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

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.