ISSN:
1432-1424
Keywords:
Ca2+ channel
;
I-V relation
;
membrane excitation
;
Nitellopsis obtusa
;
tonoplast-free cell
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary The presence of a Ca2+ channel in the plasmalemma of tonoplast-freeNitellopsis obtusa cells was demonstrated and its characteristics were studied using current- and voltage-clamp techniques. A long-lasting inward membrane current (I m ), recorded using a step voltage clamp, consisted of a single component without time-dependent inactivation. Increasing either [Ca2+] o or [Cl−] o 1) enhanced the maximum amplitude of inwardI m ((I m ) p ) and 2) shifted the peak voltage ((V m ) p ) at(I m ) p to more positive values under ramp-shaped voltage clamping and 3) depolarized the peak value of action potentials. This behavior is consistent with predictions based on the Nernst equation for Ca2+ but not for Cl−. DIDS (4,4′-diisothiocyano-2,2′-disulfonic acid stilbene) did not suppress(I m ) p in tonoplast-free cells, in contrast with its effect on normal cells. La3+ and nifedipine blocked(I m ) p irreversibly. On the other hand, Ca2+ channel agonist, BAY K 8644 irreversibly enhanced(I m ) p . Both Sr2+ influx and K+ efflux increased upon excitation. The charge carried by Sr2+ influx was compensated for by K+ efflux. It is concluded that only the Ca2+ channel is activated during plasmalemma excitation in tonoplast-free cells. In terms of the magnitude of(I m ) p , Sr2+ could replace Ca2+, but Mn2+, Mg2+ and Ba2+ could not. External pH affected(I m ) p and the membrane conductance (g m ) at(I m ) p ((g m ) p ). Increasing the external ionic strength caused increases in both(I m ) p and(g m ) p , and shifted(V m ) p to positive values. At the same time, Sr2+ influx increased. Thus Ca2+ channel activation seems to be enhanced by increasing external ionic strength. The possible involvement of surface potential is discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01869308
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