Intracellular Ca²⁺ changes induced by D<inf>2</inf>O/H<inf>2</inf>O solvent isotope exchange in devacuolated internodal cells of Chara gymnophylla

Abstract

Deuterium oxide (D2O) an isotopic analogue of water is known to cause membrane excitation through activation of Ca2+ channels in giant internodal cells of the freshwater alga Chara gymnophylla. Internally perfused devacuolated and ligated internodal cells loaded with fura-2 and overloaded with Ca2+ (pathologic or hyperexcited condition) or with normal (resting) cytosolic Ca2+ concentration ([Ca2+]i) were used to study the effect of D2O/H2O solvent isotope exchange on Ca2+ homeostasis. In spite of internal perfusion, devacuolated internodal cells showed significant cytosolic Ca2+-buffering capacity that was sensitive to external calcium and sodium azide. D2O application induced a rise in [Ca2+]i preceded by an early transient decrease. With respect to the amplitude and rate of the [Ca2+]i rise, as well as to the magnitude of early transient decay, the responses of different preparations showed a bimodal distribution (L- or S-type responses). The early transient [Ca2+]i decrease was not affected by the Ca2+ channel inhibitor, nicardipine, but the cation channel blocker, La3+ abolished it. The [Ca2+]i rising phase in L-type cells was partly channel mediated (dependent on external Ca2+, La3+ and nicardipine) and could be compared to the [Ca2+]i rise induced by KCl. Applying a hyperosmotic medium mimicked the observed D2O-induced [Ca2+]i changes, however the time-course was significantly different. The fraction of the [Ca2+]i rise manifested in S- type cells as well as after Ca2+ channel block or in Ca2+-free media, revealed a possible novel component of D2O-induced excitation ascribed to residual Ca2+ stores. © 2001 Elsevier Science Ireland Ltd.