Printed in Great Britain COMPETITION BETWEEN POTASSIUM AND RUBIDIUM IONS FOR PENETRATION OF THE MIDGUT OF HYALOPHORA CECROPIA LARVAE

The midgut of the American silkworm transports potassium actively from blood-side to lumen (Harvey and Nedergaard 1964). The use of radiopotassium to study this process is limited by the short half-life of "K (12-4 h). MRb which has a half-life of 19 days has, therefore, been used in place of potassium on the assumption that Rb and K can substitute for each other. This assumption was tested by Harvey, Haskell & Nedergaard (1968) who showed that the short-circuit current does not change significantly when all K in the bathing solution is substituted by Rb. This experiment does not, in fact, prove that the two ions compete equally. However, Nedergaard and Harvey (1968) measured the net flux of Rb and K by means of absorption spectro-photometry and found that with a K/Rb concentration ratio of I-I the mean K/Rb flux ratio was 1-4 (5 determinations). The difference was disregarded and the con-clusion made 'the midgut is unable to distinguish between potassium and rubidium'. It is obviously an advantage to use Rb-86 when working with the midgut, so we have retested the competition between K and Rb fluxes. A 100 % agreement between the fluxes could not be found, the values varied and on the average a 50—60 % Rb flu