**5.1 Sodium and chloride absorptions**

It has been determined that two carrier proteins play a role in the sodium absorption; "sodium hydrogen exchanger-2" (NHE-2) and NHE-3 which are the members of "solute carrier family-9" (SLC9) (Malakooti et al., 2011). While NHE-2 is expressed mostly in colon, NHE-3 is expressed mainly in ileum (Dudeja et al., 1996). The carrier proteins "down regulated in adenoma" (DRA) and putative anion transporter-1 (PAT-1) from SLC26 gene family have a role in chloride absorption. While PAT-1 is mainly expressed in small intestines, DRA is more expressed in colon than small intestines (Wang et al., 2002).

Probiotics such as *Lactobacillus* are used as a treatment support in diseases especially characterized by fluid loss in children. It has been determined that probiotics reduce sodium chloride and fluid loss in these diseases (Raheja et al., 2010). Furthermore it has been reported that *Saccharomyces boulardii* increases chloride net absorption from jejunum and descending colon *in vitro* (Krammer & Karbash, 1993).

To investigate the molecular mechanisms underlying the effects of probiotics on electrolyte and water absorption from intestinal tract, some *in vivo* and *in vitro* studies have been carried on. Human colon adenocarcinoma cell (Caco-2) has been used extensively as a model cell *in vitro* experiments subjected intestinal epithelium. Borthakur et al. (2008) reported that DRA activity increased in Caco-2 cells after short term *Lactobacillus acidophilus*  application and this will cause chloride absorption eventually. *Lactobacillus acidophilus* was reported to cause this effect by increasing DRA expression in apical membranes of epithelial cells. However, it has been determined that total DRA amount in the cell did not changed, only DRA expression on the surface increased and this effect was caused via phosphatidylinositol 3-kinase pathway. It has been also considered that some soluble substances secreted by *Lactobacillus acidophilus* revealed this effect*.* 

Bacteria present in the intestines are consistently interacting with epithelial cells. Therefore, it has been determined that, while *Lactobacillus acidophilus* increase DRA mRNA expression

The Impact of Probiotics on the Gastrointestinal Physiology 61

(2007) reported that fermenting calcium-fortified soymilk with some *Lactobacillus* species can

Brassart & Yey (1998) have determined that 7 *Lactobacillus* species, which were tested *in vitro*, increased the transepithelial calcium transport in Caco-2 monolayer cells. Gilman & Cashman (2006) have reported that the transepithelial calcium transport did not change in Caco-2 monolayer cells treated by *Lactobacillus salivatorius* (UCC 118) and *Bifidobacterium infantis* (UCC 35624), however UCC 118 increased calcium uptake after 24 hours. Although the differences between the results of these studies have not exactly clarified, it has been suggested that the differences may be due to the different adheration of used bacteria to epithelial cells (Gilman & Cashman, 2006). Intestinal calcium absorption increasing effect of probiotics may be also related to increased expression of calcium channels in intestinal mucosa. Vinderola et al. (2007) observed that supernatant from milk fermented by *Lactobacillus helveticus* R389 enhanced expression of TRPV6 channels in the duodenum. Enhanced expression Ca+2 channels indicate an improved capacity for dietary Ca+2 uptake.

Cholesterol entered the body via food or re-absorbed from the bile secretion to the blood, is primary factor for heart and vascular diseases. Hypercholesterolemia is one of the most significant risk factor for the cardio-vascular diseases. It has been determined that various probiotic species decrease the serum cholesterol levels in human (Larkin et al., 2009), experimental animals (Park et al., 2007) or farm animals (Özcan et al., 2003; Strompfova et al., 2006). However hypocholesterolemic effect of probiotics depends on the species of the bacteria. This hypocholesterolemic effect has been suggested to be caused by more than one mechanism. For example lactic acid bacteria exert hypocholesterolemic effect by assimilating endogenous or exogenous originated cholesterol in intestinal tract or deconjugating bile acids (Gilliland et al., 1990). In addition, it has been reported that cholesterol and free bile acids bound to the cellular surface of microorganism or co-precipitate with free bile acids by

The recent researches have revealed that probiotics affect gene expression of carrier proteins which are responsible for cholesterol absorption. The protein called Niemann-Pick C1-like 1 (NPC1L1) which is abundantly expressed on the surface of enterocytes, plays a key role on the absorption of cholesterol from intestines. Reduction or inhibition of expression levels of this protein leads to a decrease in plasma cholesterol levels. The probiotic *Lactobacillus acidophilus* (American type culture collection) ATCC 4356 reduced NPCIL-1 gene expression and inhibited the cellular uptake of micellar cholesterol in Caco-2 cells. Soluble effector molecules secreted by ATCC 4356 were shown to be responsible for the decrease in NPC1L-1. Furthermore, ATCC 4356 mediated this effect partly through the liver X receptors (LXR)

Enteric nervous system (ENS), which is located in the wall of the digestive tract, is a neural network called as second brain that is consisted of sensory neurons, motor neurons, inter neurons and glial cells. It regulates complicated reflexes, motility and secretory functions of digestive tract. Although it is connected to central nervous system, ENS can regulate the

potentially enhance the calcium bioavailability.

**5.4 The effects of probiotics on cholesterol absorption** 

probiotics (Guo & Zhang, 2010).

(Huang & Zheng, 2010).

**6. Probiotics and enteric nervous system** 

in epithelial cells of colon by transcriptional mechanisms, it is not effective in jejunum and ileum during its long-term applications. DRA is primary chloride transporter in colon, therefore significance of probiotics in chloride and water absorption has been proved (Binder & Mehta, 1989; Raheja et al., 2009). There is a limited knowledge on the molecular mechanisms underlying the effects of probiotics on sodium absorption. It is well known that probiotics produce short chain fatty acids. The short chain fatty acids increase the expression of NHE-3 which plays a main role in the absorption of sodium from ileum (Kiela et al., 2007).

#### **5.2 Na<sup>+</sup> -coupled glucose absorption**

Glucose is absorbed from intestinal brush border membrane by mainly sodium-dependent glucose co-transporters 1 (SGLT-1) and glucose transporter 2 (GLUT-2) (Shimizu et al., 2000). Absorption rate of the intestinal glucose depends on the SGLT-1 affinity and density in the membrane. High affinity SGLT-1 is primary transporter for glucose absorption.

It has been reported that sodium coupled glucose absorption increased in small intestines of pigs treated with *Saccharomyces boulardii* or *Bacillus cereus var*. *toyoi* (Breves et al., 2000). It has been also determined that *Enterococcus faecium* NCIMB 10415 used as a probiotic caused an increase in intestinal transport and barrier function and glucose absorption (Lodeman et al., 2006). Similarly, sodium coupled D-glucose absorption increase has been reported in rats orally applied *Saccharomyces boulardii* (Buts et al., 1999).

The mechanism underlying the sodium coupled glucose absorption increasing effect of probiotics in intestinal epithelium cells has not been fully defined. However it has been suggested that specific and non-specific mechanisms may be effective. It may be a nonspecific reason such as an increase in the absorptive surface or in affinity of transporters to substrates due to probiotics. On the other hand Rooj et al. (2010) have reported that supernatant obtained from lactobacilli increased the glucose transport in Caco-2 cells nongenomically and undefined metabolites produced by the probiotic caused this effect. This researchers have suggested that the metabolites produced by the probiotic cause to expression of cytosolic transporters in brush border membranes of enterocytes or to activation of transporters which were already in the membrane.

Although it has been suggested that probiotics affect intestinal glucose transport by nongenomic responses, SGLT-1 expression increases in rats applied *Saccharomyces boulardii* (Buts, 2009). Therefore, it is considered that the probiotics may be effective by changing gene expression via transcriptional or post translational mechanisms.
