Metabolic acidosis (MA) has been implicated in the pathogenesis of both osteomalacia and osteopenia. Alterations in the secretion of parathyroid hormone and in the metabolism of vitamin D may contribute to such skeletal changes. To minimize the influence of these factors, quantitative bone histology and measurements of bone formation using double tetracycline labeling were done in thyroparathyroidectomized (TPTX) rats with MA induced by ammonium chloride (TPTX-A), and in both non-acidotic TPTX (TPTX-C) and intact (C) controls. To evaluate the response of both cortical and trabecular bone to MA, histologic studies were done at three separate sites in the tibia, cortical bone from the mid-shaft, and trabecular bone from the epiphysis and from the metaphysis. Plasma pH was lower in TPTX-A, 7.24 +/- 0.10, than in either TPTX-C, 7.39 +/- 0.03, or C, 7.43 +/- 0.04, P less than 0.01, and urinary hydroxyproline excretion increased from 89.8 +/- 8.7 in TPTX-C to 150.2 +/- 25.9 micrograms/mg/creatinine in TPTX-A, P less than 0.01. Resorption surface at the epiphysis increased from 1.8 +/- 0.6% in TPTX-C to 4.0 +/- 1.6% in TPTX-A, P less than 0.05, values not different from those in C, 3.1 +/- 1.1%. Resorption surface was unchanged at other skeletal sites, but total bone volume at the metaphysis fell from 15.5 +/- 5.6% in TPTX-C to 9.0 +/- 4.3% in TPTX-A, P less than 0.05. Bone formation was reduced at each skeletal site in TPTX-A vs. TPTX-C, P less than 0.05 for all values, but histologic evidence of osteomalacia was not observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Gentamicin uptake and toxicity was studied in a nontransformed cell line obtained from the S1 segment of the proximal tubule epithelium of a transgenic mouse. Cytotoxicity was assayed using the dye 3-(4,-5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Gentamicin uptake was assayed by a fluorescence polarization assay. No differences in toxicity were found among cells incubated for 4 hours in complete culture medium, enriched Kreb's buffer alone, or enriched Krebs' buffer with added 300 micrograms/mL gentamicin, 0.5 mmol/L L-lysine, or gentamicin plus L-lysine. Uptake of 300 micrograms/mL gentamicin was minimal at zero time and increased as a function of time. Uptake of gentamicin at 4 hours was positively correlated with medium gentamicin concentration. Addition of 0.5 mmol/L L-lysine inhibited uptake of 300 micrograms/mL gentamicin 38.9 +/- 10.2%. No other amino acid, including D-lysine or arginine, significantly changed gentamicin uptake. The authors conclude that gentamicin and L-lysine share a specific uptake mechanism located in the apical membrane of renal proximal tubule cells.

To examine the mechanism responsible for enhanced calcium mobilization from bone in metabolic acidosis, we evaluated the effect of colchicine and calcitonin, two blockers of cell-mediated bone resorption, on the calcemic response to acute metabolic acidosis in thyroparathyroidectomized rats. Metabolic acidosis lasting 16 hr and induced by the feeding of NH4Cl led to a significant rise in serum calcium of 1.2 to 1.9 mg/dl. The administration of colchicine or calcitonin led to a decrement in serum calcium of 1.1 +/- 0.2 (P less than 0.01) and 0.7 +/- 0.2 mg/dl (P less than 0.05), respectively. Cyclic AMP levels in calvaria from rats with metabolic acidosis and from control rats were not different. These data suggest that mobilization of calcium from bone which occurs in metabolic acidosis is due, in part, to increased bone resorption, which is mediated by a cAMP-independent mechanism.

The sites of enhanced phosphate (PO4) reabsorption after PO4 deprivation were investigated before and after infusion of parathyroid hormone (PTH) in acutely thyroparathyroidectomized rats. Animals were fed either a control PO4 diet (1.6% P) or a low PO4 diet (0.025% P) for 2 days or 7-10 days. In control rats, PTH decreased PO4 reabsorption in the proximal tubule, loop of Henle, and distal convolution. PO4 reabsorption in the proximal tubule was enhanced after 2 days of PO4 deprivation. In this group, proximal PO4 reabsorption was decreased by PTH but remained greater than in control rats (70 +/- 6 vs. 45 +/- 6 pmol/min; P less than 0.025). After PTH, PO4 reabsorption increased in the loop of Henle from 3 +/- 0.5 to 13 +/- 2 pmol/min (P less than 0.005), whereas it was unaltered in the distal convolution in PO4-deprived rats. PTH markedly increased fractional excretion of PO4 in control rats but not in PO4-deprived rats. After prolonged PO4 deprivation, PO4 reabsorption along the nephron was unaltered by PTH. These results demonstrate that acute PO4 deprivation enhances PO4 reabsorption in the proximal tubule, although the phosphaturic effect of PTH in this segment is not abolished. Resistance to the inhibitory effect of PTH on PO4 reabsorption in some portion of the loop of Henle and possibly also in the distal convolution accounts for the absence of a significant phosphaturic effect of the hormone in acutely PO4-deprived rats. Prolongation of PO4 deprivation results in unresponsiveness to PTH extending to the proximal tubule.

The inner medullary collecting duct (IMCD) is an important site of action for arginine vasopressin (AVP). To examine the mode of action of AVP in this segment, we measured the change in transepithelial resistance of cultured rat IMCD cells grown to confluence on collagen-coated Millicell culture plate inserts in response to AVP. Resistance was measured by use of an EVOM voltage-ohm meter. AVP at 10(-11)-10(-8) M caused a fall in resistance of 6.9 +/- 1.3 to 14.0 +/- 1.4 omega.cm2 (P less than 0.05 to less than 0.01 vs. no AVP), which was reversed by removal of AVP or addition of 10(-6) M amiloride. Pretreating the apical surface of IMCD cells with trypsin had no effect on resistance but totally prevented the antidiuretic hormone-induced fall in resistance. Pretreating the apical surface with trypsin and amiloride did not prevent the fall in resistance to AVP. Addition of 10(-9) M AVP or 10(-6) M forskolin increased 2-min adenosine 3',5'-cyclic monophosphate (cAMP) accumulation by 55 or 96%, respectively. Stimulation of endogenous cAMP accumulation by forskolin or the addition of exogenous 8-bromo-cAMP caused no change in resistance. To examine the relationship between intracellular calcium [( Ca2+]i) and AVP action, the response of [Ca2+]i to AVP was measured by use of fura-2. AVP induced no change in [Ca2+]i in IMCD cells in suspension, on glass cover slips, or on permeable supports. Ionomycin (25 nM) increased [Ca2+]i in IMCD cells and lowered resistance across monolayers, but the fall in resistance was not blocked by amiloride.(ABSTRACT TRUNCATED AT 250 WORDS)