Dependence of intracellular effects of GTPγS and inositoltrisphosphate on cell membrane potential and on external Ca ions

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Abstract
Single cells from rat lacrimal glands were studied with the tighit-seal whole-cell recording method. Ca-dependent K and Cl currents were measured in response to dialysis with inositoltrisphosphate or GTPγS, two compounds known for elevating internal Ca2+ concentration. The activation of the Ca-dependent currents elicited by either compound was partially inhibited by sustained depolarization or by removal of external Ca2+. Conversely, hyperpolarization or removal of external Mg2+ led to augmentations of the Ca-dependent currents. These effects became apparent about 1 min after initiation of cell dialysis with inositoltrisphosphate-or GTPγS-containing solutions, and they further developped during the ensuing 10 min. Holding potential and external divalent cations did not affect the Ca-dependent currents elicited by dialysing the cells with strongly buffered solutions containing 0.5 μM free Ca2+. In ca2+-free external saline, cell currents were independent of the holding potential. It is suggested that InsP3 augments intracellular Ca2+ levels not only by releasing Ca2+ from internal stores but also by slowly increasing the Ca permeability of the plasma membrane. The results indicate that Ca2+ entry through the plasma membrane increases with Ca2+ entry through the plasma membrane increases with hyperpolarization. The similarity of the effects seen in InsP3- and GTPγS-dialysed cells gives support to the hypothesis that production of the former depends on a GTP-binding protein.