Cold-sensitive Ca2+ influx in Paramecium.

The concentration of intracellular calcium, [Ca2+]i, in Paramecium was imaged during cold-sensitive response by monitoring fluorescence of two calcium-sensitive dyes, Fluo-3 and Fura-Red. Cooling of a deciliated Paramecium caused a transient increase in [Ca2+]i at the anterior region of the cell. Increase in [Ca2+]i was not observed at any region in Ca(2+)-free solution. Under the electrophysiological recording, a transient depolarization of the cell was observed in response to cooling. On the voltage-clamped cell, cooling induced a transient inward current under conditions where K+ currents were suppressed. These membrane depolarizations and inward currents in response to cooling were lost upon removing extracellular Ca2+. The cold-induced inward current was lost upon replacing extracellular Ca2+ with equimolar concentration of Co2+, Mg2+ or Mn2+, but it was not affected significantly by replacing with equimolar concentration of Ba2+ or Sr2+. These results indicate that Paramecium cells have Ca2+ channels that are permeable to Ca2+, Ba2+ and Sr2+ in the anterior soma membrane and the channels are opened by cooling.