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 do indeed alter calcium currents, the inward calcium current (I_Ca) in Paramecium calkinsi was studied. First, the calcium current was characterized. The magnitude of I_Ca increased as the extracellular calcium concentration was increased from 0.5 to 50mmol l⁻¹, unlike the situation in freshwater species of Paramecium where the inward calcium current magnitude is maximal when extracellular calcium levels reach 1mmoll⁻¹. Inorganic compounds (NiCl₂ at 10 μmoll⁻¹ and CdCl₂ at 1mmol l⁻¹) and organic compounds (naphthalene sulfonamides, W-7 and W-12-Br at 100 and 2 μmol l⁻¹, respectively) reduced I_Ca. Regardless of the holding membrane potential (from −80 to −20mV), the threshold activation for I_Ca was at −10mV and the maximum value of I_Ca was reached at +20mV, suggesting that there is only one type of calcium channel in P. calkinsi. Second, we injected GTPγS, GTP and GDPβS into voltage-clamped cells while monitoring calcium and/or potassium currents. GTPγS increased the magnitude of I_Ca by 42±6% (mean ± S.D., N=5) and the effect was irreversible, GTP increased the magnitude of I_Ca by 37±4% (N=4) in a reversible manner, and GDPβS decreased I_Ca by 57±8% (N=3) irreversibly. The outward potassium currents did not change when GTPγS 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.