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Elemental distribution in Rana pipiens retinal rods: quantitative electron probe analysis.

The composition of dark‐adapted and illuminated retinal rod outer and inner segments and mitochondria was determined with electron probe X‐ray micro‐analysis of cryosections. The concentration of Ca in the outer segment was 0.4 mmol/kg dry wt. (0.1 Ca/rhodopsin) and did not measurably change upon illumination with saturating light for 5 min. The non‐mitochondrial regions of the inner segment contained the highest concentrations (up to 13 mmol/kg dry wt.) of Ca in rods; these regions probably represent the endoplasmic reticulum. The equilibrium potentials estimated from the measured elemental concentrations and the known water content of dark‐adapted outer segments were (mV): ENa = +17, EK = ‐83, ECl = ‐27. The respective values in the inner segment were: ENa = +20, EK = ‐89, ECl = ‐26. The above values were obtained in frog rods bathed in 0.18 mM‐Ca Ringer solution. In the outer segment of toad rods bathed in 1.8 mM‐Ca Ringer, ENa = +33 mV. The Mg content of the rods was high. The (computed) concentration in the dark‐adapted retinae was 11 mM in the outer segment and 24 mM in the inner segment. Illumination caused a reduction in Mg to 9 mM (outer segment) and 16 mM (inner segment). Illumination caused a highly significant reduction in Na and Cl concentrations, and an increase in K concentration in both outer and inner segments. Exposure to Na‐free (choline Ringer) solution resulted in reduction in Na to just‐detectable levels (3 +/‐ 1 mmol/kg dry wt.) in the outer segment and to 5 +/‐ 1 mM in the inner segment. This was associated with a significant loss of Cl and decrease in ECl to ‐50 mV. The low Na content of the outer segment in the Na‐depleted rods is not compatible with an extracellular concentration (105 mM) of inexchangeable Na in the intradiskal space. Mitochondrial Na and Mg paralleled the changes in the cytoplasmic concentrations: both mitochondrial Na and Mg were significantly decreased in illuminated, compared to dark‐adapted rods. There was no detectable Ca (0 +/‐ 0.2 mmol/kg dry wt.) in mitochondria of dark‐adapted rods containing high concentrations of Na; mitochondrial Ca was slightly higher (0.5 +/‐ 0.2 mmol/kg dry wt.) in the mitochondria that contained low Na following illumination.