Haemolymph composition in Formica (Hymenoptera) and urine formation by the short isolated Malpighian tubules: Electrochemical gradients for ion transport

Na, K and Cl content of the haemolymph was determined. Na and K concentrations were significantly different in the two species studied (F. cunicularia and F. polyctena), whereas Cl concentration was not. Na was predominant and was at least five times more concentrated than K. A suitable artificial Ringer was composed. Urine formation by isolated Malpighian tubules from adult worker ants (Formica, Hymenoptera) was investigated. As Formica Malpighian tubules are rather short, a method was developed to measure fluid secretion in tubules, with a low absolute rate of urine formation. The method allowed fluid secretion to be measured with a reasonable resolution in time (every 10 min) and to use each tubule as its own control. Also, continuous perfusion of the small bathing droplet permitted an easy and fast change in composition of the bathing medium and allowed the use of coarse 3 M KCl electrode as a reference in potential measurements. The rate of secretion was primarily dependent on the concentration of K and Cl. Simultaneous presence of both ions was necessary for fluid secretion to occur. With 51 mM K and 57 mM Cl in the artificial Ringer the mean secretion rate was 230 pl/min tubule. Measurement of transepithelial potential difference and X-ray analysis of secretory droplets revealed that, as expected, the tubules primarily transported KCl and concentrated both K and Cl in the lumen. K was always transported against its gradient. However Cl transport was also active, at least when basolateral [K] was low. The K content of the secreted fluid was independent of the basolateral K and Cl and amounted to a mean value of 195 mM. The Cl concentration was 20 mM higher (215 mM). The data were compared with data on other species and a working hypothesis was proposed for future research.