. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 254 THOMAS H. DIETZ AND RONALD H. ALVARADO If the inward and outward permeability coefficients for an ion are the same, then the passive diffusion of that ion should obey the flux-ratio equation (Ussing, 1949) : ^ Mo = (c0/Ci}exp(zFE/RT) (1) where the subscripts i and o refer to the inside and outside compartments, M is the flux in ^.eq/cm-'-hr; c is the ionic concentration in ^eq/cm3; £ is the valence of the ion; F is the Faraday, 96,500 joules/volt mole; E is the potential difference across the membrane in volts; R is the
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 254 THOMAS H. DIETZ AND RONALD H. ALVARADO If the inward and outward permeability coefficients for an ion are the same, then the passive diffusion of that ion should obey the flux-ratio equation (Ussing, 1949) : ^ Mo = (c0/Ci}exp(zFE/RT) (1) where the subscripts i and o refer to the inside and outside compartments, M is the flux in ^.eq/cm-'-hr; c is the ionic concentration in ^eq/cm3; £ is the valence of the ion; F is the Faraday, 96,500 joules/volt mole; E is the potential difference across the membrane in volts; R is the gas constant joule/degree mole and T is the absolute temperature. In a steady state the flux ratio (Aff/7kf0) is equal to one. If an ion is passively distributed then a calculation of the expected flux ratio should equal the observed flux ratio. For animals in PW the calculated flux ratios would be and for Na+ and Cl", respectively, assuming that the TEP is 20 mV (inside negative). Transport of both Na+ and Cl~ is thermodynamically active. The integument is the primary site of Na+ transport. This was determined by measuring the influx of Na-22 on seven worms which had their anterior and posterior ends ligated with heavy cotton string. The influx over the 10 hr interval was ± /teq Na+/10 g-hr with a net uptake of ± /*eq NayiO g-hr. Handling stress is probably responsible for the high influx. To further characterize the Na+ transport mechanism, experiments were per- formed measuring the influx of Na+ over a range of external Na+ concentrations. The basic solution for these studies was PW with different amounts of NaCl added or deleted. Worms were acclimated to PW prior to the flux measurements shown in Figure 3. The rate of Na+ transport is dependent on bath [Na"] at low concen- o> O i- Na+ CONCENTRATION (meq/l) FIGURE 3. The effect of external Na+ concentration on the influx of Na+ i
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