Previously, we had reported that ground seeds from dietary hemp (Cannabis sativa L.) more effectively attenuate metabolic disorders compared with the oil fraction from hemp seeds [9]. Now, we have further investigated the influence of dietary supplementation with hemp seeds (12% of diet) vs. corresponding concentration of hemp seed oil (4% of diet) on vascular dysfunction, blood pressure, and heart rate, the blood plasma lipid profile, oral glucose tolerance, antioxidant capacity, and renal functioning in obese Zucker rats, a model of obesity.

As was stated before, experimental supplementation neither modified the body weight gain nor the food intake of supplemented obese Zucker rats [9].

The influence of the oil from hemp seeds on lipid metabolism is well documented, contrary to the effect of the ground seeds, which is poorly studied as of yet. Therefore, we have undertaken further research. Both ground seeds and the oil were able to affect the lipid metabolism (decrease in the plasma HDL cholesterol), although the effectiveness of the seeds was much more indicated (decrease in the plasma total cholesterol). The plasma triglycerides concentration was not significantly decreased by the seeds, but it was by the oil, which is in accordance with our previous results [9]. We have further calculated a nontraditional lipid profile, which is an even better marker of atherogenicity [14,16,18]. Surprisingly, the effect of hemp oil was much more pronounced compared to the seeds, as indicated by a decrease in the AIP: log₁₀$(\frac{\mathrm{TG}}{\mathrm{HDL}})$, the cholesterol ratio: $\frac{\mathrm{TC}}{\mathrm{HDL}}$, VLDL, and by an increase in $\frac{\mathrm{nonHDL}}{\mathrm{HDL}}$ and $\frac{\mathrm{LDL}}{\mathrm{HDL}}$. Neither hemp seeds nor hemp oil improved the impaired glucose tolerance that was induced in the obese group of rats, yet these favorable effects of hemp seeds and oil were not as defined on blood glucose as those on the lipids.

Hemp seeds are a good source of fatty acids (33.2%), see Table 1. In our study, the fatty acid profile of the seeds and the oil from hemp was found to be similar. The main PUFAs determined were linoleic acid, α-linolenic acid, and γ-linolenic acid, with an abundance of ∼52%, 18%, and 4%. Moreover, hemp seeds are also a good source of protein (26.3%) and dietary fiber (27.5%), which may explain the observed differences in the blood plasma lipids. Highly digestible proteins and dietary fiber can trigger a rise in protein synthesis of smooth muscles [19] and increase the gut microbial glycolytic activity: β-glucosidase as well as α- and β-galactosidase [9].

In addition, this study is the first to describe the reactivity of isolated thoracic arteries in hemp-supplemented obese Zucker rats. This specific rat model is characterized by a number of metabolic disorders, including vascular dysfunction and increased oxidative stress. Supplementation with hemp seeds and seed oil beneficially potentiated (already decreased, Figure 8B) vasoconstriction in response to noradrenaline. However, this neither changed the membrane depolarization induced by high KCl nor the response to the thromboxane-A₂ analog, U-46619. It is worth mentioning that metabolic dysfunction observed in obese Zucker rats decreased depolarization and enhanced U-46619-induced contraction, as was presented in Figure 8A,C. Moreover, supplementation with hemp did not decrease the thromboxane-A₂ level in blood vessels under basal and acetylcholine-stimulated conditions in obese Zucker rats.

Next, we studied impaired vascular relaxation observed in obese Zucker rats. The attenuated relaxant response to acetylcholine (Figure 9A) was improved by the seeds but not by the oil. Surprisingly, the vasodilator response to exogenous nitric oxide (which is attenuated in obese Zucker rats (Figure 9B) was not modified with dietary hemp. This indicates that the sensitivity of the smooth muscles of rat thoracic aorta to nitric oxide was not modified during supplementation and that it was the endothelial functioning that was improved by the seeds but not by the oil. In rat thoracic arteries, K_ATP and BK_Ca channels are also engaged in vascular tone regulation to compensate for the attenuated vascular relaxation [2]. The relaxant response to the K_ATP channel opener was down-regulated in obese rats, which points to a decreased sensitivity (Figure 9C). Experimental supplementation with hemp improved the impaired functioning of these channels and shifted that response to the left. However, the sensitivity was not fully restored to the level observed in the lean controls. Next, we examined the impaired vasodilator response with a BK_Ca channel opener (Figure 9D). In our study, supplementation with hemp increased both the sensitivity and the maximal response, which was more pronounced in rats fed with seeds (increased sensitivity). Our results point to an improvement in the functioning of K_ATP and BK_Ca channels in response to the dietary hemp, with a more beneficial effect from the seeds than the oil.

Despite these beneficial effects on the vascular system, neither HO nor HS had any beneficial impact on impaired mean arterial pressure and heart rate of obese Zucker rats.

We noticed an increased plasma antioxidant capacity of lipid- and water-soluble compounds (Figure 4A,B) as well as MDA in the blood plasma and the heart (Figure 4I,J) in obese Zucker rats compared to the lean controls, perhaps as a response to the increased oxidative stress, which up-regulated the mechanism(s) responsible for the antioxidant defense and potentiated lipid peroxidation. Both dietary seeds and the oil decreased the lipid peroxidation in the blood plasma and in the heart. However, the effectiveness of the seeds was more indicated, which was reflected by a decrease in the plasma antioxidant capacity of water-soluble compounds. This was not observed for hemp oil and the plasma antioxidant capacity of lipid-soluble compounds.

The decreased plasma antioxidant capacity of water-soluble compounds by the seeds is associated with the decreased plasma uric acid (Figure 4C), which is strongly hydrophilic, but not with the plasma levels of albumin (Figure 4F) nor bilirubin (data not shown). All these components, together with vitamin C, are considered the main blood plasma antioxidants in humans [20,21]. Paradoxically, plasma uric acid positively correlates and predicts the development of obesity, hypertension, and cardiovascular disease [21]. Thus, the decreased plasma uric acid by the seeds in the present study can be considered beneficial for the body, especially when looking at its role in the development of gout. Support of this supposition can be found in the study by Opyd et al. [9], who showed that dietary supplementation with hemp seeds improved the antioxidant status of the liver in Zucker rats by increasing glutathione levels and decreasing a marker of lipid peroxidation. This means that a considerable decrease in the plasma antioxidant capacity and uric acid level does not automatically exclude benefits coming from hemp seed supplementation to the organ’s antioxidant defense system.

Moreover, gamma-glutamyl transferase and blood plasma creatinine were neither modified by obesity itself (Figure 4E,H) nor by hemp supplementation, as opposed to blood plasma total protein level, which was decreased by HO. It is worth mentioning that obesity increased blood plasma albumins, total protein, and urea (Figure 4D,F,G,).