T. Schettino

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 (γ′K=γ′K′), the K+ concentration is 135.0 mmol/l. The K+ equilibrium potential,EK, was calculated as −90.0 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 external sodium on intracellular chloride activity in the surface cells of frog gastric mucosa

The intracellular chloride activity and its dependence on ionic substitutions in the bathing media was studied in individual surface cells of resting gastric mucosa using conventional and Cl− selective microelectrodes. When the tissue was perfused with control NaCl-Ringer the cell membrane p.d.s, cell-lumen (Ψcm) and cell-serosa (Ψcs) were −40.9±0.6 mV and −66.8±0.5 mV (n=175) respectively specitively and the p.d. measured by the Cl− selective microelectrodes across the serosal membrane (ΨcsCl-) averaged −32.4±0.7 mV (n=138). From these values an intracellular Cl− activity (acCl−) of 15.3 mmol/l can be estimated. The data indicate that chloride ion is distributed close to equilibrium at the luminal membrane while it is accumulated by an energy requiring step at the serosal membrane. Reduction (2 mmol/l) or absence of chloride from the luminal bath did not result in any detectable change ofacCl−; on the other hand, after removal of Cl− from the serosal bath the intracellular Cl− activity fell to 7.1 mmol/l.When the tissue was exposed to serosal Na+-free Ringer (Na+ replaced by choline or TMA), although theacCl− remained unaffected, a marked reduction of the electrochemical gradient for Cl− at the serosal membrane was observed.These data indicate that: (i) chloride is accumulated in the surface cells against its electrochemical potential difference at the serosal membrane; (ii) the luminal membrane has a negligible conductance to Cl−, while the serosal membrane represents a conductive pathway to chloride; (iii) the uphill entry of chloride at the serosal membrane seems to be, at least partially, Na+-dependent.

On the luminal membrane permeability to Cl− of necturus gastric surface cells

The surface cells (SEC) luminal membrane conductance to Cl− was tested on the resting Necturus gastric mucosa by applying conventional and Cl−-sensitive microelectrodes under luminal low-Cl− conditions. Reduction of chloride (2 mmol/l) in the luminal bath did not result in any detectable reduction of intracellular Cl−. Therefore, in analogy to previous findings on the frog SEC, also the luminal membrane of the Necturus surface cells appears to have a negligible conductance to Cl−. The data speak against a relevant role of the amphibian SEC in the non-acidic Cl− secretion.

Nonelectrolyte fluxes across gastric mucosa in relation to gastric stimulation

The effects of histamine and thiocyanate, added to the serosal bathing solution, on unidirectional fluxes of some nonelectrolytes (thiourea, methylated thiourea derivates, mannitol), and on H+, pepsinogen and mucous secretion were investigated in frog (Rana esculenta) fundic gastric mucosa. Histamine (10−4 M) increases significantly the outfluxes (serosa to mucosa fluxes) of only thiourea and its derivates (but not mannitol) and the stimulation is the greater the more lipidsoluble the nonelectrolyte is. Influxes (mucosa to serosa fluxes) of the same molecules are not affected. In parallel histamine stimulates H+-secretion but does not modify pepsinogen and mucous secretion.SCN− (10−2M) inhibits the histamine effect on thiourea outfluxes and on H+-secretion, while pepsinogen and mucous secretion are not affected.Colchicine (10−4 M) pretreatment inhibits the histamine effect on outfluxes and H+-secretion.It is concluded that: (1) histamine induces a secretion of nonelectrolytes towards the lumen; (2) such secretion is correlated with the hormone-induced secretion of HCl and fluid; (3) this process is mediated by an exocytotic mechanism.