Figure 1.

Membrane voltage drives oscillations in cytosolic free [Ca²⁺] ([Ca²⁺]_i). Concurrent records of voltage, [Ca²⁺]_i, and clamp current (B and C) from three Vicia guard cells bathed in 5 mM Ca²⁺-Mes, pH 6.1, with 10 mM KCl. Time scales, 1 min. (A) Rise in [Ca²⁺]_i and [Ca²⁺]_i oscillations (lower trace) coincident with spontaneous membrane hyperpolarizations and oscillations in free-running voltage (upper trace). Data from one cell. (B) Cyclic increases in [Ca²⁺]_i (lower trace) evoked under voltage clamp by 20-s steps from −50 to −200 mV (top trace). Clamp current (middle trace) shows inactivation of the inward-rectifying K⁺ channel current (I_K,in) with each [Ca²⁺]_i rise. Note the initial I_K,in activation during the first 1–2 s of each step, and the early inactivation of I_K,in during the later voltage steps to −200 mV before full [Ca²⁺]_i recovery. (C) [Ca²⁺]_i increase (Left, lower trace) evoked by transfer to 0.1 mM KCl in 5 mM Ca²⁺-Mes, pH 6.1, to hyperpolarize the free-running voltage (Left, upper trace). No change in [Ca²⁺]_i was observed when membrane voltage was clamped to −75 mV during exposure to 0.1 mM KCl (Right, upper and lower traces). Clamp current (Right, middle trace) was dominated by outward-rectifying K⁺ channels (not shown; see ref. 20). Open bars (above traces) indicate periods of exposure to 0.1 mM KCl.