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We studied the role of renal prostaglandins in the regulation of glomerular filtration rate (GFR) and renal blood flow (RBF) in the isolated dog kidney. Indomethacin or meclofenamate, 2 mg/kg of body weight, suppressed renal prostaglandin E2 (PGE2) secretion, measured by radioimmunoassay, to zero within 20 minutes; the effect persisted for the duration of the study. When renal arterial pressure (PRA) was maintained at 104 mm Hg both drugs caused a sharp decrease in sodium excretion and RBF with redistribution of flow from inner to outer cortes we examined autoregulation of GFR and RBF over the pressure ranges of 150-100 and 150-75 mm Hg, respectively, after inhibition of PGE2 secretion and under control conditions. deltaGFR/deltaPRA (ml/min per mm Hg) was 0.020 +/- 0.017 in the indomethacin group, 0.152 +/- 0.055 in the meclofenamate group, and 0.086 +/- 0.017 in the control group. The change in GFR for the indomethacin group was significantly less than that for meclofenamate (P less than 0.01) and control groups (P less than 0.025); the latter two groups were not statistically different from each other (P greater than 0.1). There was no significant difference (P greater than 0.1) between the three groups with respect to deltaRBF/deltaPRA, which measured 0.288 +/- 0.046, 0.370 +/- 0.112, and 0.438 +/- 0.123 ml/min per mm Hg in the indomethacin, meclofenamate and control groups, respectively. Renal was lowered from 150 to 75 mm Hg. The observation that inhibition of prostaglandin synthesis promotes a redistribution of RBF from inner to outer cortex suggests that renal prostaglandins may participate in the regulation of medullary blood flow. However, since autoregulation of GFR and RBF remained intact despite inhibition of prostaglandin secretion, these data argue against a role for renal prostaglandins in regulating whole kidney GFR and RBF.

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The intracellular distribution of rat and human liver isocitric dehydrogenase is essentially the same as that of mouse and rabbit liver. This enzyme is largely cytoplasmic but washing does not remove all of the activity from the mitochondria. A method is outlined using permeability considerations as a test of whether an enzyme is within the mitochondria or adsorbed thereon. This method, when applied to washed liver mitochondria, indicates that some isocitric dehydrogenase activity is within the liver mitochondria of rat and rabbit. The results were not conclusive with human liver.

1. The freezing-points of white and yolk in the hen's egg gradually approach equality when the egg is kept for long periods; and the rate of the process of equilibration is rapid at first but becomes very slow as equality is more closely approached. 2. Between 0° and 25° C. the rate of equilibration has a temperature coefficient (Q 10) of from 1.5 to 2. At 25° C. equality of freezing-points is reached after about 70 days. 3. Equilibration is achieved partly by the passage of water across the vitelline membrane from white to yolk, but partly also by more complicated changes of osmotic concentration occurring more or less independently in white and yolk. 4. The recovery of hypertony by a yolk, previously diluted by immersion in water, when it is replaced in egg-white can, be explained on the basis of a temporary heterogeneity of the diluted yolk, and this explanation is supported by experimental evidence. 5. The rate of equilibration is much greater when the separated yolk is placed in mixed egg-white than in the intact egg, but since it is also greater in thin white than in thick white, and greater again in the white dialysate, the structure and viscosity of the white are probably important factors. 6. There is evidence of an appreciable resistance to water-movement both in egg-white and in egg-yolk. 7. In hypotonic or hypertonic aqueous solutions of glucose or glycerol, or in Ringer's solution, the rate of equilibration is greater than in egg-white and many times greater than in the intact egg. Water is taken up by the yolk both from hypotonic and hypertonic solutions of Ringer, within the range δ = 0.10° to 1.20° C., at a rate which increases the further the solution is removed from the point of isotony. 8. Evidence that the apparent disequilibrium in intact eggs is not a steady state maintained by a "Lebenswirkung," is afforded by: (i) the form of the equilibration curves, which strongly suggest the slow attainment of an equilibrium by diffusion, rather than a steady state terminated by death; (ii) the temperature relations of equilibration, which are consistent with the former assumption, but which do not agree at all with the effect of temperature on the viability of fertile eggs; (iii) the absence of any tendency of the yolk to maintain its hypertony when the white is concentrated by rapid evaporation; (iv) the alternative explanation for the recovery of hypertony by diluted yolks, which was the most crucial evidence for the existence of a steady state maintained by the expenditure of energy.

Washing the sediment or the mitochondrial fraction of rabbit cerebral cortex homogenates in the process of tissue fractionation increases its isocitric dehydrogenase activity thus giving anomalous recovery results. This increase on washing is not due to the removal of a soluble inhibitor of the dehydrogenase itself. Isocitric dehydrogenase activity is associated with the mitochondrial fraction and soluble fraction of rabbit cerebral cortex homogenates. It is unlikely that the mitochondrial or soluble activities have been derived from one another, or are due entirely to contamination of one fraction with another. The enzymes themselves from these sources have not been shown to be either the same or different enzymes. The activation of the particulate activity by freezing and thawing has been used as a roughly quantitative test for particulate bound activity. Mitochondrial bound enzyme may be activated by freezing and thawing, exposure to sonic vibrations, the shearing effect of Waring Blendor blades, and partially by washing. The latter may be explained on the basis of permeability factors aggravated by removal of isocitrate within the mitochondria by aconitase. Rabbit cerebral cortex mitochondria are not easily ruptured by homogenization in the Potter and Elvehjem homogenizer in sucrose suspension. The increase in specific activity of the nuclear fraction due to freezing and thawing is larger than that of the mitochondrial fraction, though the actual increase in activity is much smaller. The presence of a nuclear isocitric dehydrogenase has not been ruled out. A highly active isocitric dehydrogenase in soluble form has been prepared from rabbit cerebral cortex mitochondria.

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