Treatment of DSS rats fed a high salt diet with the exenatide analogue AC3174, captopril, or AC3174 plus captopril markedly attenuated the development of hypertension and cardiomyopathy. The overall efficacy of AC3174 was comparable to captopril. However, the effect of AC3174 was additive with captopril in normalizing LV mass. Thus, AC3174 had cardiovascular benefits beyond those achieved via ACE inhibition alone, either by enhancing captopril effectiveness, or more likely, by an independent mechanism. One possible mechanism for the additive effects of AC3174 and captopril could be increased concentrations of circulating angiotensin(1-7) . In spontaneously hypertensive rats, angiotensin(1-7) was the only component of the renin-angiotensin system that was elevated compared with normotensive rats. In addition, chronic ACE inhibition resulted in elevated angiotensin(1-7) levels in both hyper- and normotensive rats.
Overall, AC3174 monotherapy or AC3174 plus captopril was more efficacious than GLP-1. In a previously reported study, GLP-1 infusion prevented cardiac hypertrophy, reduced cardiac fibrosis, lowered MAP, and partially restored endothelial function in isolated aortic rings from high salt DSS rats . In salt-sensitive obese diabetic mice, exenatide attenuated the development of hypertension and body weight gain, and increased urinary sodium excretion . Additionally, in a rat model of metabolic syndrome, exenatide reversed corticosterone-induced elevations in BP independent from weight change . Exenatide reduced corticosterone-induced hypertension by 86%. Finally, in a pig model of acute myocardial infarction, exenatide treatment before reperfusion improved cardiac function, reduced infarct size, and decreased myocyte apoptosis within the ischemic infarct site . Taken together, these data suggest that GLP-1 receptor agonists such as exenatide and AC3174 have potential as therapeutic agents for preventing and attenuating the development of hypertension and cardiac hypertrophy.
DSS rats fed a high salt diet rapidly developed profound hypertension, LV hypertrophy, insulin resistance, and renal pathology leading to early-onset mortality from hypertension-induced stroke, with 50% mortality before week 5 and zero survival by week 7. In contrast, none of the low salt DSS rats died during the 8 week observation period. AC3174, captopril, and AC3174 plus captopril all lengthened survival among high salt DSS rats, with the combination being the most effective. Previous data have demonstrated that although acute exposure to exenatide decreases food intake in the short-term, this effect disappears after a week with chronic exenatide exposure . Thus, rats treated with the exenatide analogue AC3174 were exposed to equivalent salt content from food intake as other treatment groups. In contrast, GLP-1 did not improve survival. This resulted in fewer GLP-1 animals being available for end-of-study analyses.
DSS rats fed a high salt diet develop cardiac dysfunction leading to failure characterized by LV hypertrophy, increased LV wall stress, and LV fibrosis [13, 26, 28]. DSS rats fed a high salt diet from 7 weeks of age develop hypertension antecedent to compensatory LV hypertrophy with LV relaxation abnormalities by 13 weeks of age . By 17 weeks of age, there was further progression of LV hypertrophy combined with fibrosis and myocardial stiffening. Overt diastolic heart failure, increased LV filling pressure, and pulmonary congestion occurred by approximately 20 weeks of age; followed shortly by death [26, 28].
There is a strong correlation between insulin-resistance and hypertension in humans [38–40]. Insulin resistance has been linked to cardiac hypertrophy and fibrosis, endothelial dysfunction, and renal glomerulosclerosis. Hyperinsulinemia may contribute to the development of hypertension by promoting endothelial dysfunction and stimulating renal tubule reabsorption of sodium . Conversely, increasing insulin sensitivity is associated with increased cardioprotection, normalized endothelial function, and reduced MAP [13, 25, 40].
The DSS rat model of hypertension exhibits insulin-resistance exacerbated by salt intake and age . One mechanism by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway . Fujii et al.  reported that fatty acid oxidation and insulin-stimulated glucose uptake were impaired in high salt DSS rats with cardiac hypertrophy. Further, glucose uptake was maximally stimulated under basal conditions in the hypertrophied heart, with no residual capacity to increase glucose uptake in response to insulin administration. The new findings reported here are that treatment of high salt DSS rats with AC3174, captopril, or the combination reduced fasting insulin and insulin resistance with no effect on fasting glucose, similar to GLP-1. Chronic treatment with exenatide or GLP-1 is associated with increased insulin sensitivity in preclinical models [17, 42]. Further, these changes were at least as great as the change associated with metformin, TZDs, SFUs, or insulin therapy. Given the correlation between insulin resistance, hyperinsulinemia, and hypertension leading to more serious cardiovascular disease , these data lend further support for the potential of AC3174 therapy in this setting. After a glucose load (postprandial state), ambient glucose concentrations remained elevated for an extended period of time in high salt DSS rats and to a slightly lesser extent in GLP-1 treated high salt rats. This was in sharp contrast to all other treatment groups where more rapid normalization of glucose levels occurred. The insulin response to a glucose load was muted in high salt rats receiving vehicle or captopril. In contrast, the insulin response in high salt rats treated with AC3174 monotherapy or AC3174 plus captopril was 3-fold more vigorous within 15 minutes post-glucose load. These data point to the ability of a GLP-1 receptor agonist to independently stimulate an insulinotropic response to glucose loading in the context of advanced cardiac and renal disease.
The RAAS plays a significant role in the emergence of LV dysfunction by instigating myocardial fibrosis, volume overload, vasoconstriction, and cardiovascular tissue inflammation . After 7 weeks on a high salt diet, DSS rat kidneys are characterized by decreased function, increased proteinuria, glomerulosclerosis, increased adrenomedullin and atrial natriuretic peptide concentrations compared with salt-resistant rats . DSS rats on a high salt diet develop a renal pathology resembling that observed in patients with diabetic nephropathy and hypertensive-induced end stage renal disease [13, 30, 43]. Supporting these findings is the demonstrated ability of ACE inhibitors, ARBs, and aldosterone antagonists to delay renal disease progression in this model [28, 43]. Captopril administration to male high salt DSS rats prevented a further increase in systolic BP, but had no effect on plasma sodium concentration, plasma osmolality, or hematocrit . Long-term captopril infusion slowed the expected rise in systolic BP, increased urinary protein excretion, and slowed renal glomerular sclerosis .
Under the conditions used in the study reported here, AC3174, captopril, or the combination improved glomerular filtration rate more than GLP-1 in DSS rats fed a high salt diet. Yu et al.  reported that intravenous infusion of GLP-1 into DSS rats fed a high salt diet for 2 weeks attenuated the development of hypertension, renal proteinuria, and renal albuminuria. GLP-1 also reduced glomerulosclerosis, renal tubule necrosis, and the degree of renal interstitial fibrosis in the outer medulla. Taken together, these data suggest that GLP-1 receptor agonists can promote renal salt and water excretion by inhibiting tubular reabsorption of sodium, a mechanism contributing to the antihypertensive effects in this model.
In the kidneys of high salt DSS rats, more than 75% of the area of the glomerular capillaries can be filled with matrix material, indicating a high degree of glomerular injury . This pathology is accompanied by a marked necrosis of renal tubules and formation of protein casts in the outer medulla. In one study, GLP-1 normalized renal morphology to a degree comparable to kidneys in low salt DSS rats . As reported here, the morphology of high salt DSS rat kidneys showed a high degree of sclerosis that was improved comparably by AC3174 or captopril monotherapy. Notably, combination treatment with AC3174 and captopril further improved renal sclerosis and this effect was better than that induced by GLP-1 monotherapy. Since the anti-hypertensive effects were comparable among AC3174, captopril, and AC3174 plus captopril, the renal histopathological improvements from AC3174 plus captopril suggest an additional direct effect of AC3174 on the heart and kidney combined with the possibility of indirect effects from lowered BP.