Increased Activity of cGMP-Specific Phosphodiesterase (PDE5) Contributes to Resistance to Atrial Natriuretic Peptide Natriuresis in the Pregnant Rat

Abstract

Abstract. Increased cGMP-specific phosphodiesterase (PDE5) activity in renal inner medullary collecting duct (IMCD) cells contributes to resistance to atrial natriuretic peptide (ANP) and the excessive sodium retention seen in experimental nephrotic syndrome and liver cirrhosis. Normal pregnancy is also accompanied by sodium retention and plasma volume expansion, and pregnant rats are resistant to the natriuretic action of ANP. The authors investigated a possible role of increased renal PDE5 activity in the physiologic sodium retention of normal rat pregnancy. The natriuresis and increased urinary cGMP excretion (U cGMP V) evoked by acute volume expansion (a measure of renal responsiveness to endogeneous ANP) was blunted in 16-d pregnant versus virgin rats, despite equivalent increases in circulating ANP in pregnants and virgins. The ANP-dependent cGMP accumulation in isolated IMCD cells from pregnants was blunted versus virgins and restored by the PDE5-selective antagonist DMPPO (10 Ϫ7 mol/L). PDE5 activity in vitro and PDE5 protein abundance in IMCD were greater in pregnants. Four days postpartum, volume expansion natriuresis, U cGMP V, and PDE5 protein levels in IMCD cell homogenates had returned to virgin values. These results demonstrate that normal rat pregnancy leads to in vivo and in vitro renal resistance to ANP, in association with heightened activity of the cGMP-specific PDE5 in IMCD. This may contribute to the physiologic sodium retention of normal pregnancy.Normal pregnancy is characterized by marked maternal hemodynamic changes, including a profound plasma volume expansion. In women, a maximum increment of approximately 50% occurs; in the pregnant rat, an increase of 80 to 100% is normal (1-4). This large plasma volume expansion represents an "optimal, physiologic" response; in women, failure to volume expand is associated with poor reproductive performance and is also a feature of preeclampsia (1-3). The plasma volume expansion results from a slow, cumulative net renal sodium retention, the mechanism of which is unclear because there are many conflicting signals to the kidney in pregnancy. For example, large increases occur in circulating angiotensin and aldosterone levels, ureteral pressure increases and systemic BP falls, all of which will promote net sodium retention. On the other hand, several natriuretic systems are also activated, including the large (30 to 50%) increase in GFR, high progesterone levels that exert a marked antimineralocorticoid action, increased plasma atrial natriuretic peptide (ANP) concentration, increased renal production of nitric oxide (NO), and decreased renal Na,K-ATPase abundance and activity, (3-7).In the case of ANP, there is evidence of a selective renal resistance to the natriuretic actions of ANP in normal rat pregnancy (8), which could contribute to a "permissive" renal sodium retention and plasma volume expansion. Renal resistance to ANP-mediated natriuresis coupled with cumulative sodium retention and volume expansion are also seen in several pathologic cond...

Chronic vasodilation produces plasma volume expansion and hemodilution in rats: consequences of decreased effective arterial blood volume

“…These changes closely resemble the responses in normal pregnancy. We also found that these changes are accompanied by an increase in renal medullary PDE5 mRNA and protein expression, similar to the changes in PDE5 protein abundance we previously observed in normal pregnant rats (23). Therefore, these data suggest that a primary underfill signal causes an increase in renal inner medullary PDE5, leading to loss of tubular natriuretic responses.…”

“…In IMCD cells isolated from pregnant rats, there is blunting of the cGMP response to both atrial natriuretic peptide (ANP) and a nitric oxide (NO) donor (sodium nitroprusside), and this is normalized by inhibition of PDE5 (23,29). We suggest that this increased inner medullary PDE5 inhibits all cGMP-mediated natriuretic responses, contributing to the volume expansion of normal pregnancy.…”

“…We previously reported that the activity and abundance of the cGMP-degrading enzyme phosphodiesterase-5 (PDE5) are selectively increased in the inner medullary collecting duct (IMCD) in normal pregnancy (23). In IMCD cells isolated from pregnant rats, there is blunting of the cGMP response to both atrial natriuretic peptide (ANP) and a nitric oxide (NO) donor (sodium nitroprusside), and this is normalized by inhibition of PDE5 (23,29).…”

Chronic vasodilation increases renal medullary PDE5A and α-ENaC through independent renin-angiotensin-aldosterone system pathways

“…We found no change in renal cortical NCC (an aldosterone responsive sodium transporter in the distal tubule) with NIF-induced chronic vasodilation in the present study, although Wang et al did report increased NCC using a much higher dose of NIF (37). However, in normal pregnancy, we also observed that cortical NCC was unchanged (38); thus, the chronic systemic vasodilation in virgin female rats in the present study recapitulates the changes seen in normal pregnancy (24,38). The renal sodium retention of normal pregnancy is accompanied by early and sustained activation of the RAAS (5, 17, 34), a physiological response to an "underfill" signal.…”

“…We have reported increased abundance and activity of phosphodiesterase 5A (PDE5A) in the inner medulla of the normal pregnant kidney (24). Increased PDE5A activity in the inner medulla blunts the cGMP-mediated natriuretic actions of atrial natriuretic peptide (ANP) and nitric oxide (NO) in the pregnant rat (15,24,33).…”