Effects of gastrointestinal hormones on transport by peritoneal dialysis

Changes in Peritoneal Transport during the First Month of Peritoneal Dialysis

Objective

To determine if peritoneal transport characteristics change during the initial month of peritoneal dialysis.

Design

Retrospective review of peritoneal equilibration test (PET) results in patients who received their first PET during the first two weeks of peritoneal dialysis (early PET group) versus patients who received their first PET between four and 28 weeks after the initiation of dialysis (late PET group). The initial PET values were compared to subsequent PET results obtained approximately seven months after the initial PET.

Setting

Peritoneal dialysis unit of a tertiary medical center.

Outcome Measures

PET results and calculated mass transfer area coefficient (MT AC) values.

Patients

Thirty-four peritoneal dialysis patients in the early PET group and 17 peritoneal dialysis patients in the late PET group.

Results

In the early PET group, there was a statistically significant increase from the initial to follow-up values for both dialysate-to-plasma (DIP) creatinine and MTAC creatinine (p < 0.01) as well as a significant decrease for four-hour dialysate to initial dialysate ratios (DID) glucose (p = 0.08) and MTAC glucose (p < 0.05). In the late PET group, there was no significant change in any of these parameters with time. However, in the late PET group, there was a significant decrease in DIP urea values with time (p < 0.01), but not with MTAC urea. In addition, there were no differences over time in either group for serum albumin or hematocrit values.

Conclusion

During the first two weeks of peritoneal dialysis, there tends to be a change in peritoneal transport characteristics in some patients. PET data obtained during this time period should be interpreted as preliminary.

Pathophysiology of Ultrafiltration in Peritoneal Dialysis

Pathophysiology of peritoneal ultrafiltration is analyzed in the present study. Peritoneal equilibration test is the easiest procedure to study in detail the possible causes of failure to control the ultrafiltration rate in patients undergoing peritoneal dialysis. Membrane failure, reduction in peritoneal blood flow, excessive lymphatic reabsorption catheter malposition, and fluid sequestration are the most common causes of ultrafiltration loss. Pharmacologic manipulation of peritoneal membrane, correction of mechanical inconvenients, reduction in peritonitis rate and in the level of immunostimulation of the mesotelial macrophages, together with a careful policy in terms of glucose concentration in the dialysate and dwell times may contribute not only to treat different forms of ultrafiltration loss but also to prevent their incidence.

Pharmacologic Modification of Transperitoneal Movement of Water

Investigations concerning the influence of pharmacologic agents on transperitoneal water movement are predominantly undertaken in the hope that their results can help in a restoration of net ultrafiltration (UF) volume toward normal in cases with declining UF during long-term dialysis treatment. Net UF volume represents the difference between net transcapillary UF and lymphatic absorption.

The choice of a pharmacological agent for enhancing UF depends on the mechanisms responsible for net UF loss, which include: (a) early dissipation of the transperitoneal osmotic gradient; (b) decrease in the peritoneal surface area; (c) Iymphomonokine overproduction; (d) enhanced lymphatic absorption; (e) high residual volumes left in the peritoneal cavity; or (f) a combination of these factors. Leakage of dialysate to the abdominal wall sometimes occurring in peritoneally dialysed patients (1), according to a definition of net UF volume, cannot be regarded as a true UF loss.

Errors in estimates of peritoneal fluid volume

Inherent limitations in the suitability of drainage volumes for monitoring intraperitoneal fluid volume have resulted in the frequent use of indicator dilution techniques, but little attention has been given to confirming the adequacy of the estimates that volume markers provide. In a series of experimental exchanges in rats, volume estimates were compared based on the dilution of blue dextran and hemoglobin with direct collections of surgically exposed intraperitoneal fluid. Significant systematic and random errors in the indicator dilution volume estimates were observed. The systematic errors appeared to be due to the rapid removal of a fixed amount of marker from peritoneal fluid, while the random errors were caused by the rapid appearance of a variable amount of endogenous chromogen. The behavior of the markers observed in this study was not consistent with the assumptions commonly used to analyze volume transport in peritoneal dialysis.

Influence of shaking on peritoneal transfer in rats

To determine the role played by stagnant peritoneal fluid layers in the diffusion of solutes between peritoneal cavity and blood, we measured peritoneal transfer of urea, creatinine, [14C]-L-glucose and protein in anesthetized rats shaken at varying rates on an orbital platform shaker. The diffusion transfer rates of the low molecular weight solutes increased dramatically with shaking, with near maximal values obtained at a shaking rate of 250 RPM. The permeability area product (PA) for each of the low molecular weight solutes increased about fourfold with rapid shaking while the PA of protein increased by only about 50%. It seems likely while the PA of protein increased by only about 50%. It seems likely that shaking increased PA primarily via reduction of the thickness of stagnant peritoneal fluid layers, although increases in surface area or changes in tissue permeability cannot be excluded with certainty. We conclude that stagnant fluid layers probably are the rate limiting step in diffusive peritoneal transfer of low molecular weight solutes in stationary rats.

The mechanism of dextrose-enhanced peritoneal mass transport rates

The mechanism whereby hypertonic dextrose affects peritoneal transport was investigated in a short-term model of peritoneal dialysis using alert intact rabbits. During control (1.5% dextrose) dialyses osmotic ultrafiltration was 0.28 mg/kg/min, the clearance of potassium was 0.98, urea 0.54, phosphate 0.32, and dextrose (reverse) 0.21 ml/kg/min. With 4.25% dextrose, the ultrafiltration rate increased to 0.73 ml/kg/min (P less than 0.02), but solute transport did not increase despite the added convective flux. The posthypertonic exchanges did not differ from control despite the effect of residual dialysate contaminating this peritoneal lavage. By indicator dilution residual volume averaged 12% of total dialysate volume. Acute volume expansion by intravenous dextrose after desoxycorticosterone acetate (DOCA) pretreatment increased the ultrafiltration coefficient, potassium and urea clearances significantly, and DOCA alone was ineffective. It is suggested that in uremic humans hypertonic dextrose dialysis increases peritoneal mass transport rates because the absorbed dextrose causes extracellular volume expansion that cannot be eliminated promptly. No evidence of a direct effect of dextrose augmenting peritoneal permeability was detected.

Amphotericin selectively increases peritoneal ultrafiltration

Because amphotericin B is known to affect transport rates across biologic membranes, the effects of this agent on transport parameters in an animal model of peritoneal dialysis were investigated. When amphotericin B in doses ranging from 0.5 to 25 mg/kg was instilled intraperitoneally with commercial dialysis solution, diffusive clearances of phosphate and urea did not differ from control values measured in the same animals, and only a modest increase in potassium clearance was detected. Ultrafiltration due to the osmotic gradient induced by the dextrose content of the dialysis solution increased significantly to 0.31 mL/kg/min with amphotericin B, compared with control values of 0.18 mL/kg/min. The drug did not affect dextrose transport and the osmotic gradient did not differ in the two groups. Hence, the ultrafiltration coefficient was higher with amphotericin B (14 microL/kg/min/mosm), than during control dialyses (6 microL/kg/min/mosm). Increased water flux was detected at the lowest dose and there was no dose relationship over the range studied. Amphotericin B may be the type of agent that will be clinically useful in patients with reduced peritoneal ultrafiltration capacity, and safer analogues should be explored.

Acceleration of peritoneal mass transport by drugs and hormones

The major restrictions to the transport of solute and solvent across the peritoneum are the limited peritoneal blood flow, area and permeability. Recent investigations have demonstrated that several vasoactive drugs influence transport parameters. Isoproterenol, nitroprusside, dipyridamole and dopamine exemplify drugs that dilate the splanchnic vasculature, thereby augmenting transport, whereas vasoconstriction induced by l-norepinephrine decreases clearances. The tissue prostaglandins affect peritoneal mass transport in accord with their known vasoactive effects, suggesting a role in modulating peritoneal blood flow. The gastrointestinal hormones vasodilate the splanchnic circulation. Exposure of the endothelial surface to glucagon markedly increases peritoneal mass transport, while secretin increases the ultrafiltration rate significantly. These preliminary studies suggest the possible future clinical use of drugs and hormones to augment the efficiency of peritoneal dialysis.