Changes in Vascular Nitric Oxide/Cyclic Guanosine Monophosphate System in Salt‐Loaded Stroke‐Prone Spontaneously Hypertensive Rats. Satomi Kagota and Masaru Kunitomo. Department of Pharmacology, School of Pharmaceutical Sciences, Mukogawa Women's University, Nishinomiya, Hyogo 663‐8179, Japan.Excessive dietary salt causes downregulation of soluble guanylate cyclase (sGC) in aortas of spontaneously hypertensive rats (SHR) followed by reduced cyclic guanosine monophosphate (cGMP) production leading to impairment of vascular relaxation in response to nitric oxide (NO) (Br J Pharmacol 2001; 134:737–744). Furthermore, dysfunction of this NO/cGMP system has been found to result from a direct effect of salt itself not from increased blood pressure due to high salt intake (J Pharmacol Exp Ther 2002; 302:344–351). Therefore, the present study was designed to determine whether salt loading alters the NO/cGMP system in aortas from stroke‐prone spontaneously hypertensive rats (SHRSP), in which salt is known to accelerate the development of hypertension.Male Wistar Kyoto rats (WKY/Izm) (n = 6) and SHRSP/Izm (n = 6) at 8 weeks of age were divided into two groups each. One group of each rat strain received normal drinking water ad libitum for 4 weeks, and the other group received 1% NaCl as drinking water. All animals were maintained on a regular diet (RC‐4, Japan SLC, Hamamatsu, Japan). At the end of the experiment, blood samples were taken from the abdominal aorta under anesthesia with pentobarbital sodium. The thoracic aortas were immediately removed and placed in Krebs‐Henseleit solution. The aortic rings were prepared and fixed vertically under a resting tension of 1.0 g in a 10‐mL organ bath filled with the solution described above. The rings were preconstricted with phenylephrine (0.1–0.3 µM), and then acetylcholine, adenosine diphosphate, sodium nitroprusside or nitroglycerin was cumulatively added to the bath medium. The isometric tension change was measured with a force–displacement transducer (Model t‐7, NEC San‐Ei, Tokyo, Japan) coupled to a dual channel chart recorder (Model 8K21, NEC San‐Ei). The protein levels of endothelial NO synthase (eNOS) and sGC in the aortas were determined by western blot analysis. The plasma NO2− plus NO3− level were determined using a commercial kit (NO2/NO3 Assay Kit‐C, Dojindo Laboratories, Kumamoto, Japan) based on the Griess reaction.In aortic rings from the salt‐loaded SHRSP, endothelium‐dependent relaxations in response to acetylcholine and adenosine diphosphate were significantly impaired as compared to those in the control SHRSP. Endothelium‐independent relaxations in response to sodium nitroprusside and nitroglycerin were also significantly impaired. However, no significant differences were found in the protein levels of eNOS or sGC between aortas from the salt‐loaded SHRSP and control groups. Plasma NO2− plus NO3− levels were significantly increased in the salt‐loaded SHRSP as compared to those in the control. In contrast, these changes by salt loading were not observed in WKY.These results indicate that in aortas of SHRSP, as found with SHR, dietary salt leads to impairment of relaxation responses to NO, which may be due to dysfunction of the NO/cGMP system in smooth muscle cells. Thus, the impairment may not be due to a decrease in sGC protein expression as observed in SHR, but to reduced activity of sGC or accelerated degradation of cGMP.