Quantitative electron probe microanalysis of leech photoreceptors

Summary

The photoreceptive microvilli in the visual cells of the leech protrude into a large ‘intracellular vacuole’ which is but an extracellular compartment (ionic composition unknown), because it communicates with the extracellular space by narrow (≅ 20 nm) clefts (septate junctions) of unknown permeability properties. Application of Thiéry's cytochemical silver proteinate method reveals that the ‘vacuole’ contains carbohydrate-rich material. We used electron probe microanalysis of dry, ultrathin cryosections to determine quantitatively the elemental (K, Na, Cl, Mg, Ca, P, S) composition of the cytoplasm, ‘vacuole’ and extracellular space.

The composition of the ‘vacuole’ is similar to that of the extracellular space, as shown by the comparable Na/K (11 to 13) and K/Ca (1.8 to 2.2) ratios in these two compartments. There are neglible concentration gradients for Na, K and Cl between ‘vacuole’ and extracellular space. The ‘vacuole’ has a high S content and a relatively large deficit of Cl compared to [Na]+[K]+2 [Ca]. Thus the data indicate that the ‘vacuole’ is in ionic communication with the extracellular space and contains sulfonated glycoprotein(s) that can partially exclude Cl; electroneutrality is maintained in part by these organic anions. The cytoplasmic K concentration (393±30 mmol/kg dry wt) is comparable to that in other nerve cells. The cytoplasmic Cl concentration (216±14 mmol/kg dry wt) is relatively high: significantly (P<0.001) higher than the cytoplasmic Na (130±15 mmol/kg dry wt). The high cytoplasmic Cl content is in excess of that predicted by passive distribution, and suggests the operation of a Cl pump.