Effects of partial and total colectomy on mineral and acid-base homoeostasis in the rat: magnesium deficiency, hyperphosphaturia and osteopathy, in the presence of high serum 1,25-dihydroxyvitamin D but normal parathyroid hormone

The effects of colectomy on acid-base status, extra-osseous and bone minera ls, calciotropic hormones and bone morphology have not yet been studied. To rectify this, groups of normally fed male rats were subjected to distal (n = 11), proximal (n = 12) or total (n = 12) colectomy. Sham-operated rats ( n = 12) served as controls. At 112 (+/-2) days after colectomy the followin g changes were noted: (1) weight gain was delayed; (2) faecal excretion of calcium and phosphorus was normal, whereas that of magnesium was increased; (3) intestinal calcium secretion and absorption of calcium and phosphorus were normal, but magnesium absorption was decreased; (4) urinary excretion of magnesium was also decreased, that of phosphorus was increased, and that of pyridinium and deoxypyridinium tended to be high; (5) the serum levels of ionized magnesium, total calcium, 25-hydroxyvitamin D and parathyroid ho rmone were normal, while that of 1,25-dihydroxyvitamin D was markedly eleva ted; and (6) bone magnesium and phosphorus content were decreased, but bone calcium was normal, and thus the bone calcium/phosphorus ratio was high. T hese abnormalities were associated with moderate metabolic acidosis, as ref lected by high urinary ammonium, low citrate and low total CO2, but normal blood gases. Significant structural abnormalities of bone were not detectab le, but trabecular bone tended to show rarefication. Distal colectomy had t he least effect, whereas proximal and total colectomies had a distinct effe ct, on these parameters. It is concluded that colectomy in the rat causes: (1) a syndrome of magnesium deficiency of intestinal origin, compensated me tabolic acidosis, urinary phosphorus loss, and high circulating 1,25-dihydr oxyvitamin D levels, with the degree depending on the extent of surgical re section; acid (2) brittle bones, a feature characteristic of low bone magne sium and more generalized magnesium deficiency. The mechanisms leading to t his syndrome are unknown, but altered tissue levels of magnesium and phosph orus may play a key role.