PATCH-CLAMP STUDIES OF LIGHT-INDUCED CURRENTS ACROSS THE THYLAKOID MEMBRANE OF ISOLATED-CHLOROPLASTS

Light-induced currents through thylakoid membrane in isolated chloroplasts of Peperomia metallica were measured with suction electrodes under clamped membrane potential (MP). Comparison of the photocurrents and MP photoresponses on the same chloroplast indicated that the recording pipette had a low resistance access to a thylakoid lumen. Two components in the membrane photocurrent were distinguished. The fast inward current rose to an initial peak and declined within seconds to a steady level of about 50 pA. This component was not significantly affected by MP change in the range from -10 to +20 mV and seemed to reflect a light driven H+ influx into thylakoids. The slow component was only evident under certain conditions, e.g., in the presence of 30 nM nigericin or substrate amounts of ATP or ADP with phosphate. It was directed either inward or outward depending on the sign of a set MP. With increasing length of light pulses both outward and inward currents increased in extent and exhibited a slowing down of decay kinetics. It is concluded that slow currents, irrespective of their direction, are carried through the same pathway by the same ion species. An apparent similarity was observed between photocurrents and MP photoresponses measured under voltage clamp and current clamp conditions. A strong sensitivity of photocurrent kinetics to variations in MP suggested that the baseline dark MP is an important factor affecting the time-course of the MP formation during illumination.