Fig. 9. Anoga-DH₄₄ does not selectively stimulate Na⁺ transport after phosphodiesterase activity is inhibited by IBMX. Fluid secretion and urine Na⁺ and K⁺ concentrations were first measured under control conditions (30 min) and then in the presence of 100 µmol l^–1 IBMX (50 min). Subsequently, the tubules were challenged with 1 µmol l^–1 Anoga-DH₄₄ alone (40 min) and then in combination with 1 µmol l^–1 Anoga-DH₃₁ (40 min) in the continued presence of IBMX. Fluid secretion and ion concentrations were measured at the end of each collection period. Bars represent the means + S.E.M. for eight tubules. Fluid secretion (open bars) increases in the presence of IBMX, but the urine [Na⁺]:[K⁺] ratio (solid bars) is unchanged. Anoga-DH₄₄ promotes a further increase in secretion but has no effect on the [Na⁺]:[K⁺] ratio, whereas both parameters increase dramatically after the addition of Anoga-DH₃₁. Identical letters indicate values that do not differ significantly (P>0.05).