Analog circuit of theAcetabularia membrane

Abstract

The high membrane potential of Acetabularia (Em=-170mV) is due to an electrogenic pump in parallel with the passive diffusion system (Ed=-80mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive force Ep with two elements P1 and P2 in series; P2 is shunted by a large capacitance (Cp=3mF cm-2). The nonlinear current-voltage relationship of P1 (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value of Ep(-190 mV) indicates a stoichiometry of 2:1 between electrogenically transported charges and ATP. The electrical energy normally stored in Cp, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship of P2 (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 muW cm-2) amounts to some percent of the total energy turnover. The high Cl- fluxes (1 nmol cm-2 sec-1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl- pathway between the vacuole and outside is postulated to exist.

Current-voltage relationships and voltage sensitivity of the Cl− pump inHalicystis parvula

“…In both genera the measured R,, (100f~.cm 2 in Acetabularia) is too low to account for the pump-generated PD by Ohm's law, and Rm increases during metabolic inhibition. Like H. parvula, the I-V curve of Acetabularia under standard conditions exhibits a conspicuous negative-slope region which is abolished by low temperature (Gradmann & Klempke, 1974;Gradmann, 1975). Both organisms also show similar early transient currents following an abrupt depolarization (Fig.4, this paper; Gradmann & Klempke, 1974), which may be the result of hysteresis in the voltage-sensitive C1-pump.…”

“…Therefore, the C1-pumps in Halieystis and Acetabularia have similar properties and thus may have similar molecular mechanisms. Gradmann (1975) has constructed an analog circuit for the cell membrane of A.cetabularia. Based primarily on electrical measurements Gradmann concludes that the negative slope region of the I-V curve is due to a voltage-sensitive C1-pump.…”

“…In separate theoretical models Finkelstein (1964) and Rapoport (1970) proposed that an electrogenic pump may be voltage sensitive and consequently contribute to the membrane conductance. There is experimental evidence for the existence of voltagesensitive, electrogenic pumps in the marine alga, Acetabularia (Gradmann, 1975), the freshwater alga, Nitella (Spanswick, 1972), a molluscan nerve (Kostyuk, Krishtal & Pidoplichko, 1972) & Curran, 1973). In the frog skin direct flux measurements have been made to support this hypothesis, but the direction of voltage sensitivity and the anatomical complexity of the skin make the interpretation equivocal.…”

Interpretation of steady-state current-voltage curves: Consequences and implications of current subtraction in transport studies

“…This problem has been most acutely felt in studies of plant cells and microorganisms for which, unlike most animal tissues (cf. Marmor, 1971;Kostyuk et al, 1972;Mandel & Curran, 1973;Fuchs et al, 1977;Roy & Okada, 1978;Goudeau et al, 1982; see also Chapman et al, 1979), voltage spans of 200 to 300 mV are generally accessible (Gradmann, 1975;Gradmann et al, 1978;Felle, 1981aFelle, , 1982, and from which a corresponding degree of kinetic information might be gained.…”

“…The difference between the two curves is then obtained across the available voltage spectrum by subtracting the respective currents at every membrane potential. Difference I-V characteristics derived in this fashion have been used to characterize both primary electrogenic ion pumps and secondary transport of organic and inorganic solutes in the fungus Neurospora (Gradmann et al, 1978;Sanders et al, 1983;Blatt et al, 1984;, the giant algae Acetabularia (Gradmann, 1975) and Chara (Walker et al, 1979;Beilby & Walker, 1981 ;Beilby, 1984;Kishimoto et al, 1984;Takeuchi et al, 1985), and a liverwort, Riccia (Felle, 1980(Felle, , 1981aJohannes & Felle, 1985). Current subtractions of steady-state I-V curves have also been used to extract voltage-dependent features for active Na + transport in snail neurones (Kostyuk et al, 1972), a putative K + conductance in L-cells (Roy & Okada, 1978), and ionic components of transcellular pathways in frog skin (Fuchs et al, 1977;Goudeau et al, 1982).…”