An in situ ligated loop procedure was applied to dissect transmural calcium transport in the intestine into two components, a saturable and a nonsaturable process. The existence of two such processes was confirmed in the duodenum, but ileal calcium transport was devoid of the saturable component. There was a small saturable component in the upper jejunum. The level of CaBP, the vitamin D-dependent cytosolic calcium-binding protein (Mr, approximately or equal to 9,000), corresponded to the magnitude of the saturable component. No CaBP was detected in the ileum. Vitamin D dependence of the saturable component was established by inducing it in the duodenum of vitamin D-deficient animals following intraperitoneal injection of 1,25-dihydroxyvitamin D3. In these same animals, conversely, the ileum did not respond to exogenous 1,25-dihydroxyvitamin D3. This confirms the absence in the ileum of the saturable component of transmural calcium movement and the fact that the nonsaturable component is not vitamin D dependent. Everted sac experiments also showed that duodenal sacs from vitamin D-replete or -repleted animals transported calcium against a chemical gradient, whereas ileal sacs did not. Vitamin D regulation of intestinal calcium absorption thus occurs only in the proximal intestine, even though calcium is absorbed down its chemical gradient all along the small intestine.
To investigate the nonsaturable, paracellular pathway of intestinal Ca absorption, the luminal contents of 12-cm segments of the intestine of 8-wk-old male Sprague-Dawley rats were analyzed for pH, sojourn time and soluble and insoluble Ca over a 24-h period. The rats had been fed one of two high Ca diets for 2 wk: 1.5% Ca (diet group 3a) and 3.1% (diet group 5a). The pH of the small intestine increased from < 6.6 to > 8.0 from duodenum to ileum; transit time increased from 2.5 min in the duodenum to 58 min in the distal ileum, with the entire ileum accounting on the average for 74% of the transit time of 3 h. The amount of Ca solubilized throughout the intestine was 32 +/- 3.3 mumol in diet group 3a and 53 +/- 5.3 mumol in diet group 5a, i.e., 2.7% and 2.0% of the total luminal Ca. Because absorption by diet group 3a was 1.45 +/- 0.23 mmol/d and that by diet group 5a was 2.50 +/- 0.18 mmol/d, the amounts absorbed were 45.3 and 47.1 times greater than present in the lumen in soluble form at any one time. Thus, over a 24-h period, an average of 3.2% (46.2/1440) of the soluble Ca present in the lumen at any time was absorbed per min. Calculations involving the gradient between luminal and plasma Ca show that the rate of Ca diffusion from lumen to blood is < 2% of what it would be if the paracellular path were unrestricted. Thus, intestinal sojourn time, Ca solubility and mucosal permeability to Ca are factors that determine the rate of passive Ca absorption.
Calcium absorption was studied by an in situ ligated-loop procedure in 9-wk-old male Wistar rats that had been placed from weaning on one of three semisynthetic regimens, 0.17% Ca, 0.44% Ca, or 0.44% Ca plus lactose. Lactose was added because it is known to increase intestinal calcium retention. When the amount of calcium absorbed was expressed as a function of calcium instilled in the loop, it became possible to describe absorption as the sum of a hyperbolic and a linear function, equivalent to a saturable and a nonsaturable process, respectively. The slope of the nonsaturable component was independent of prior calcium intake, while the maximum saturable flux (Jmax) decreased as calcium intake increased. Analysis of the duodenal content of the vitamin D-dependent calcium-binding protein (CaBP, Mr congruent to 10(4)) revealed a positive relation between Jmax and CaBP. Thus, vitamin D appears to be implicated in the saturable, but not in the nonsaturable, component of calcium absorption.
Duodenal calcium transport was resolved into a saturable and a nonsaturable process by means of an in situ ligated loop procedure applied to Wistar rats at 3, 12, 19, 24, 30, 40, 60, 110, and 150 days of age. All postweaning animals were males that had been placed on a 1.5% calcium, 1.5% phosphorus semisynthetic diet. Duodenal calcium-binding protein (CaBP) levels were determined at all ages. The newborn rat had no saturable transport component and no CaBP. Its nonsaturable component was very high. With increasing age the saturable component and CaBP varied biphasically, increasing steeply until the animals were about 35 days old; thereafter, each decreased to low but detectable values. The nonsaturable component, on the other hand, decreased in near-linear fashion in the first 35 days; in animals beyond that age it remained invariant. The difference in age dependence between the saturable and nonsaturable components may be considered to constitute additional evidence for the existence of the two transport processes. CaBP and the saturable transport process were highly correlated, further proof that both are vitamin D dependent. Histological studies have revealed the presence of many vacuoles in the intestinal cells of the very young rats; these vacuoles were absent in rats older than 35 days. It is suggested that these vacuoles may be implicated in a pinocytosislike nonsaturable transport that is superimposed on the nonsaturable, non-vitamin D-dependent calcium transport found in all enterocytes.
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