Calcium Currents in Isolated Taste Receptor Cells of the Mudpuppy

Recent studies have shown that taste receptor cells are electrically excitable and possess voltage-dependent K+, Na+, and Ca'+ currents.' Although taste cells generate action potentials in response to a variety of chemical stimuli, the role of voltage-dependent conductances in taste transduction remains unclear. In this study we have characterized the voltage-dependent Ca2+ current (I,) because it represents an important link between the receptor potentials and transmitter release in taste cells. Taste cells were isolated from the surrounding lingual epithelium as previously described.' Whole-cell I, was studied by bathing cells in amphibian saline containing 10 mM tetraethylammonium (TEA) and 1 pm TTX, and by using an intracellular pipette solution containing CsCl. Current was activated by step depolarizations to 2 10 mV from a holding potential of 100 mV. I, reached a peak in 5-15 msec and slowly inactivated with a time constant of approximately 60 msec (FIG. 1A). Substitution of Ba2+ for Ca'+ caused the current to increase in magnitude and shifted the activation approximately 20 mV in the hyperpolarizing direction. Macroscopic kinetics were similar in Ca2+ and Ba2+, implying that inactivation is voltage-dependent, rather than Ca'+-dependent. The effect of brief (2-sec) depolarizing prepulses on inactivation is illustrated in FIGURE 1B. I, was approximately half-inactivated by a prepulse to -40 mV and completely inactivated by a prepulse to 0 mV. When the half-inactivated current was scaled to the same magnitude as the control current (trace 3, FIG. lB), it decayed at approximately the same rate as the control current. This observation suggests that in taste cells is mediated by a single type of Ca'+ channel. I, was reversibly blocked by 150 pM verapamil(80% block), 100 p M cadmium, and by substitution of Mg'+ for Ca'+. I, was little affected by bath addition of oconotoxin VIC ( 1 pM), 8-bromo-CAMP (1 mM) or 8-bromo-cGMP ( 1 mM). The effects of dihydropyridine drugs are illustrated in FIGURE 2. When taste cells were held at a potential that approximately half-activated L, both ( + ) PN 200110 and BAY K 8644 (10 p M ) blocked I,-. by approximately 80%. Little effect of these