Phosphodiesterase 5A inhibition decreases NHE-1 activity without altering steady state pH(i): role of phosphatases

This study aimed to identify the signaling pathway for the proposed link between phosphodiesterase-5A (PDE5A) inhibition and decreased cardiac Na(+)/H(+) exchanger (NHE-1) activity.

NHE-1 activity was assessed in rat isolated papillary muscles by the Na(+)-dependent initial pH(i) recovery from a sustained acidosis (ammonium prepulse). ERK1/2, p90RSK and NHE-1 phosphorylation state during acidosis was determined.

PDE5A inhibition (1 μmol/L sildenafil, SIL) did not modify basal pH(i) but significantly blunted pH(i) recovery after sustained acidosis. Although preventing ERK1/2- p90RSK signaling pathway (10 μmol/L U0126) mimicked SIL effect, SIL did not blunt the acidosis-mediated increase in kinases activation. SIL+U0126 did not show additive effect on NHE-1 activity. Then, we hypothesized that SIL could be activating phophasatases (PP1 and/or PP2A) to directly dephosphorylate NHE-1 despite preserved ERK1/2-p90RSK activation. Non-specific phosphatases inhibition (1 μmol/L okadaic acid) canceled SIL effect on pH(i) recovery from acidosis. Same result was observed by inhibiting PP2A either with a lower dose of okadaic acid (1 nmol/L) or, more specifically, with 100 μmol/L endothall. Consistently, NHE-1 phosphorylation at Ser703 increased after acidosis, SIL prevented this effect and PP2A inhibition (endothall) reverted SIL effect.

We suggest that PDE5A inhibitors decrease NHE-1 phosphorylation and activity through a mechanism that involves PP2A activation.