X-ray microanalysis of Rb+ entry into cricket Malpighian tubule cells via putative K+ channels

Elemental imaging by X-ray microanalysis of fully frozen-hydrated samples shows that when Malpighian tubules of the black field cricket (Teleogryllus oceanicus L.) are incubated in saline in which Rb(+) has been substituted for K(+), Rb(+) replaces much of the cellular K(+) in the main segment of control Malpighian tubules and this is prevented by incubation in saline containing Ba(2+), a non-selective K(+) channel blocker. Similarly the amount of cellular K(+) is greatly reduced when tubules incubated in normal, i.e. K(+) containing, saline are exposed to Ba(2+). By considering the amounts of cellular K and Rb remaining in the main segments of tubules incubated in either K(+) containing saline or Rb(+) containing saline after Ba(+) treatment, it is suggested that: (a) a major part (56%) of cellular K(+) enters by Ba(2+) sensitive K(+) channels and that Rb(+) can also enter by this route; (b) a smaller fraction (26%) of cellular K(+) enters by a previously proposed Na(+)/K(+)/2Cl(-) co-transporter, which can also transport Rb(+); (c) a previously proposed Na(+)/K(+)-ATPase is responsible for maintaining a K(+) pool (18%) in the tubule cells that is not exchangeable by Rb(+); and (d) entry by this Na(+)/K(+)-ATPase is not available to Rb(+).