Benefits of omega-3 fatty acid against bone changes in salt-loaded rats: possible role of kidney

Abstract

There is evidence that dietary fats are important components contributing in bone health and that bone mineral density is inversely related to sodium intake. Salt loading is also known to impose negative effects on renal function. The present study aimed to determine the effect of the polyunsaturated fatty acid omega-3 on bone changes imposed by salt loading, highlighting the role of kidney as a potential mechanism involved in this effect. Male Wistar rats were divided into three groups: control group, salt-loaded group consuming 2% NaCl solution as drinking water for 8 weeks, and omega-3-treated salt-loaded group receiving 1 g/kg/day omega-3 by gavage with consumption of 2% NaCl solution for 8 weeks. Systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), and heart rate (HR) were recorded. Plasma levels of sodium, potassium, calcium, inorganic phosphorus (Pi), alkaline phosphatase (ALP), creatinine, urea, 1,25-dihydroxyvitamin D [1,25(OH)2D3], and transforming growth factor-beta1 (TGF-β1) were measured. The right tibia and kidney were removed for histologic examination and renal immunohistochemical analysis for endothelial nitric oxide synthase (eNOS) was performed. The results revealed that omega-3 reduced SBP, DBP, and MAP and plasma levels of sodium, potassium, Pi, creatinine, urea, and TGF-β1, but increased plasma levels of calcium, ALP, and 1,25(OH)2D3 as well as renal eNOS. Omega-3 increased cortical and trabecular bone thickness, decreased osteoclast number, and increased newly formed osteoid bone. Renal morphology was found preserved. In conclusion, omega-3 prevents the disturbed bone status imposed by salt loading. This osteoprotective effect is possibly mediated by attenuation of alterations in Ca(2+), Pi, and ALP, and improvement of renal function and arterial blood pressure.

Keywords: Bone; omega-3; renal function; salt intake.

Figure 1

Metaphysis of upper end of tibia of: (A and B) Control group: (A) showing irregular cancellous bone trabeculae with bone marrow (B.M.) tissue in-between (H&E 200 × ). (B) Showing osteocytes (curved arrow), osteoprogenitor cells (↑), and osteoblasts (Ob) in relation to bone trabeculae (H&E 640 × ). (C and E) Salt-loaded group: (C) showing apparent thinning of bone trabeculae and widening of B.M. tissue in-between them (H&E 200 × ), (D) showing eroded surface of the bone trabeculae with osteoclast (▲) and osteoprogenitor cells (↑) lining the endosteum on the other side of the trabeculum (H&E 640 × ), (E) showing osteoclasts (▲) lining the resorption area on bone surface (H&E 1500 × ). (F and G) Omega-3-treated salt-loaded group: (F) showing apparent thickening of bone trabeculae compared to salt-loaded group with B.M. tissue in-between them (H&E 200 × ), (G) showing osteoprogenitor cells (↑) and osteoblasts (Ob) lining endosteum of bone trabeculae (H&E 640 × ).

Figure 2

Metaphysis of upper end of tibia of: (A) Control group showing newly formed osteoid bone with light green color (↑). (B) Salt-loaded group showing apparent less newly formed osteoid bone. (C) Omega-3-treated salt-loaded group showing apparent increased newly formed osteoid bone (↑) compared to salt-loaded group (Masson's trichrome 400 × ).

Figure 3

Renal cortex of: (A) Control group showing renal corpuscle with normal glomerulus (G). The juxtaglomerular apparatus (↑) is seen. Notice normal pattern of proximal convoluted (P) and distal convoluted (D) tubules. (B and C) Salt-loaded group: (B) showing most of the corpuscles (G) with high cellularity and obliterated capsular space. Proximal convoluted tubules (P) show destructed epithelial lining. Notice congested blood vessels (▲), thickened arterioles (↑), and cellular infiltration (C.I.). (C) Showing high cellularity of renal corpuscles (G) and extra-mesangial cells (↑) of juxtaglomerular apparatus. Notice destructed epithelial lining of distal convoluted tubules (D). (D) Omega-3-treated salt-loaded group showing preservation of normal pattern of renal corpuscles (G) and juxtaglomerular apparatus (↑). Both proximal (P) and distal (D) convoluted tubules appear nearly normal (H&E 400 × ).

Figure 4

Renal cortex of: (A) Control group showing eNOS-positive immunoreactivity of endothelial cells of both glomeruli and intertubular capillaries. (B) Salt-loaded group showing apparent less intense eNOS immunoreactivity. (C) Omega-3-treated salt-loaded group showing apparent increased eNOS immunoreactivity compared to salt-loaded group (eNOS 400 × ).