Tubular Mechanism for the Spontaneous Hypercalciuria in Laboratory Rat

Abstract

A B S T R A C T Recently it has been observed that Ca excretion in laboratory rats does not follow a Gaussian distribution, with -10% of them excreting Ca at a rate of 2 SD above the group mean. This phenomenon has been described as spontaneous hypercalciuria (SH). Our studies were designed to define its mechanism. 48 Wistar rats were subjected to metabolic studies to identify SH, prospectively defined as Ca excretion 2 SD above the group mean during 7 d of dietary Ca deprivation (<0.03% by analysis), in the absence of hypercalcemia, P04 depletion, or exaggerated natriuresis.Progenies from SH rats were found to have significantly higher urine Ca/creatinine (micrograms per milligram) (male = 38 vs. 23, P < 0.05; female = 79 vs. 60, P < 0.005) with 7/20 males and 9/26 females having values 2 SD above the means of normal. After a 12-h fast and during 10% volume expansion with saline, clearance and micropuncture studies were performed on three groups of acutely parathyroidectomized female rats; (a) normocalciuric (N) progenies from the normal, (b) normocalciuric (NC) progenies from SH, and (c) hypercalciuric (HC) progenies from SH rats. Among these groups, there was no significant difference in body weights, glomerular filtration rate, plasma ultrafiltrable Ca (4.5, 4.6 vs. 4.7 mg/100 g), P04, and the fractional excretion (FE) late distal tubule (6.9 vs. 6.9 and 7.6%, P = NS). Although FECa exceeded late distal FDCa in all three groups, the increment was significantly greater in HC rats (7.02%) than both N (3.4, P < 0.05) and NC rats (3.05, P < 0.02).The effects of chlorothiazide (27.5 mg/kg/d, i.p. X 7 d) were evaluated in the female offsprings of the SH rats. Before chlorothiazide, average urine Ca/creatinine (253 vs. 77.2) and cyclic AMP (26.6 vs. 13.4 jAmol/mg creatinine, P < 0.001) on days 7 and 8 of the Ca-deprived diet were higher than the normal. On days 6 and 7 of chlorothiazide, average cyclic AMP (cAMP) excretion fell to normal range (11.7 vs. 12.7 ,umol/mg creatinine) as Ca excretion was reduced to normal (62 vs. 59.4 tsg Ca/mg creatinine).We conclude: (a) SH, as defined in this study, is an inheritable biochemical marker and renal in origin. (b) The hypercalciuria is independent of parathyroid hormone, changes in plasma Ca and tubular handling of Na. (c) As studied in the PTX and volume expanded conditions of our experiments, decreased Ca reabsorption in superficial proximal convoluted tubule is demonstrable, but the hypercalciuria is probably mediated by diminished Ca transport by the deep nephron. The unlikely possibility of increased secretion by the terminal nephron, however, remains to be excluded. (d) In normal rats, there is internephron heterogeneity in regard to Ca transport during saline loading.

Mechanism of hypercalciuria in genetic hypercalciuric rats. Inherited defect in intestinal calcium transport.

“…Also, serum 1,25(OH)2D3 levels in our hypercalciuric animals were equivalent (females) or lower (males) than controls, not higher as they report. The breeding experiments reported in the present study and those described previously (25,26) appear to have resulted in different mechanisms of hypercalciuria through selection of different defects in calcium metabolism.…”

“…The increased calcium excretion in these rats was thought to be a function of diminished calcium reabsorption in the deep (inaccessible to micropuncture) nephrons (25). The decreased renal tubular reabsorption of calcium was supported by persistent hypercalciuria in vitamin D-deficient animals whose serum 1,25(OH)2D3 and intestinal calcium transport rates were markedly reduced and were comparable to normocalciuric controls (26).…”

“…In the vast majority of patients with idiopathic hypercalciuria serum parathyroid hormone and urinary cAMP are normal (7,9,18,20,22 (25). The increased calcium excretion in these rats was thought to be a function of diminished calcium reabsorption in the deep (inaccessible to micropuncture) nephrons (25).…”

“…An animal model would be potentially valuable to test hypotheses regarding genetic and environmental components of hypercalciuria. Hypercalciuria occurs spontaneously among rats (24), and mating of hypercalciuric rats results in enrichment of hypercalciuria among offspring (25,26). Our approach has been to establish a colony of genetically hypercalciuric rats through selective inbreeding and use them to explore the pathogenetic role of intestinal calcium absorption and 1,25(OH)2D3 in hypercalciuria.…”

Pathophysiology of spontaneous hypercalciuria in laboratory rats. Role of deranged vitamin D metabolism.

“…Vitamin D deprivation is only feasible in laboratory animals. Rats with spontaneous hypercalciuria (SH), recently described (32) and investigated by us (33), provide a useful model, since the hypercalciuria is inheritable (17,24,33) and it is associated with fasting normocalcemia (33), proximal tubular dysfunctions (33), parathyroid hormone (PTH)-independent defects in the reabsorption of Ca (33) and P04 (34), as well as normal response to thiazide (33) orthophosphate and PTH (35). The 7th generation normocalciuric (NC) and SH Wistar rats were therefore subject to the following studies to evaluate the role of deranged vitamin D metabolism.…”