Two Voltage-Gated, Calcium Release Channels Coreside in the Vacuolar Membrane of Broad Bean Guard Cells

Voltage-gated, Ca2+ release channels have been characterized at the vacuolar membrane of broad bean guard cells using patch clamps of excised, inside-out membrane patches. The most prevalent Ca2+ release channel had a conductance of 27 pS over voltages negative of the reversal potential (Erev) (cytosol referenced to vacuole), with 5,10, or 20 mM Ca2+ as the charge carrier on the vacuolar side and 50 mM K+ on the cytosolic side. The single-channel current saturated at ~2.6 pA. The relative permeability of the channel was in the range of a Pca2+:Pk+ ratio of 6:1. Divalent cations could act as charge carriers on the vacuolar side with a conductance series of Ba2+ > Mg2+ > Sr2+ > Ca2+ and a selectivity sequence of Ca2+ [approximately equals to] Ba2+ [approximately equals to] Sr2+ > Mg2+. The channel was gated open by cytosol-negative (physiological) transmembrane voltages, increases in vacuolar Ca2+ concentration, and increases in the vacuolar pH. The channel was potently inhibited by the Ca2+ channel blockers Gd3+ (half-maximal inhibition at 10.3 [mu]M) and nifedipine (half-maximal inhibition at 77 [mu]M). The stilbene derivative 4,4[prime]-diisothiocyano-2,2[prime]-stilbene disulfonate was also inhibitory (half-maximal inhibition for a 4-min incubation period at 6.3[mu]M). The 27-pS channel coresides in individual guard cell vacuoles with a less frequently observed 14-pS Ca2+ release channel that had similar, although not identical, voltage dependence and gating characteristics and a lower selectivity for Ca2+ over K+. The requirement for two channels with a similar function at the vacuolar membrane of guard cells is discussed.