Evidence That Reduced Renal Medullary NOS Activity of Dahl S Rats Enables Small Elevations of Avp to Produce Sustained Hypertension
It remains unclear why sustained elevations of plasma arginine vasopressin (AVP), a potent vasoconstrictor and fluid retaining hormone, do not generally result in hypertension. Related to this, there have been 4 general observations: 1) AVP is elevated in many forms of human and experimental hypertension, including Dahl S rats; 2) AVP can stimulate nitric oxide synthases (NOS) and nitric oxide (NO) production in the renal medulla of normal rats; 3) AVP stimulated NO production can buffer AVP induced reductions of medullary blood flow; 4) partial reduction of medullary NOS activity (via medullary L-NAME infusion) unmasks chronic hypertensive effects of small elevations of plasma AVP (Hypertension. 2000; 35:740-745). In the present study, we hypothesize that Dahl salt-sensitive rats (DS) have reduced capacity to synthesize medullary NO which sensitizes them to the hypertensive effects of small elevations of circulating AVP. DS and Brown Norway (BN) rats with implanted arterial and venous catheters were fed a 0.4% salt diet and infused continuously for 14 days with a chronic “subpressor” dose of AVP (2 ng/kg/min). Conscious mean arterial pressure (MAP) was measured 2 hours daily with rats maintained in their home cages. MAP in DS rats increased during day1 of AVP infusion from a control level of 127 ± 0.9 mmHg to an average of 147 ± 1.6 mmHg after 14 days. MAP did not return to control values even within three days following the end of AVP infusion. BN rats showed no changes of MAP during 14 days of AVP infusion (90.4 ± 0.6 mmHg and 92.3 ± 0.4 mmHg). Northern blot analysis of renal tissue from vehicle (saline) infused rats demonstrated that NOS I and NOS III mRNA expression was significantly less in DS rats in the renal outer medulla compared to BN rats. We conclude that small, normally subpressor elevations of plasma AVP can produce chronic hypertension in DS rats, a phenomenon probably related to reduced renal medullary NO synthesis.