Guanine nucleotides modulate calcium currents in a marine Paramecium

Voltage-dependent calcium channels play a critical role in many cell functions and in many cell types ranging from protozoa to vertebrates. We have shown previously that guanine nucleotides modulate the calcium action potential and the duration of backward swimming in Paramecium, both indirect measurements of calcium channel function. To determine whether guanine nucleotides to indeed alter calcium currents, the inward calcium current (ICa) in Paramecium calkinsi was studied. First, the calcium current was characterized. The magnitude of ICa increased as the extracellular calcium concentration was increased from 0.5 to 50 mmol l-1, unlike the situation in freshwater species of Paramecium where the inward calcium current magnitude is maximal when extracellular calcium levels reach 1 mmol l-1. Inorganic compounds (NiCl2 at 10 mumol l-1 and CdCl2 at 1 mmol l-1) and organic compounds (naphthalene sulfonamides, W-7 and W-12-Br at 100 and 2 mumol l-1, respectively) reduced ICa. Regardless of the holding membrane potential (from -80 to -20 mV), the threshold activation for ICa was at -10 mV and the maximum value of ICa was reached at +20 mV, suggesting that there is only one type of calcium channel in P. calkinsi. Second, we injected GTP gamma S, GTP and GDP beta S into voltage-clamped cells while monitoring calcium and/or potassium currents. GTP gamma S increased the magnitude of ICa by 42 +/- 6% (mean +/- S.D., N = 5) and the effect was irreversible, GTP increased the magnitude of ICa by 37 +/- 4% (N = 4) in a reversible manner, and GDP beta S decreased ICa by 57 +/- 8% (N = 3) irreversibly. The outward potassium currents did not change when GTP gamma S was injected into the cells. These results support the hypothesis that injection of guanine nucleotides modulates the voltage-dependent calcium channel in P. calkinsi, presumably by activating G-protein-dependent processes.