Total nutritional support through the peritoneal cavity.

We prepared ten dogs with intraperitoneal catheters to allow total nutritional support for 30 days by constant infusion of a solution containing 1.5% amino acid, 10% glucose, and 1% lipids at a rate of 4 ml/kg/hr. Seven dogs survived in apparent good health but with a 13.9 +/- 1.3% weight loss. There was no correlation between actual caloric input and percentage of weight loss. Serum albumin concentration fell to 2.6 +/- 0.3 gm/dl but other chemistries remained near normal. The peritoneum showed significant inflammatory reaction but this resolved by 30 days. Three dogs died in the first week from what appeared to be circulatory collapse from rapid fluid shifts. We conclude that a significant amount of nutrients can be delivered through the peritoneal cavity but whether total nutritional support is feasible remains to be proven.

Role of the medullary collecting duct in potassium excretion in potassium-adapted animals.

Previous studies have demonstrated that the rate of urinary potassium excretion is markedly greater in animals fed a high potassium diet than in animals on a control diet before and during the acute infusion of potassium salts. Because the contribution of the medullary collecting duct to urinary potassium under these conditions has not been determined, microcatheterization studies were performed to elucidate the role of that nephron segment in regulating potassium excretion in controls and in animals on a potassium-enriched diet. Although there was no evidence of net transport of potassium by the medullary collecting duct in control animals under basal conditions, net secretion of potassium by that nephron segment accounted for nearly one half of the increased rate of urinary excretion in animals fed a high potassium diet. During acute infusion of potassium salts, potassium secretion by the medullary portion of the collecting duct contributed approximately one half of the potassium in the final urine samples in both controls and animals fed a high potassium diet. Because the rate of urinary excretion was twofold greater in experimental animals during acute potassium loading, absolute potassium secretion in medullary collecting duct was twice the control rate in animals adapted to a high potassium dietary intake. These data demonstrate, therefore, that the medullary collecting duct plays an important role in regulating the excretion of potassium when the excretory load is either acutely or chronically increased.

Electrical properties of the medullary collecting duct in the rat.

Studies were performed in the rat to determine the transepithelial potential difference (PD) of the medullary collecting duct (MCD). With the microcatheter as an exploring electrode the PD from duct lumen to the reference electrode, placed in peritoneal fluid, was approximatley 1 mV, lumen positive, in the MCD. It was unlikely that the polarity and small magnitude of PD was due to an artifact created by the microcatheter, since essentially similar results were obtained by transmural puncture of the inner third of the duct using Ling-Gerard glass electrodes and glass microelectrodes, tip OD 3-5 micron. Moreover, a PD of a few millivolts, lumen positive, was observed in experimental states characterized by marked increases in potassium and hydrogen ion secretion and sodium absorption. Further studies showed that the diffusion potential existing within the interstitial fluid compartment, between papilla and reference electrode, was approximately +2 mV, owing to the higher NaCl concentration in the papillary area. These data demonstrate, therefore, that the transepithelial PD in MCD approaches zero in control animals, and is not substantially altered by marked changes in net electrolyte transport.

Evidence for active chloride transport in dogfish rectal gland.

To evaluate the characteristics of the transport process for Na and C1 by the rectal gland of the dogfish shark, experiments were performed with an in vitro perfused model. Changes in glandular secretion rate and net electrical potential difference (PD) were measured during independent alterations of the Na and C1 concentrations of the perfusate solution. The rate of net secretion was enhanced by increasing the concentration of either Na or Cl in the perfusate, demonstrating a substratedependent process. When tetramethyl-ammonium was substituted for Na, the rate of secretion fell in association with a rise in net PD from -8 mV to -17 mV (lumen negative). When SO4 was substituted for Cl, secretion rate also fell, but in association with a fall in net PD. Despite almost complete replacement of Cl with SO4, net PD was not reversed. Although the mechanism responsible for Na secretion remains unclear, these data suggest that chloride is transported by an active process in the dogfish rectal gland.