**4. Roles of ω-6 and ω-3 PUFAs in physiology**

#### **4.1 Platelet physiology**

*Biochemistry and Health Benefits of Fatty Acids*

fattyvacids, such as EPA and/or saturated fatty acids [25, 26]. As a whole, the physicochemical properties of the fatty acids may affect the functions of these molecules

*= eicosapentaenoic acid, DHA = docosahexaenoic acid. Double bonds of the unsaturated fatty acids are denoted by red color (in A) (B). Because of the presence of double bond(s) along the long axis of (poly)unsaturated fatty acids, they occupy more space when they are esterified in the phospholipid bilayer and loosely align with 3D cholesterol, and increase the degree of disorder (membrane fluidity). However, when straight-chained saturated fatty acids like PLA highly align (stacks) with 3D cholesterol, the degree of packing in the bilayer* 

Fatty acids are usually oxidized by most of the cells of tissues in the body, except the RBCs. The cells of the central nervous system also do not use fatty acids for their energy requirements, using instead carbohydrates or ketone bodies. Heart cell fully depends on energy derived from fatty acid oxidation. Fatty acids constitute the principal source of energy for cells between meals, during hypoglycemia, and/or in diabetes. Beta-oxidation of fatty acids takes place in the mitochondria and, to some extent, in the peroxisomes, particularly the very long chain fatty acids [28]. Unlike in the mitochondria, beta-oxidation of fatty acid in the peroxisomes is not coupled to ATP; the high-potential electrons are rather transferred to O2, yielding hydrogen peroxide (H2O2) and generating heat. The enzyme catalase, found exclusively in peroxisomes, converts H2O2 into water and oxygen. H2O2 is also used intracellularly to digest unwanted wastes like proteins and/or to defend against intracellular foreign particles including toxins or microorganisms. All fatty acids are not oxidized at the same rates, which implicates that the purposes of cellular accumulation of fatty acids are not the same for all cells. Some fatty acids might have been exploited for energy purposes, some of them might be exploited for the structural purposes, and some of them (or their derivatives) might help the cell for the signal transductions. For example, 30–40% of all esterified fatty acids in the neural plasma membrane phospholipids consist of DHA [29], while EPA constitutes only a tiny percent of the brain total fatty acid. Among the saturated fatty acids, lauric acid (12:0) is oxidized

[27], ultimately leading to altered functions of the cells and organisms.

*increases (tightens); hence, the membrane bilayer becomes more rigid, that is, less fluid.*

*The 3D structural features of the most common fatty acids (A). PLA = palmitic acid, STA = stearic acid, OLA = oleic acid, LLA = linoleic acid, LLN = α-linolenic acid, AA = arachidonic acid, EPA* 

**3. Fatty acid oxidation**

**Figure 5.**

**16**

Platelets are derived from megakaryocytes and cause aggregation and play important roles in physiological conditions and pathological conditions as well. Fatty acids are enriched in the plasma membranes of platelets and thus may contribute to the physiology and pathology of platelets. Oral administration of ω-3 PUFAs to rats decreases the degree of platelet aggregation both in rats and humans [38, 39]; hence, it is evident that fatty acids may affect the platelet physiology and atherosclerosis. The mechanisms through which PUFA affects the platelet aggregation is unclear; however, it is assumed that ω-3 PUFA deceases the levels of atherogenic ω-6 PUFA particularly platelet membrane-AA, which acts as a proaggregatory fatty acid. Therefore, ω-3 prevents platelet aggregation by inhibiting PLA2 and interrupting the prostaglandin/thromboxane pathways [40, 41]. In addition, ω-3 PUFAs modulate the platelet membrane fluidity [42], specific lipid domains that hold the receptors for a variety of aggregation factors, such as ADP, thrombin, fibrin, etc. [37], and doing so, they decrease platelet aggregation.

### **4.2 Effects of fatty acids on hypertension**

The effect of fish oil on hypertension came into light when the Norwegian under Nazi invasion had to consume more fish rather than land-based food items during WWII [43]. The Norwegian had low blood pressure, low degree of platelet aggregation, and hypocholesterolemia as well. Afterwards, in studies on the Greenlandic Eskimos, Dyerberg and Bang [44] and Fischer et al. [45] reported that the Eskimos had also a low incidence hypertension and blood cholesterol levels. Then, oil components of marine animals and fish, in particular EPA and DHA, were attributed to lower incidence of cardiovascular risk factors, such as hypertension, hypercholesterolemia, and platelet hyperaggregation. We have previously reported that oral administration of EPA and DHA to rats

(hypercholesterolemic) decreased the hypertension [46] and hypercholesterolemia [47]. The results were consistent with many other published reports [48]. To understand the mechanism(s) of action of these PUFAs, we also pretreated the rat thoracic endothelial cells with these PUFAs and some interesting data emerged from our experiments. For example, the EPA and DHA increased the plasma levels of nucleotide products including ATP, ADP, AMP, and adenosine. The blood vessels of the PUFA-fed rats exhibited less sensitivity to noradrenaline and had caused an increased release of the total purines (ATP + ADP + AMP + Adenosine), concurrently with less contractility [47]. We hypothesized that these nucleotides and their derivatives decreased the noradrenaline sensitivity to purine-receptors of the blood vessels and decreased the blood pressure. The mechanism also might be related to the EPA/DHA-induced increase in the membrane fluidity of the endothelial cells (ECs). These hypotheses led us to preincubate the cultured ECs with EPA and DHA. As expected, the PUFAs increased the membrane fluidity of the ECS [49]. The inhibitory effects of fish oil ω-3 polyunsaturated fatty acids (PUFAs) have also been reported on the expression of endothelial cell adhesion molecules [50]. Hence, the ω-3 PUFAs might have played beneficial roles in reducing hypertension in the animal models as well as in human cases who consumed fish/marine animals' oils in their everyday life.

## **4.3 Effects of fatty acids on hepatic functions**

Saturated and/or unsaturated fatty acids are indispensable for the functions of all tissues in the mammalian body. However, an adequate balance between saturated and (poly)unsaturated and between ω-6 and ω-3 PUFAs is essential to the proper functioning of the cells. Fatty acids after their absorption in the intestinal epithelial cells are first carried to the liver, which acts as a distribution center for the whole body. Therefore, fatty acids can affect the liver functions. Inadequate amounts of essential fatty acids may cause disorders of the liver, such as fatty liver, liver cirrhosis, metabolic syndromes, hyperlipidemia, hypercholesterolemia, and other liver problems [51, 52].

Oral administration of ω-3 DHA decreases the plasma as well as hepatic cholesterol and triacylglycerol levels [53]. The mechanism through which ω-3 PUFAs decrease the plasma cholesterol is not clear; however, it is attributed to the inhibition of hepatic HMG-CoA reductase by the PUFAs, including EPA and DHA. To prove the mechanism, we determined the levels of hepatic mRNA levels of HMG-CoA reductase (yet unpublished) of the DHA-fed rats. DHA decreased the expression of HMG-CoA reductase. Our results were also consistent with numerous other published reports [54–56]. The beneficial effects also emerged at lower levels of LDL-C and TG and high levels of HDL-C. The oral administration of DHA also increased the levels of ω-3 PUFAs and decreased the levels of ω-6 AA both in the plasma and liver tissues. It might be suggested that the oral administration of PUFAs like DHA increases the degrees of oxidative stress and mammalian tissues, including the liver. However, the levels of lipid peroxide (LPO) and reactive oxygen species (ROS) were not increased, thus demonstrating that the feeding of DHA does not pose an oxidative stress to the tissues. We suggest that the oral administration of DHA rather increases the levels of antioxidative enzymes, including glutathione peroxidase and catalase, and antioxidant substrate like GSH [53]. In a similar study, the levels of antioxidative enzymes and GSH increased in the brains of hypercholesterolemic aged rats after oral administration of DHA [57]. However, there are also contradictory results where consumption of PUFA was reported to promote oxidative stress [58]. Furthermore, we isolated and purified the canalicular plasma membranes of the hepatic cells of EPA/DHA-fed rats.

**19**

**Figure 6.**

*plasma membrane of hepatic cells).*

*Fatty Acids: From Membrane Ingredients to Signaling Molecules*

These membranes allow the transport and pump bile components in-and-out of the hepatic cells. The levels of PUFAs increased in the canalicular plasma membranes, concurrent with increases in the activities of membrane-bound enzymes such as Mg2<sup>−</sup>-ATPase, 5′-nucleotidase. Membrane fluidity also increased in these membranes, thus suggesting that an increased fluidity might have helped in the pumping out the cholesterol via the bile (**Figure 6**). Otherwise, the levels of fecal cholesterol could have not been increased in the feces of the fish-oil-fed rats [53].

ω-6 PUFA like arachidonic acid (AA, C20:4, ω-6) generates 2-series prostanoids, namely prostaglandins PGE2, PGI2, PGD2, and PGF2*α* (largely produced by monocytes and macrophages) and thromboxanes TXA2 and TXB2 by COX-1/COX-2 enzymes. Prostaglandin PGI2/PGE2 has proinflammatory effects. AA by the action of LOX also produces leukotrienes such as 5-HETE and 5-HPETE, LTE4, LTB4, LTC4, and LTD4. They are strong proinflammatory agents and have vasoconstriction effects and platelet- and/or neutrophil- and macrophage-activating effects [59–61]. Interestingly, the eicosanoids derived from the action COX and/or LOX on EPA and DHA produces 3-series prostaglandins and thromboxanes and 5-series leukotrienes, and they are less inflammatory and even have anti-inflammatory effects, as compared to the eicosanoids derived from AA. These lipid mediators antagonize the effects of those derived from AA, thus conferring beneficial effects

Skeletal muscle is the largest organ in the human body, comprising approximately 40% of total body weight [63]. This muscle has a plastic-like property and has adapting capability to physical activity. Strenuous muscle exercise increases muscle fatigue and decreases muscle strength, leading to an increase in muscle oxidative stress. It is believed that the response of skeletal muscle to exercise can be modified by the nutritional status of the muscles. There

*Effects of oral administration of EPA and DHA on the plasma and hepatic lipid profile (TC = total cholesterol, TG = triacylglycerol), LPO = lipid peroxide, ROS = reactive oxygen species, CanPM = canalicular* 

*DOI: http://dx.doi.org/10.5772/intechopen.80430*

**4.4 Anti-inflammatory responses**

on inflammatory responses [62].

**4.5 Effects on skeletal muscles**

*Fatty Acids: From Membrane Ingredients to Signaling Molecules DOI: http://dx.doi.org/10.5772/intechopen.80430*

These membranes allow the transport and pump bile components in-and-out of the hepatic cells. The levels of PUFAs increased in the canalicular plasma membranes, concurrent with increases in the activities of membrane-bound enzymes such as Mg2<sup>−</sup>-ATPase, 5′-nucleotidase. Membrane fluidity also increased in these membranes, thus suggesting that an increased fluidity might have helped in the pumping out the cholesterol via the bile (**Figure 6**). Otherwise, the levels of fecal cholesterol could have not been increased in the feces of the fish-oil-fed rats [53].

### **4.4 Anti-inflammatory responses**

*Biochemistry and Health Benefits of Fatty Acids*

fish/marine animals' oils in their everyday life.

**4.3 Effects of fatty acids on hepatic functions**

problems [51, 52].

(hypercholesterolemic) decreased the hypertension [46] and hypercholesterolemia [47]. The results were consistent with many other published reports [48]. To understand the mechanism(s) of action of these PUFAs, we also pretreated the rat thoracic endothelial cells with these PUFAs and some interesting data emerged from our experiments. For example, the EPA and DHA increased the plasma levels of nucleotide products including ATP, ADP, AMP, and adenosine. The blood vessels of the PUFA-fed rats exhibited less sensitivity to noradrenaline and had caused an increased release of the total purines (ATP + ADP + AMP + Adenosine), concurrently with less contractility [47]. We hypothesized that these nucleotides and their derivatives decreased the noradrenaline sensitivity to purine-receptors of the blood vessels and decreased the blood pressure. The mechanism also might be related to the EPA/DHA-induced increase in the membrane fluidity of the endothelial cells (ECs). These hypotheses led us to preincubate the cultured ECs with EPA and DHA. As expected, the PUFAs increased the membrane fluidity of the ECS [49]. The inhibitory effects of fish oil ω-3 polyunsaturated fatty acids (PUFAs) have also been reported on the expression of endothelial cell adhesion molecules [50]. Hence, the ω-3 PUFAs might have played beneficial roles in reducing hypertension in the animal models as well as in human cases who consumed

Saturated and/or unsaturated fatty acids are indispensable for the functions of all tissues in the mammalian body. However, an adequate balance between saturated and (poly)unsaturated and between ω-6 and ω-3 PUFAs is essential to the proper functioning of the cells. Fatty acids after their absorption in the intestinal epithelial cells are first carried to the liver, which acts as a distribution center for the whole body. Therefore, fatty acids can affect the liver functions. Inadequate amounts of essential fatty acids may cause disorders of the liver, such as fatty liver, liver cirrhosis, metabolic syndromes, hyperlipidemia, hypercholesterolemia, and other liver

Oral administration of ω-3 DHA decreases the plasma as well as hepatic cholesterol and triacylglycerol levels [53]. The mechanism through which ω-3 PUFAs decrease the plasma cholesterol is not clear; however, it is attributed to the inhibition of hepatic HMG-CoA reductase by the PUFAs, including EPA and DHA. To prove the mechanism, we determined the levels of hepatic mRNA levels of HMG-CoA reductase (yet unpublished) of the DHA-fed rats. DHA decreased the expression of HMG-CoA reductase. Our results were also consistent with numerous other published reports [54–56]. The beneficial effects also emerged at lower levels of LDL-C and TG and high levels of HDL-C. The oral administration of DHA also increased the levels of ω-3 PUFAs and decreased the levels of ω-6 AA both in the plasma and liver tissues. It might be suggested that the oral administration of PUFAs like DHA increases the degrees of oxidative stress and mammalian tissues, including the liver. However, the levels of lipid peroxide (LPO) and reactive oxygen species (ROS) were not increased, thus demonstrating that the feeding of DHA does not pose an oxidative stress to the tissues. We suggest that the oral administration of DHA rather increases the levels of antioxidative enzymes, including glutathione peroxidase and catalase, and antioxidant substrate like GSH [53]. In a similar study, the levels of antioxidative enzymes and GSH increased in the brains of hypercholesterolemic aged rats after oral administration of DHA [57]. However, there are also contradictory results where consumption of PUFA was reported to promote oxidative stress [58]. Furthermore, we isolated and purified the canalicular plasma membranes of the hepatic cells of EPA/DHA-fed rats.

**18**

ω-6 PUFA like arachidonic acid (AA, C20:4, ω-6) generates 2-series prostanoids, namely prostaglandins PGE2, PGI2, PGD2, and PGF2*α* (largely produced by monocytes and macrophages) and thromboxanes TXA2 and TXB2 by COX-1/COX-2 enzymes. Prostaglandin PGI2/PGE2 has proinflammatory effects. AA by the action of LOX also produces leukotrienes such as 5-HETE and 5-HPETE, LTE4, LTB4, LTC4, and LTD4. They are strong proinflammatory agents and have vasoconstriction effects and platelet- and/or neutrophil- and macrophage-activating effects [59–61]. Interestingly, the eicosanoids derived from the action COX and/or LOX on EPA and DHA produces 3-series prostaglandins and thromboxanes and 5-series leukotrienes, and they are less inflammatory and even have anti-inflammatory effects, as compared to the eicosanoids derived from AA. These lipid mediators antagonize the effects of those derived from AA, thus conferring beneficial effects on inflammatory responses [62].

#### **4.5 Effects on skeletal muscles**

Skeletal muscle is the largest organ in the human body, comprising approximately 40% of total body weight [63]. This muscle has a plastic-like property and has adapting capability to physical activity. Strenuous muscle exercise increases muscle fatigue and decreases muscle strength, leading to an increase in muscle oxidative stress. It is believed that the response of skeletal muscle to exercise can be modified by the nutritional status of the muscles. There

#### **Figure 6.**

*Effects of oral administration of EPA and DHA on the plasma and hepatic lipid profile (TC = total cholesterol, TG = triacylglycerol), LPO = lipid peroxide, ROS = reactive oxygen species, CanPM = canalicular plasma membrane of hepatic cells).*

are numerous reports on the beneficial effects of EPA and DHA on muscle. Therefore, the effects of these PUFAs on muscle strength have been investigated with increasing interest. Hess et al. [64] reported that dietary algae and marine fish increase the levels of EPA and DHA in the equine skeletal muscles. Guen et al. [65] reported that DHA-enriched supplementation improves endurance exercise capacity and skeletal muscle mitochondrial function in murine skeletal muscle. Stebbins et al. [66] reported that DHA + EPA enhances skeletal-muscle blood and vascular conductance in active skeletal muscle (especially type I and IIa fibers) and that the increase in muscle blood is due to an increase in cardiac output secondary to increases in vascular conductance [66]. However, we believe that there are differential effects of PUFAs on the muscle [67]. AA deposition in the fast-twitch muscle of aging rats reduced cell volume with an increase in oxidative stress [68].
