Central opiate mu-receptor-mediated suppression of tissue protein synthesis

Chronic alcohol accentuates simian acquired immunodeficiency syndrome-associated wasting

BACKGROUND

Survival following human immunodeficiency virus (HIV) infection has improved significantly following the advent of highly active antiretroviral therapy. A large percentage of HIV-infected patients consume and abuse alcohol. Erosion of lean body mass is an important contributing factor to patient morbidity and mortality, and is a common feature of both chronic alcohol (ALC) consumption and acquired immunodeficiency syndrome (AIDS). We hypothesized that alcohol-induced loss in lean body mass is likely to exacerbate the AIDS wasting syndrome, particularly at the terminal stage of AIDS (SAIDS).

METHODS

This study examined the impact of chronic, intra-gastric ALC (5 h/d x 4 d/wk; blood alcohol levels = 55 mM to 60 mM) administration on body composition and muscle mass in simian immunodeficiency virus (SIV)-infected male Rhesus macaques in contrast to SIV-infected isocaloric (22 kcal/kg/d) sucrose (SUC)-infused control animals at the terminal stage of SIV infection.

RESULTS

At terminal stage, ALC/SIV+ animals had significantly lower body weight, body mass index, and limb muscle area than SUC/SIV+ animals. Both ALC/SIV+ and SUC/SIV+ animals had suppressed expression of insulin-like growth factor-I and increased expression of the ubiquitin ligase muscle-specific RING finger-1 mRNA. ALC increased mRNA expression of atrogin-1 (pre-SIV and at SAIDS) and tumor necrosis factor (TNF)-alpha (SAIDS). These changes were not associated with significant differences in fractional rates of muscle protein synthesis or in overall survival rate. These data show that chronic ALC exacerbated the loss of muscle mass at terminal SAIDS.

CONCLUSION

Our findings suggest the involvement of TNF-alpha and increased muscle proteolysis via atrogin-1 for the greater erosion of lean body mass at terminal SAIDS in ALC-treated Rhesus macaques.

Response of protein synthesis to hypercapnia in rats: independent effects of acidosis and hypothermia

Acute metabolic acidosis has been shown to inhibit muscle protein synthesis, although little is known on the effect of acidosis of respiratory origin. The aim of this study was to investigate the effect of acute respiratory acidosis on tissue protein synthesis. Rats (n = 8) were made acidotic by increasing the CO2 content of inspired air to 12% for 1 hour. Similar rats breathing normal air served as controls (n = 8). Muscle and liver protein synthesis rates were then measured with L-[ 2H5 ]phenylalanine (150 micromol per 100 g body weight, 40 mol%). The results show that protein synthesis is severely depressed in skeletal muscle (-44% in gastrocnemius, -39% in plantaris, and -24% in soleus muscles, P < .01) and liver (-20%, P < .001) in acidotic animals. However, because breathing CO2 -enriched air was found to lower body temperature by approximately 2 degrees C, in a second experiment (n = 10), the difference in body temperature between treated and control animals was minimized by gently wrapping rats breathing CO2 -enriched air in porous cloths. This second experiment confirmed that respiratory acidosis depresses protein synthesis in muscle (-22% in gastrocnemius, P < .001; -19% in plantaris, P < .01; and -4% in soleus, P = NS). However, no effect on liver protein synthesis could be detected, suggesting that liver protein synthesis may be sensitive to changes in body temperature but is not affected by acute respiratory acidosis for 1 hour. The results show that respiratory acidosis inhibits protein synthesis in skeletal muscle and indicates that acidosis, whether of metabolic or respiratory origin, may contribute to loss of muscle protein in patients with compromised renal or respiratory function.

Acute metabolic acidosis inhibits muscle protein synthesis in rats

In this study, we investigated the effect of acute metabolic acidosis on tissue protein synthesis. Groups of rats were made acidotic with intragastric administration of NH(4)Cl (20 mmol/kg body wt every 12 h for 24 h) or given equimolar amounts of NaCl (controls). Protein synthesis in skeletal muscle and a variety of different tissues, including lymphocytes, was measured after 24 h by injection of l-[(2)H(5)]phenylalanine (150 micromol/100 g body wt, 40 moles percent). Results show that acute acidosis inhibits protein synthesis in skeletal muscle (-29% in gastrocnemius, -23% in plantaris, and -17% in soleus muscles, P < 0.01) but does not affect protein synthesis in heart, liver, gut, kidney, and spleen. Protein synthesis in lymphocytes is also reduced by acidosis (-8%, P < 0.05). In a separate experiment, protein synthesis was also measured in acidotic and control rats by a constant infusion of l-[(2)H(5)]phenylalanine (1 micromol.100 g body wt(-1).h(-1)). The results confirm the earlier findings showing an inhibition of protein synthesis in gastrocnemius (-28%, P < 0.01) and plantaris (-19%, P < 0.01) muscles but no effect on heart and liver by acidosis. Similar results were also observed using a different model of acute metabolic acidosis, in which rats were given a cation exchange resin in the H(+) (acidotic) or the Na(+) (controls) form. In conclusion, this study demonstrates that acute metabolic acidosis for 24 h depresses protein synthesis in skeletal muscle and lymphocytes but does not alter protein synthesis in visceral tissues. Inhibition of muscle protein synthesis might be another mechanism contributing to the loss of muscle tissue observed in acidosis.

Enhanced synthesis of albumin and fibrinogen at high altitude

The acute effects of active and passive ascent to high altitude on plasma volume (PV) and rates of synthesis of albumin and fibrinogen have been examined. Measurements were made in two groups of healthy volunteers, initially at low altitude (550 m) and again on the day after ascent to high altitude (4,559 m). One group ascended by helicopter (air group, n = 8), whereas the other group climbed (foot group, n = 9), so that the separate contribution of physical exertion to the response could be delineated. PV was measured by dilution of (125)I-labeled albumin, whereas synthesis rates of albumin and fibrinogen were determined from the incorporation of isotope into protein after injection of [ring-(2)H(5)]phenylalanine. In the air group, there was no change in PV at high altitude, whereas, in the foot group, there was a 10% increase in PV (P < 0.01). Albumin synthesis (mg. kg(-1). day(-1)) increased by 13% in the air group (P = 0.058) and by 32% in the foot group (P < 0.001). Fibrinogen synthesis (mg. kg(-1). day(-1)) increased by 40% in the air group (P = 0.068) and by 100% in the foot group (P < 0.001). Hypoxia and alkalosis at high altitude did not differ between the groups. Plasma interleukin-6 was increased modestly in both groups but C-reactive protein was not changed in either group. It is concluded that increases in PV and plasma protein synthesis at high altitude result mainly from the physical exercise associated with climbing. However, a small stimulation of albumin and fibrinogen synthesis may be attributable to hypobaric hypoxia alone.