Dissociation of cellular K+ accumulation from net Na+ transport by toad urinary bladder

Abstract

Summary.A number of published data suggest a variable stoichiometry between the rates of cellular potassium uptake and net sodium transport (JNa) across the urinary bladder of the toad. This problem was examined by simultaneously studying the iutracellular chemical activity of potassium (aK) with open-tip K+-selective microelectrodes and micropipets, and monitoring JNa by measuring the short-circuit current (SCC). When bathed in the short-circuited state with solutions containing an a• of 2.7 mrs, the mean + SEM values for intracellular aK were 43 _+ 0.6 m•.Ouabain, at a concentration of 10 -2 M, reduced intracellular aK by 56-67% and SCC by 96-100%. At 5 x 10 .4 M, ouabain reversibly reduced intracellular aK by 40-55%, and SCC by 63-68%; the inhibition of SCC was only partly reversible during the period of observation.Removal of external potassium reduced intracellular aK by 69-80% and SCC by 51-76%. Restoration of external potassium entirely returned intracellular aK to its control value, but only Partially reversed the inhibition of SCC during the period of study. Furthermore, recovery of aK began 19-43rain before that of SCC; recovery of aK was 90-97% complete before any increase in SCC could be measured. Although other interpretations are possible, the simplest interpretation of the data is that the processes responsible for potassium accumulation and transepithelial sodium transport are not identical. We propose the existence of a separate transfer mechanism at the basolateral cell membrane, responsible for accumulating intracellular potassium, and not directly coupled to active sodium transport.

Electrophysiologic changes associated with potassium depletion of frog skin

“…These observations complement the results of recent studies of the intracellular fluids of toad urinary bladder, using K +-selective microelectrodes (DeLong & Civan, 1978, 1979, 1980 and using electron probe X-ray microanalysis . After incubation in K +-free media for 1 89 to 2 hr, the transporting cells of toad bladder lose only some 20% of their intracellular potassium (Robinson & Macknight, 1976a;Civan et al, 1980).…”

“…Recent electronmicroprobe analysis of toad bladder is in qualitative agreement with previous estimates based on chemical analysis of mucosal cell scrapings, but indicates that 0022-2631/80/0057-0235 $01.40 9 1980 Springer-Verlag New York Inc. more sodium is actually gained by the transporting epithelial cells than had previously been estimated; the accumulation of sodium and loss of potassium are comparable in magnitude (Civan, Hall & Gupta, 1980). Thus, given the uncertainties of the experimental techniques, and the possibility that the stoichiometry of the pump might undergo modest changes even under physiologic conditions, the dissociation between the time courses of intracellular potassium activity and short-circuit current noted by DeLong and Civan (1978) might be consistent with the operation of a single Na-K rheogenic pump at the basolateral membrane.…”

“…In the classical formulation of Koefoed-Johnsen and Ussing (1958), the extrusion of sodium was considered to be stoichiometrically linked to potassium accumulation into the cell from the submucosal or inner bathing medium. Although a number of published data have supported this concept (Biber, Aceves & Mandel, 1972;Finn & Nellans, 1972;Nielsen, 1979), many other experimental results have suggested that coupling between potassium uptake and sodium extrusion is not fixed (Essig & Leaf, 1963;Curran & Cereijido, 1965;Essig, 1965;Biber et al, 1972;Candia & Zadunaisky, 1972;Giebisch, Sullivan & Whittembury, 1973;Nellans & Schultz, 1976;Robinson & Macknight, 1976 b;Kimura & Fujimoto, 1977;Kimura, Urakabe, Yuasa, Miki, Takamitsu, Orita & Abe, 1977;Valenzeno & Hoshiko, 1977;DeLong & Civan, 1978;Nagel, 1980). A particularly striking example of the apparent dissociation of cellular potassium accumulation from net sodium transport was provided by the different time courses of intracellular potassium activity and short-circuit current across toad urinary bladder following restoration of external potassium to potassium-depleted tissues (DeLong & Civan, 1978).…”

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

“…Since epithelial cells contain more sodium than sartorius muscle fibres ai would not be expected to exceed 80-90 mm. On the other hand, values of 39-43 mm for ai found previously in frog and toad bladder (Kimura & Fugimido, 1977;DeLong & Civan, 1978) would seem to err on the other side, leaving a large osmotic deficit within the cells. DeLong & Civan (1979 have since reported ai values of 80 and 70 mm from experiments with double-and single-barrelled micro-electrodes respectively.…”

“…This may be due to an ionic gradient through to tissue (Zeuthen, 1978) or to a difference in treatment of the tissue. It may also reflect the apparent compartmentalization ofintracellular potassium (DeLong & Civan, 1978) in epithelial tissues. The increase in a' found by us in the presence of amiloride although contrary to findings in toad bladder (MacKnight, Civan & Leaf, 1975) may occur in the rapidly exchanging potassium pool (MacKnight et al 1980) reported to have a lower potassium content than the slowly exchanging pool.…”