Fig. 4. Effects of amiloride 10^–5 mol l^–1 on fluid secretion rate, basolateral membrane potential and intracellular pH in 10^–6 mol l^–1 5-HT stimulated Malpighian tubules. (A) Mean fluid secretion rates (± s.e.m., N=7) are shown for tubules exposed to 10^–5 mol l^–1 amiloride (filled symbols) or to the vehicle alone (open symbols). The arrow indicates the addition of amiloride. (B) Representative recording of basolateral membrane potential. Downward-pointing arrow indicates impalement of the cell and upward-pointing arrow the removal of the microelectrode from the cell. Tubules were exposed to 10^–5 mol l^–1 amiloride for the period indicated by the horizontal bar. (C) pH_i was measured before (control) and 2 min after addition of amiloride (C, N=4 tubules).