Summary:Guanidinoacetic acid, a precursor of creatine, is an essential substrate for muscle energy metabolism. Since guanidinoacetic acid has been reported to be synthesized from arginine and glycine by glycine amidinotransferase (transamidinase) in kidney homogenates or slices, the purpose of this study was to provide evidence of guanidinoacetic acid synthesis in isolated tubules from rat kidneys, and to clarify the mechanism regulating it. Isolated rat tubules were incubated with various substrates. Guanidinoacetic acid was separated by high performance liquid chromatography and measured fluorometrically.Results obtained were as follows:(1) Guanidinoacetic acid was synthesized from arginine or canavanine and glycine in isolated rat tubules.(2) Z),Z,-Norvaline, ornithine and methionine suppressed guanidinoacetic acid synthesis.(3) Creatine suppressed guanidinoacetic acid synthesis, i.e. creatine was a negative feedback inhibitor of guanidinoacetic acid synthesis in this in vitro system.(4) Guanidinoacetic acid was not synthesized from hydroxyurea, citrulline, argininosuccinic acid or canaline.These data demonstrate that guanidinoacetic acid is synthesized only from arginine or canavanine and glycine, and that the guanidine cycle may not function fully in the rat renal tubule.

Urinary guanidinoacetic acid (GAA) is a sensitive marker for gentamicin nephrotoxicity in rats. This study assesses the usefulness of GAA concentrations in the diagnosis of renal tubular injury in diabetic nephropathy. Serum, urine, and renal cortex samples were obtained from rats 1, 2, and 3 weeks after streptozotocin injection (65 mg/kg body weight). Guanidinoacetic acid levels were measured by high-performance liquid chromatography. N-acetyl-beta-D-glucosaminidase (NAG) activity in urine was determined by an enzymatic method. GAA levels in serum, urine, and renal cortex were significantly decreased in diabetic rats compared with those in control rats. In contrast, urinary NAG activity was significantly increased in diabetic rats. Decreases in serum, urine, and renal cortical GAA levels were attenuated by insulin treatment. These results indicate that a high serum glucose level may affect GAA synthesis in the renal cortex and that urinary GAA may be a clinically useful indicator of renal tubular injury in diabetic nephropathy.

The kidney is the main site of guanidinoacetic acid synthesis and excretion. The aim of this study was to examine whether urinary guanidinoacetic acid is a sensitive indicator for diagnosis of early-stage gentamicin nephrotoxicity. Early-stage renal injury was induced in rats by a single intravenous injection of 5, 10, or 30 mg/kg body weight gentamicin. Twenty-four hours after injection all rats were killed. Blood, urine and tissue guanidino compound concentrations were determined by high performance liquid chromatography. Glycine amidinotransferase activity in tissues was assayed according to the method of Pilsum. Urinary guanidinoacetic acid excretion was decreased in 5 mg/kg gentamicin-treated rats in comparison to that in control rats, whereas urine N-acetyl-beta-D-glucosaminidase activity and beta2-microglobulin were unchanged. Guanidinoacetic acid concentration and glycine amidinotransferase activity in the kidney were significantly decreased in 5, 10, and 30 mg/kg gentamicin-treated rats; the decreases were dose-dependent. These results suggest that the urine guanidinoacetic acid concentration is a more sensitive indicator of renal injury than conventional indicators such as urine N-acetyl-beta-D-glucosaminidase and beta2-microglobulin.

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A 46-year-old man developed neuroleptic malignant syndrome with acute myoglobinuric renal failure after the discontinuation of sulpiride and maprotiline treatment. He showed the characteristic features of hyperpyrexia, altered consciousness, muscle rigidity, and autonomic dysfunction. Laboratory data showed lysis of skeletal muscle cells and renal impairment. Muscle biopsy revealed necrosis and regenerative changes in muscle fibers. Renal biopsy showed focal tubulitis and interstitial infiltration of small inflammatory cells. The combination of sulpiride and maprotiline has not previously been reported to be the cause of neuroleptic malignant syndromeand acute myoglobinuric renal failure.