| Abstract: | Until recently, polarographic methods for measuring the time course of transient changes in the rate of oxygen consumption (ΔQO2) have been applied only to tissue preparations containing thousands of cells. Here, we describe ΔQO2 measurements on the lateral ocellus of the barnacle (Balanus eburneus) which contains only three photoreceptor cells. The decrement of partial pressure of oxygen (ΔPO2) elicited by an 80 ms flash of light was measured near the cells with a microelectrode and the ΔQO2 was calculated from the ΔPO2 using a model of diffusion with spherical symmetry. As shown by mathematical simulation, the exact shape of the preparation is not crucial for our measurements of the time course of the ΔQO2. For a given ΔQO2, the model describes correctly the attenuation of the ΔPO2 measured at increased distances from the preparation. To know more about the mechanisms controlling the ΔQO2, we compared it with the electrical response of the photoreceptor cells: both responses have a similar spectral dependence, but only the ΔQO2 was abolished by a 10-min exposure to 50 μM dinitrophenol or to 3 mM amytal. We conclude that the ΔQO2 reflects an increase in mitochondrial respiration and that it is initiated by the phototransformation of rhodopsin, as was already found in the honeybee drone retina (Dimitracos and Tsacopoulos, 1985; Jones and Tsacopoulos, 1987). |
