Chotoo I. Bhagat

Studies on the inhibitory effect of bacitracin on 125I-labelled insulin internalization in the rat hepatocyte

Previous studies have suggested that transglutaminase has a role in the internalization of some polypeptide hormones and is inhibited by the antibiotic, bacitracin. Bacitracin has been used in insulin-receptor studies to inhibit extracellular degradation of 125I-labelled insulin. The aim of this study was to investigate bacitracins effect on 125I-labelled insulin-receptor interactions in isolated rat hepatocytes. 1 g/l bacitracin increased cell-associated 125I-labelled insulin insulin at 20, 30 and 37 degrees C (P less than 0.001, 0.0005 and 0.0005, respectively). At 5 and 15 degrees C (internalization does not occur), bacitracin did not affect cell-associated 125I-labelled insulin. The bacitracin effect was concentration dependent, increasing to 2 g/l. Scatchard analysis showed that bacitracin did not alter insulin receptor affinity or number. 1 g/l bacitracin abolished the effect of chloroquine. The increased cell-associated radioactivity with bacitracin was surface-bound in nature. 0.5 g/l bacitracin decreased 125I-labelled insulin degradation in hepatocyte suspensions (P less than 0.001) and in buffer previously incubated with hepatocytes (P less than 0.0005). More 125I-labelled insulin remained associated with cells during dissociation studies at 37 degrees C when the buffer contained 1 g/l bacitracin. Label that appeared in the buffer after 60 min was significantly more intact in the presence of bacitracin (P less than 0.025). These results suggest that bacitracin retards the internalization of 125I-labelled insulin in isolated rat hepatocytes.

Tubular maximum for calcium reabsorption: lack of diagnostic usefulness in primary hyperparathyroidism and familial hypocalciuric hypercalcaemia

The theoretical tubular maximum for calcium reabsorption was calculated and its usefulness assessed in the diagnosis and differential diagnosis of primary hyperparathyroidism and familial hypocalciuric hypercalcaemia. The sensitivity of the test in the diagnosis of primary hyperparathyroidism was only 12%. The theoretical tubular maximum for calcium reabsorption was recalculated after correction of calcium concentration in plasma for albumin concentration and for urinary sodium excretion. Despite these corrections, the sensitivity improved to only 44%. This contrasts with a sensitivity of 80% for the plot of fasting calcium excretion against calcium concentration in plasma in primary hyperparathyroidism. The calculation of theoretical tubular maximum for calcium reabsorption cannot be recommended as a useful test for distinguishing between primary hyperparathyroidism and familial hypocalciuric hypercalcaemia. The simple calculation of fractional excretion of calcium was a better test in distinguishing familial hypocalciuric hypercalcaemia from primary hyperparathyroidism.

Acute Effects of an Oral Calcium Load in Pregnancy and Lactation: Findings on Renal Calcium Conservation and Biochemical Indices of Bone Turnover

An oral calcium (Ca) tolerance test was used to compare the acute calcaemic, calciuric, parathyroid and bone turnover responses in 21 women at 36 weeks of pregnancy, 27 breast-feeding women studied 22 weeks postpartum and 27 control women. In all groups the oral Ca load increased plasma Ca and urinary calcium excretion (CaE), reduced intact PTH concentration (and consequently reduced renal phosphate and cyclic AMP excretion) and reduced hydroxyproline excretion (HypE, a biochemical index of bone resorption). There were no changes in the biochemical indices of bone formation, serum osteocalcin (elevated in the lactating group) and alkaline phosphatase, in any group. The pregnant women had the same fall in HypE and a greater calcaemic response than the controls. These results suggest that there is increased intestinal Ca absorption efficiency and a normal rate of bone resorption in late pregnancy. In contrast, the lactating women had a greater fall in HypE (from a baseline twice that of controls) and a significantly lower (p less than 0.001) rise in CaE, despite a calcaemic response similar to that of controls. Therefore, in lactation there is increased bone turnover and an increased capacity to reabsorb Ca in the renal tubule, compared to controls. An oral calcium supplement may benefit breast-feeding women, by reducing lactation-related elevated rate of bone resorption and consequent loss of trabecular bone.