Thyroid replacement in thyroparathyroidectomized dogs.

The thyroparathyroidectomized dog is an important experimental model of hypoparathyroidism and has been widely utilized for acute and chronic studies of the physiologic role of parathyroid hormone on systemic and renal acid-base, electrolyte and vitamin D physiology. Despite widespread use of this model, the appropriate thyroid hormone replacement dose necessary for achievement of postoperative euthyroidism has not been established for this species. Accordingly, serum thyroxine (T4) concentration was measured prior to and following chronic thyroparathyroidectomy in dogs given thyroid hormone replacement at different doses and routes of administration: sodium levothyroxine, 2.4 micrograms/kg daily in one or two divided subcutaneous doses (group I, n = 9), 20 micrograms/kg daily in two divided oral doses (group II, n = 8), and a wide range of intravenous doses (group III, n = 3). Group I dosage was based on the reported T4 production rate in dogs and is slightly greater than the reported production rate in man. Group II dosage was based on published clinically derived estimates of required replacement amounts. With subcutaneous replacement (group I) serum T4 concentration decreased from a preoperative value of 1.80 +/- 0.20 to 0.60 +/- 0.10 microgram/100 ml (p less than 0.001) following thyroparathyroidectomy. With oral replacement (group II), serum T4 concentration after thyroparathyroidectomy was not significantly changed from control (1.88 +/- 0.22 vs. 1.66 +/- 0.43 microgram/100 ml). Group III studies revealed that both total and free serum T4 concentration could be normalized after thyroparathyroidectomy with an intravenous dose of 10 micrograms/kg daily. As found with subcutaneous administration, intravenous replacement with lesser amounts of T4 than 10 micrograms/kg daily (i.e. 2.5 micrograms/kg) resulted in significant decreases in serum T4 concentration from control.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal and systemic acid-base effects of chronic hypoparathyroidism in dogs.

MEtabolic alkalosis has been reported in patients with chronic hypoparathyroidism under conditions of uncontrolled diet and medication intake. Hypoparathyroidism has also been reported to result in increased renal bicarbonate reabsorptive capacity in acutely bicarbonate-loaded dogs. However, the acid-base effects of experimentally induced chronic hypoparathyroidism have not been investigated in any species. Accordingly, we investigated the chronic effects of hypoparathyroidism by thyroparathyroidectomy (TPTX) plus thyroxine replacement on renal regulation of plasma acid-base composition under metabolic balance conditions of normal dietary acid load (group I) and alkali load (group II, 9.0 meq/kg HCO3(-) daily) in dogs ingesting a normal Cl-, high Ca2+ diet throughout study. For groups I and II pre-TPTX: [HCO3(-)]p, 19.7 +/- 1.0, 20.1 +/- 0.9 meq/liter. Plasma acid-base composition (days 5-10) was unchanged by TPTX: delta [HCO3(-)]p, -0.7 +/- 0.4, 0.0 +/- 0.2 meq/liter; delta [H+]p, 0 +/- 1, -1 +/- 0 neq/liter, NS from control. A reduction in plasma total calcium concentration ([CaT]p) occurred and persisted (group I: [CaT]p, -1.6 +/- 0.2 mg/100 ml, P less than 0.01, day 1 and -1.2 +/- 0.9, days 5-10; group II: -1.4 +/- 0.3 mg/100 ml, P less than 0.01, day 1 and -2.3 +/- 0.4, days 5-10). No significant change in net acid or Cl- excretion occurred following TPTX. Thus, chronic hypoparathyroidism characterized by a chronic reduction in [CaT]p does not result in significant alterations in renal regulation of plasma acid-base composition in the dog.

Renal and systemic acid-base effects of chronic spironolactone administration.

Studies in dogs were carried out to investigate the effects of chronic administration of the mineralcorticoid antagonist spironolactone (15 mg/kg orally) on renal and systemic acid-base metabolism. In adrenalectomized dogs administered fixed mineralocorticoid and glucocorticoid replacement, spironolactone resulted in a definite renal antimineralocorticoid effect, as evidenced by natriuresis and chloruresis, and sustained metabolic acidosis and hyperkalemia due in part to impaired renal secretion of hydrogen and potassium. In adrenalectomized dogs receiving physiological glucocorticoid without mineralocorticoid, metabolic acidosis also occurred, but a marked stimulatory effect of spironolactone on net acid excretion occurred in association with increased urinary SO4-2 and total nitrogen excretion. Accordingly, spironolactone results in sustained renal tubular acidosis when administered in the presence of constant physiological levels of mineralocorticoid and glucocorticoid steroids. When administered under conditions of complete lack of mineralocorticoid activity, spironolactone exerts systemic and renal acid-base effects similar to those of a glucocorticoid steroid, namely, increased protein catabolism and sulfuric acid production with resultant extrarenal metabolic acidosis associated with increased net acid excretion.

Effects of glucocorticoid steroids on renal and systemic acid-base metabolism.

Clinical states of hyperglucocorticoidism are associated with renal metabolic alkalosis, yet the systemic and renal acid-base response to chronic administration of glucocorticoid steroids (dexamethasone, triamcinolone) possessing little or no mineralocorticoid activity has not been investigated. In balance studies studies in dogs administration of triamcinolone (Tcn), 1.0 mg . kg-1 . day-1 for 6-9 days (group I, n = 5), resulted in a persistent reduction in urine pH and increase in net acid excretion (NAE), and in the excretion of urinary unmeasured anions (C+NH4,Na;K minus A-Cl,HCO3,Pi), which were identified as organic anions and sulfate. A significant degree of metabolic acidosis occurred initially (delta [HCO3-]p, -3.4 meq/liter, P less than 0.05, day 1). As Tcn administration was continued, the cumulative increment in net acid excreted exceeded the cumulative increment in urinary unmeasured anion excreted and [HCO-3]p returned to pre-Tcn control values and remained stable thereafter. In the steady state of Tcn administration plasma potassium concentration and renal potassium clearance were not significantly different from pre-Tcn control, in contrast to the findings of hypokalemia and increased renal potassium clearance during chronic administration of deoxycorticosterone (DOC). Triamcinolone did not result in antinatriuresis or antichloruresis. Chronic administration of a 10-fold smaller dose of Tcn (0.1 mg . kg-1 . day-1) in an additional group (group III) also resulted in a persisting reduction in urine pH and an increase in net acid excretion that exceeded unmeasured anion excretion and resulted in a small increase in steady-state plasma bicarbonate concentration. These results suggest that chronic administration of potent glucocorticoid steroids results in 1) a persisting increase in endogenous acid production, and 2) stimulation of renal hydrogen ion secretion that was of greater degree than accounted for by the increment in endogenous acid production and that was not accompanied by renal mineralocorticoid effects on sodium and potassium transport.

Lipoid nephrosis appearing as acute oliguric renal failure.

Acute oliguric renal failure previously was reported to develop in patients with preexisting idiopathic nephrotic syndrome in association with clinical evidence of vascular volume depletion. We describe an 81-year-old man without recent proteinuria or evidence of preexisting nephrotic syndrome in whom acute oliguric renal failure developed. Renal biopsy disclosed minimal change disease. Nephrotic range proteinuria without severe hypoalbuminemia was detected during the 25-day course of oliguric renal failure. Renal vein thrombosis was excluded. Urine sodium concentration and fractional sodium excretion were reduced, yet left ventricular filling pressure was not subnormal and could be increased to supernormal levels without improvement in glomerular filtration rate. Oliguria and azotemia were corrected following initiation of glucocorticoid therapy. This case suggests that lipoid nephrosis can appear as acute oliguric renal failure without historical or physical evidence of preexisting nephrotic syndrome.

Effect of methotrexate and 5-fluorodeoxyuridine on ribonucleotide reductase activity in mammalian cells.

A number of studies in bacteria have indicated that deoxythymidine 5'-triphosphate may be a repressor or corepressor of ribonucleotide reductase. For determination of whether a similar regulating mechanism exists in mammalian cells, HeLa cells and partially hepatectomized rats were treated with either methotrexate, 5-fluorouracil, or 5-fluorodeoxyuridine in order to block thymidylate synthesis and consequently lower the intracellular pools of deoxythymidine 5'-triphosphate. In HeLa cells there was a significant (360 to 400 percent) increase in reductase activity in both the methotrexate and 5-fluorodeoxyuridine-treated cells. The administration of methotrexate to partially hepatectomized rats resulted in a 2.7-fold enhancement of the hepatectomy-induced increase in reductase activity, and the 5-fluorouracil treatment yielded a 60 percent increment in the increase of ribonucleotide reductase activity after partial hepatectomy. Cycloheximide prevented the increase in reductase activity after the exposure of HeLa cells to methotrexate and 5-fluorodeoxyuridine, indicating that the stimulation of ribonucleotide reductase activity was the result of enhancement of de novo enzyme synthesis rather than of enzyme activation. The data support the thesis that deoxythymidine 5'-triphosphate or a thymidylate metabolite may be involved in the regulation of ribonucleotide reductase levels in mammalian cells.