METABOLIC DEPRESSION DURING ENVIRONMENTAL-STRESS - THE ROLE OF EXTRACELLULAR VERSUS INTRACELLULAR PH IN SIPUNCULUS-NUDUS

Environmental stresses such as hypoxia or hypercapnia are known to cau se acid-base disturbances and in several organisms they lead to metabo lic depression, The present study was undertaken to quantify the influ ence of these changes in acid-base parameters on metabolic rate, We de termined the rate of oxygen consumption in a nonperfused preparation o f the body wall musculature of the marine worm Sipunculus nudus at var ious levels of extra- and intracellular pH (pHe and pHi, respectively) , P-CO2 and [HCO3-]. The acid-base status of the tissue was modified a nd clamped by long-term exposure to media set to specific values of ex tracellular pH, P-CO2 and [HCO3-]. At a pHe of 7.90, which is equivale nt to the normoxic normocapnic in vivo extracellular pH, and an ambien t P-CO2 of 0.03 kPa (control conditions), pHi was 7.26+/-0.02 (mean +/ - S.D,, N=5), A reduction of extracellular pH from 7.90 to 7.20 result ed in a significant decrease of pHi to 7.17+/-0.05 at 0.03 kPa P-CO2 ( normocapnia) and to 7.20+/-0.02 at 1.01 kPa P-CO2 (hypercapnia), At th e same time, the rate of oxygen consumption of the tissue was signific antly depressed by 18.7+/-4.7% and 17.7+/-3.0%, respectively. A signif icant depression of oxygen consumption by 13.7+/-4.7% also occurred un der hypercapnia at pHe 7.55 when pHi was elevated above control values (7.32+/-0.01), No significant changes in oxygen consumption were obse rved when pHe was either drastically elevated to 8.70 under normocapni a (pHi 7.36+/-0.05) or maintained at 7.90 during hypercapnia (pHi 7.37 +/-0.03). ATP and phospho-L-arginine concentrations, as well as the Gi bbs free energy change of ATP hydrolysis (dG/d xi(ATP)), were maintain ed at high levels during all treatments, indicating an equilibrium bet ween energy supply and demand, We conclude that the depression of aero bic energy turnover in isolated body wall musculature of S. nudus is i nduced by low extracellular pH, A model is proposed which could explai n a reduced ATP cost of pHi regulation during extracellular acidosis, thus contributing to metabolic depression.