Electron microprobe and ion-selective microelectrode studies of fluid secretion in the salivary glands of Calliphora.

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 (raM kg" wet weight): Na 20, K 115, Cl 33 in unstimulated glands and Na 15, K 125, Cl 23 after stimulation with io" 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 H4O into the canaliculi are discussed.