1. The concentrations of sodium, potassium, chloride and water in the cells, secretory canaliculi and the lumen of Calliphora salivary glands have been measured by microprobe analysis of frozen-hydrated sections. Free concentrations of K+ and Cl- in all three compartments were measured with ion-selective microelectrodes.

  2. Most of the quantitative data from the two techniques are in close agreement.

  3. Microprobe values for the cytoplasm are (mm kg−1 wet weight):Na 20, K 115, Cl 33 in unstimulated glands and Na 15, K 125, Cl 23 after stimulation with 10−8 M 5-hydroxytryptamine (5-HT). The water content is 85% and 77·5% respectively.

  4. The highest concentrations are found in the canaliculi. The results indicate that the fluid in the canaliculi is 60–80 m-osmol more concentrated, thus supporting the hypothesis that canaliculi are the sites of solute-water coupling by some form of local osmosis.

  5. Both the microprobe and microelectrode measurements show that 5-HT stimulation causes a rise in the level of intracellular potassium and a fall in that of chloride.

  6. On 5-HT stimulation there is a rise in the luminal concentrations of K and Cl and little change in sodium level.

  7. In K-free medium the potassium in all tissue compartments is replaced with sodium; nearly normal rates of fluid secretion are maintained.

  8. The possible routes for the entry of K, Cl and H2O into the canaliculi are discussed.

© 1978 by Company of Biologists
1978
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