demonstrate that in rats NaCl supplementation during the last week of pregnancy induces preeclampsia-like symptoms including elevation of BP, proteinuria, suppression of PRA and fetal growth retardation. Ele- vations of BP are associated with enhanced excretion of MBG, which makes MBG a potential target for therapy. Key Words: NaCl, Pregnancy induced hypertension, Na,K-ATPase in- hibitors P-319 CROSS TALK BETWEEN CENTRAL AND PERIPHERAL DIGITALIS-LIKE SODIUM PUMP LIGANDS (SPL) IN DAHL-S RATS (DS) FOLLOWING NACL LOADING Olga V Fedorova, Igor A Zhuravin, Natalia I Agalakova, Mark I Talan, Edward G Lakatta, Alexei Y Bagrov. Laboratory of Cardiovascular Science, National Institute on Aging, NIH, Baltimore, MD. In DS on a high NaCl intake, endogenous brain ouabain (OU), contrib- utes to the onset of hypertension via an angiotensin (AT) sensitive pathway. Recently, we observed, that NaCl loading of DS stimulates a bufadienolide SPL, marinobufagenin (MBG), a natriuretic and a vaso- constrictor, and that a transient OU elevation precedes a sustained in- crease in MBG (Circulation, 105: 1122-7, 2002). We hypothesized that, brain OU increases AT, which stimulates MBG. In 10 wk old DS treated with vehicle, antibodies to MBG (MBGab) or OU (OUab), or Losartan (LOS, 25 mg/kg), we studied effect of 3 hr NaCl loading (12.5 mmol/kg, IP) on SPLs, excretion of Na, blood pressure (BP), and the Na pump in proximal convoluted tubules. NaCl loading induced peak transient 3-fold increases of OU in amygdala and hippocampus (15 min; P0.01), 2-fold increases of OU in the hypothalamic supraoptic nucleus and pituitary (30 min; P0.01) followed by a transient 2-fold increase in pituitary ATII (60 min), and sustained increases in MBG excretion and BP, and natri- uretic responses, and inhibition of the renal Na pump. The MBGab blocked MBG excretion, and the BP and natriuretic responses, and restored the renal Na pump activity. The OUab blocked both OU and MBG excretion and natriuresis, and increased Na pump activity. LOS blocked MBG, while did not affect OU excretion. ATII (1 nmol/L) doubled MBG production by primary culture of adrenocortical cells from DS. Thus, in response to NaCl loading, brain OU stimulates MBG via AT 1 signalling, and MBG inhibits the renal Na pump and raises BP, i.e., behaves like a putative natriuretic hormone. The elaboration of OU in limbic structures in response to a NaCl, suggests that brain OU may be a factor linking behavioral and environmental mechanisms in the patho- genesis of NaCl sensitive hypertension. Key Words: Na/K-ATPase, Marinobufagenin, AT signalling P-320 GUANYLYL CYCLASE CROSSTALK IN SPONTANEOUSLY HYPERTENSIVE RATS Sandrine Besnard, Pavel Hamet, Johanne Tremblay. Research Center, CHUM-Hotel Dieu, Montreal, Quebec, Canada. Particulate (GC-A) and soluble (sGC) guanylyl cyclases are two impor- tant cGMP synthesizing enzymes. Both of these genes have been pro- posed as candidate genes of hypertension. They both map to the rat chromosome 2, within a blood pressure QTL identified in several rat genetic models of hypertension and defects of both sGC and GC-A activity have been found to be associated with impaired vasodilation and salt-sensitive hypertension in rodents. Here, we investigated the two guanylyl cylcase pathways in aortic smooth muscle cells (ASMC) from normotensive (BN) and hypertensive (SHR) rats. We used RIA to measure cGMP levels in cell culture medium, as a determinant of guanylyl cyclase activity. Cells were studied under basal conditions or after a 90 min stimulation with 100 nM ANP, the biological ligand of GC-A, or 100M SNP, a NO donor, ligand of sGC, in presence of 30 M L-NMMA or 10 M ODQ, a sGC inhibitor. We also per- formed semi-quantitative RT-PCR in order to assess mRNA levels of sGC. We observed a significant increase in guanylyl cyclase activity in SHR compared to BN rats for both basal and in response to SNP (basal cGMP levels, SHR: 119.6131.21 versus BN: 3.760.76, p0.005; and SHR: 5800700 versus BN: 300 50, p0.005 after SNP activation). This result was associated with a swich in the expression of sGC subunits (from alpha2 to alpha1) in ASMC from hypertensive rats, whereas beta1 subunit expression did not differ from normotensive rats. We then tested the activity of sGC in SHR rats by using inhibitors of its pathway. We did not observe any significant effect of L-NMMA on basal cGMP levels, whereas a 400% increase in cGMP levels was detected in presence of ODQ. These results were confirmed after gua- nylyl cyclase stimulation: SNP induced a significantly higher cGMP increase in presence of L-NMMA, whereas ODQ abolished its effect. Interestingly, ANP enhanced cGMP levels in presence of ODQ, not L-NMMA. Since alpha1/beta1sGC heterodimer is more effective than alpha2/ beta1, our results suggest that the change in sGC alpha1 subunit expres- sion could explain the increased sGC activity observed in ASMC of SHR rats. Furthermore, our study strongly suggests a crosstalk between sGC and GC-A, sGC seems to be involved in the negative regulation of GC-A activity. Supported by CIHR and FRSQ-INSERM. Key Words: guanylyl cyclase, hypertension P-321 MECHANISMS INVOLVED IN NOREPINEPHRINE- INDUCED HYPERTENSION IN RATS Erika I Boesen, Michelle M Kett, Warwick P Anderson. Department of Physiology, Monash University, Melbourne, Victoria, Australia. Chronic norepinephrine (NE) infusion in rats results in an increase in blood pressure, but the mechanisms responsible for this effect remain unclear. To investigate these mechanisms and determine whether angio- Non- Pregnant Pregnant Pregnant, 0.9% NaCl Pregnant 1.8% NaCl SBP (mm Hg) 128  1 105  1** 129  3† 139  3‡ Na excretion (mmol per 24 hrs) 1.5  0.1 0.64  0.12* 9.1  0.6‡ 25.6  3.0‡ MBG excretion (pmol per 24 hrs) 89  5 140  18* 197  21† 294  28‡ OLC excretion (pmol per 24 hrs) 30  5 60  12* 88  24 66  12 Plasma renin activity (Ang I pmol/ml/hr) 3.9  0.4 8.1  1.2* 5.4  1.0 4.4  0.7† Fetal weight (grams) 5.05  0.08 5.08  0.10 4.41  0.14† One-way ANOVA followed by Newman-Keuls test. * -P0.05, * * -P0.01 vs. nonpregnant rats; † -P0.05, ‡ -P0.01 vs. pregnant rats without NaCl supple- mentation. n=12–15 for each group. Baseline Vehicle NaCl MBGab NaCl OUab NaCl LOS Systolic BP (mm Hg) 125  4 160  3** 117  5‡ 125  2‡ 134  2‡ Na excretion (mmol/kg/hr) 0.5  0.4 2.8  0.3** 1.5  0.4‡ 2.0  0.2† 1.2  0.2‡ Na pump (nmol Rb/mg prot/min) 44  3 25  2** 39  3‡ 32  2† 33  1*† MBG excretion (pmol/kg/hr) 1.5  0.3 6.1  1.1* 2.0  0.4‡ 1.8  0.4‡ 2.0  0.5† OU excretion (pmol/kg/hr) 2.01  0.1 5.3  1.3* 4.3  1.0* 0.8  0.4‡ 4.1  0.9* * -P0.05, * * -P0.01 vs. baseline; † -P0.05, ‡ -P0.01 vs. vehicle; one- way ANOVA and Newman-Keuls test. n=8 –10 for each group. 152A AJH–May 2003–VOL. 16, NO. 5, PART 2 POSTERS: Experimental Hypertension