THE COMPOSITION OF THE INTERSTITIAL FLUID IN THE RETINA OF THE HONEYBEE DRONE - IMPLICATIONS FOR THE SUPPLY OF SUBSTRATES OF ENERGY-METABOLISM FROM BLOOD TO NEURONS

Ion-selective microelectrodes were used to measure extracellular free ion concentrations in the retina of the drone honeybee, Apis mellifera male. Mean values were (in millimoles per litre): Na+, 196; K+, 10.2; Ca2+, 2.0; pH 6.9. The elemental composition of fluid that rose into a micropipette inserted in the retina was obtained by electron microprobe X-ray analysis: from the concentrations of Na and K it was estimated that this fluid was 91% interstitial fluid. Amino acids and carbohydrates were analysed by chromatography. Four amino acids had concentrations > 20 mM: proline (109 mM), glutamine (38 mM) alanine (31 mM) and beta-alanine (24 mM). These concentrations were higher than in the haemolymph. Other amino acids had concentrations of less than 3 mM. The identified carbohydrates were trehalose, glucose, pyruvate and fructose. All of these were less concentrated than in the haemolymph. These results: (ij show that the ion concentrations of previously used Ringer solutions were reasonably correct; (ii) demonstrate properties of the blood-retina barrier; (iii) suggest that the extracellular concentration of alanine is ample for it to serve as a major substrate of neuronal energy metabolism in this tissue.