to an efflux rate at the basolateral border of 30.1 X 10(-9) mol cm-2 min-1 (48 microA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. Reduction of sodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 0.1 mM caused aiNa to fall in stages to 2 mM. Because psi ac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

Microelectrode study of K+ accumulation by tight epithelia: I. Baseline values of split frog skin and toad urinary bladder

Toad bladder and split frog skin were impaled with fine-tipped single- and double-barrelled K+-selective microelectrodes. In order to circumvent membrane damage induced by impaling toad bladder, a null point method was developed, involving elevations of mucosal potassium concentration. The results suggest that intracellular potassium activity of short-circuited toad bladder is approximately 82 mM, twice as large as earlier estimates. Far more stable and rigorously defined intracellular measurements were recorded from short-circuited split frog skins. The intracellular positions of the micropipette and microelectrode tips were verified by transient hyperpolarizations of the membrane potential with mucosal amiloride or by transient depolarizations with serosal barium or strophanthidin. Simultaneous impalement of distant cells with separate micropipettes demonstrated that both the baseline membrane potentials and the responses to depolarizing agents were similar, further documenting that frog skin is a functional syncytium. Measurements with double-barrelled microelectrodes and simultaneous single-barrelled microelectrodes and reference micropipettes suggest that the intracellular potassium activity is about 104 mM, lower than previously reported. Taken together with measurements of intracellular potassium concentration, this datum suggests that potassium is uniformly distributed within the epithelial cells.

Microelectrode study of K+ accumulation by tight epithelia: II. Effect of inhibiting transepithelial Na+ transport on reaccumulation following depletion

The effects of restoring serosal potassium to potassium-depleted toad urinary bladders have been re-examined using double-barrelled microelectrodes. The data confirm the existence of a time-lag phenomenon, a dissociation between potassium reaccumulation and restoration of short-circuit current. Returning serosal potassium stimulates an increase in intracellular potassium activity 21-26 min before any increase can be detected in short-circuit current. The reaccumulation of potassium has been further studied using split frog skin, a far more suitable preparation for electrophysiologic study than toad bladder. Under baseline short-circuited conditions, potassium is accumulated against an electrochemical gradient of 22 +/- 4 mV. Reaccumulation of potassium by potassium-depleted tissues can be blocked by inhibiting the Na,K-exchange pump with high concentrations of ouabain. On the other hand, blocking apical sodium entry by the addition of 10(-4) M amiloride to the outer bathing medium does not interfere with reaccumulation of potassium. The data support the concept that the time-lag phenomenon of toad bladder reflects stimulation of potassium reaccumulation by the sodium pump in exchange for the extrusion of excess cell sodium collected during the period of potassium depletion. This reaccumulation of potassium can proceed before the entry of significant added amounts of sodium across the apical plasma membrane.

Intracellular ionic activities in frog skin

Intracellular Na+, K+, and Cl- activities (aiNa, aiK, aiCl) and transapical membrane potentials (V0) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23 degrees C in normal amphibian Ringer's solution. Under control conditions aiNa = 14 +/- 3 mM, aiK = 132 +/- 10 mM and aiCl = 18 +/- 3 mM (SD). The value of aiCl is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl- is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2--5 microM) to the apical bathing medium, aiK, aiNa, and aiCl were essentially unchanged although V0 had hyperpolarized by about 30--40 mV. During long-term exposure to amiloride aiK and aiCl did not change significantly, V0 depolarized by about 16 mV from the maximal value and aiNa decreased to 8 +/- 3 mM. Immediately after exposure to amiloride the transmembrane driving force for Na+ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in both V0 and aiNa, this driving force remained virtually constant. Since Isc during this period was close to zero, it is suggested that the observed driving force for Na+ under these condition approximates the maximal driving force generated by the Na+--K+ ATP-ase pump in the basolateral cell membrane.

Electrophysiologic changes associated with potassium depletion of frog skin

Skins from the frog Rana pipiens pipiens were studied under short-circuited conditions during the course of removing and replacing potassium in the inner bathing media in 14 experiments. The intracellular potential (Vsc), fractional resistance (FR), short-circuit current (Isc) and total tissue conductance (gr) were constantly monitored during impalements of the epithelial cells. The mean value (+/- SE) for Vsc was --79 (+/- 3) mV under baseline conditions. Removal of potassium from the inner bathing solution transiently stimulated the short-circuit current and hyperpolarized the basolateral membrane; with sufficiently long incubations, the basolateral membrane was eventually depolarized. Restoration of potassium to the inner solution within 43 min after initiating the perfusion with K+-free solution depolarized the basolateral membrane. However, restoration of potassium after at least 1 1/2 hr of incubation hyperpolarized the membrane. Ouabain consistently depolarized the basolateral membrane, even after extended periods of potassium depletion as long as 320 min. In the presence of ouabain, restoration of potassium depolarized the basolateral membrane. The data provide further evidence for the concept that the Na--K exchange pump of frog skin is rheogenic. Furthermore, the results suggest that the pump continues to be active even during prolonged periods of potassium depletion, reaccumulating potassium which has leaked out of the epithelial cells.

Intracellular potassium activity in epithelial cells of frog fundic gastric mucosa

Microelectrodes were used to measure membrane potential and intracellular potassium activity in surface epithelial cells (SEC) of frog (Rana esculenta) fundic gastric mucosa in vitro. Separate measurements were carried out by applying fine-tipped, single barrelled, KCl filled non-selective electrodes and liquid K+ -selective electrodes. Membrane potentials with respect to the mucosal and serosal surfaces, measured with non-selective electrodes, were -54.5 +/- 1.0 S.E mV (n = 59) and -73.0 +/- 1.1 S.E. mV (n = 59) respectively. The electrical potential difference referred to the mucosal surface, when measured with K+ -sensitive electrodes, was +21.2 +/- 0.8 S.E mV (n = 35), and intracellular K+ activity was 98.5 mmol/l. Assuming that intracellular and extracellular K+ activity coefficients are equal (gamma K = gamma K), the K+ concentration is 135.0 mmol/l. The K+ equilibrium potential, EK, was calculated as -90. mV i.e. more negative than both membrane potentials. This result indicates active potassium accumulation in the SEC and provides direct evidence of the presence of an active K+ pump in either both or in only one of the cell membranes.

Effect of intracellular potassium upon the electrogenic pump of frog retinal pigment epithelium

We have studied the hyperpolarizing, electrogenic pump located on the apical membrane of the retinal pigment epithelium (RPE) in an vitro preparation of bullfrog RPE-choroid. Changes in RPE [K+]i alter the current produced by this pump. Increasing [K+]o in the solution perfusing the basal membrane increases RPE [K+]i (measured with a K+-specific microelectrode), and also depolarizes the apical membrane. The depolarization is due to a decrease in electrogenic pump current flowing across the apical membrane resistance, since it is abolished when the pump is inhibited by apical ouabain, by cooling the tissue, or by 0 mM [K+]o outside the apical membrane. Removal of Cl- from the solution perfusing the basal membrane abolishes the K+-evoked apical depolarization by preventing the entry of K+ (as KCl) into the cell. We conclude that the increase in [K+]i causes the decrease in pump current. This result is consistent with the finding that [K+]i is a competitive inhibitor of the Na+ - K+ pump in red blood cells. It is possible that the light-evoked changes in [K+]o in the distal retina could alter RPE [K+]i, and thus could affect the pump from both sides of the apical membrane. Any change in pump current is likely to influence retinal function, since this pump helps to determine the composition of the photoreceptor extracellular space.