Small Intestinal Calcium Absorption in the Rat with Experimental Diabetes

Abstract

We previously studied downhill calcium absorption in the diabetic rat (luminal concentration 3.4 mM, serum ionized calcium, 1-1.1 mM): Duodenal absorption by diabetics was half that of controls whereas ileal absorption was normal (1). Although vitamin D intake and absorption of fat are normal in diabetic rats (l), we subsequently found vitamin D metabolism to be abnormal: serum concentrations of 1,25-dihydroxyvitamin D (1,25-(OH),D) in diabetics are one-eighth the levels in controls (2). Since 1,25-(OH)2D is the active metabolite of vitamin D that acts directly on the gut, the depression of 1,25-(OH),D concentration explains the decreased duodenal calcium absorption. Unexplained is the lack of effect of diabetes on ileal calcium absorption, since the ileum responds to vitamin D depletion and repletion (3). Since we had studied only downhill calcium absorption and had examined diabetics during the 5-to 10-day time span after injection (l), it was possible that we did not detect an effect of diabetes on ileal transport because of the luminal calcium concentration used or the time span after induction of diabetes selected for study (1). We therefore examined the effects of diabetes on duodenal and ileal calcium transport early (4 days) and late (11 days) after induction of diabetes and at luminal calcium concentraions below and above serum ionized calcium.Materials and Methods. We used male Sprague Dawley derived rats (Simonsen Labs, Gilroy, CA) housed in air-conditioned quarters and fed Teklad diet (1.5% calcium; 1.3% phosphorus). Animals to be made diabetic received intraperitoneal streptozotocin freshly prepared in citrate buffer, pH 4.5. The initial dose was 100 mg/kg body wt and 24 hr later they received a second injection of 25 mg/kg. Controls received similar injections of buffer alone. Diabetes was assessed by loss of body weight, glycosuria and elevated fasting blood glucose. To measure absorption, rats were anesthetized with intraperitoneal Nembutal and perfused in vivo by recirculating 10 ml of solution at a pump rate of 2.0 ml/min for 2 hr. Absorption was studied following an overnight fast at 4 days and I 1 days after injection of streptozotocin. The first 10 cm of duodenum and last 15-20 cm of ileum were perfused in each rat, and control and diabetic rats were always studied at the same time. Perfusates contained 0.8 mM calcium or 3.4 mM calcium, 45Ca (12 &liter, initial specific activity 2000 pCi/mg, New England Nuclear Corp.), 165 mM NaC1, and 50 mg/l phenol red.At the conclusion of perfusion blood was drawn from the inferior vena cava to measure glucose in serum (4). The duodenal and ileal segments were removed, measured for length, weighed, and the mucosa scraped from the underlying tissue. Total wet weight and total length of the small intestine were also obtained. Tissues were dried for 24 hr at 100" in a vacuum oven and reweighed to obtain dry weight. Calcium was measured by atomic absorption spectrometry in test solutions and luminal perfusates. 45Ca was measured by liquid scintillation countin...

Development of bone mineral loss in insulin‐treated diabetes: a 1 1/2 years follow‐up study in sixty patients

“…The finding of normal [24] or low [25, 261 serum immunoreactive parathyroid hormone concentration in serum from insulin treated diabetics, and high urinary excretion rates of calcium [25,261 favour the hypothesis that the bone mineral loss is due to a primary disturbance in bone metabolism [24,251 rather than secondary to a renal loss of calcium which would lead to secondary hyperparathyroidism [27]. Normal intestinal calcium absorption has been reported in a few human diabetics [24,28], whereas low or absent calcium absorption in the duodenum was found in diabetic rats [29]. In a few studies serum I ,25-dihydroxyvitamin D has been measured and was found to be normal [24] or decreased [30].…”

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Cav1.3 and TRPV6

“…Of note with reference to the debate over the role of calbindin‐D 9K , knockdown of calbindin‐D 9K in Caco‐2 cells had no effect on prolactin‐stimulated Ca 2+ absorption 130 . In addition, there is evidence from in vivo perfusion experiments that active absorption is present in ileum, where calbindin‐D 9K is very low 10,191,192 ; moreover, changes in absorption can precede changes in calbindin synthesis 193–195 . Taken in conjunction with the knockout mice results, the argument that low calbindin‐D 9K in ileum dictates a paracellular linear component of absorption seems no longer tenable 9,28 …”