The charge-pulse relaxation spectrum of nonperfused and perfused (turgescent) cells of the giant marine alga Ventricaria ventricosa showed two main exponential decays with time constants of approximately 0.1 msec and 10 msec, respectively, when the cells were bathed in artificial sea water (pH 8). Variation of the external pH did not change the relaxation pattern (in contrast to other giant marine algae). Addition of nystatin (a membrane-impermeable and pore-forming antibiotic) to the vacuolar perfusion solution resulted in the disappearance of the slow exponential, whereas external nystatin decreased dramatically the time constant of the fast one. This indicated (by analogy to corresponding experiments with Valonia utricularis, J. Wang, I. Spieß, C. Ryser, U. Zimmermann, J. Membrane Biol. 157: 311–321, 1997) that the fast relaxation must be assigned to the RC-properties of the plasmalemma and the slow one to those of the tonoplast. Consistent with this, external variation of [K+] o or of [Cl−] o as well as external addition of K+- or Cl−-channel/carrier inhibitors (TEA, Ba2+, DIDS) affected only the fast relaxation, but not the slow one. In contrast, addition of these inhibitors to the vacuolar perfusion solution had no measurable effect on the charge-pulse relaxation spectrum. The analysis of the data in terms of the ``two membrane model'' showed that K+- and (to a smaller extent) Cl−-conducting elements dominated the plasmalemma conductance. The analysis of the charge-pulse relaxation spectra also yielded the following area-specific data for the capacitance and the conductance for the plasmalemma and tonoplast (by assuming that both membranes have a planar surface): (plasmalemma) C p = 0.82 * 10−2 F m−2, R p = 1.69 * 10−2Ω m2, G p = 5.9 * 104 mS m−2, (tonoplast) C t = 7.1 * 10−2 F m−2, R t = 14.9 * 10−2Ω m2 and G t = 0.67 * 104 mS m−2. The electrical data for the tonoplast show that (in contrast to the literature) the area-specific membrane resistance of the tonoplast of these marine giant algal cells is apparently very high as reported already for V. utricularis. The exceptionally high value of the area-specific capacitance could be explained — among other interpretations — by assuming a 9-fold enlargement of the tonoplast surface. The hypothesis of a multifolded tonoplast was supported by transmission electronmicroscopy of cells fixed under maintenance of turgor pressure and of the electrical parameters of the membranes. This finding indicates that the tonoplast of this species exhibited a sponge-like appearance. Taking this result into account, it can be easily shown that the tonoplast exhibits a high-resistance (1.1 Ω m2).
The Journal of Membrane Biology – Springer Journals
Published: Mar 15, 1999
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