1. The role of the large intestine in the maintenance of K+ balance in uraemic patients established on long-term dialysis was studied with a rectal dialysis technique in 14 normal subjects, ten normokalaemic patients undergoing chronic ambulatory peritoneal dialysis (CAPD), and seven patients undergoing haemodialysis. Dietary K+ intakes in the normal subjects, CAPD patients and haemodialysis patients were 80–100 mmol/24 h, 70–80 mmol/24 h and 60–70 mmol/24 h, respectively.
2. At an initial intraluminal K+ concentration of 45 mmol/l, rectal K+ secretion in the CAPD patients (2.4 ± 0.4 μmol h−-1cm−-2) was greater than in normal subjects (1.2 ± 0.2 μmol h−-1 cm−-2, P < 0.02). Under similar conditions, rectal K+ secretion was also greater in the haemodialysis patients than in normal subjects, both pre-dialysis (3.7 ± 0.4 μmol h−-1 cm−-2, P < 0.001) and post-dialysis (2.4 ± 0.5 μmol h−-1 cm−-2, P < 0.05), even though haemodialysis decreased plasma K+ concentration from 5.3 ± 0.1 mmol/l to 3.5 ± 0.2 mmol/l (P < 0.001).
3. There were no significant differences in rectal Na+ absorption, rectal potential difference, plasma aldosterone concentration, or total body K+ content (measured by whole-body counting of 40K), between the normal subjects and either the CAPD or the haemodialysis patients.
4. These results indicate that K+ homoeostasis is maintained in uraemic patients undergoing long-term dialysis by a combination of K+ losses during dialysis, and enhanced large intestinal K+ excretion. The role of the large intestine appears to be particularly important at higher dietary K+ intakes, and the K+ secretory process is sensitive to changes in plasma K+ concentration.