THE EFFECT OF THYROID AND THYROXINE ON THE CONCENTRATION OF CREATINE IN THE HEART, MUSCLE, LIVER, AND TESTES OF THE ALBINO RAT

The distribution, metabolism and function of creatine in the male mammalian reproductive tract: a review

Creatine is widely distributed throughout the male reproductive system in several mammalian species, and proteins involved in its metabolism and transport have been reported in a number of cell and tissue types. Creatine is synthesized within some organs, incorporating nitrogen from amino acid metabolism. Although creatine metabolism is obligatory for the motility of sea urchin spermatozoa, this does not appear to be the case for mammals. The possible functions of creatine within the reproductive tract are discussed.

Thyroid hormones and the creatine kinase system in cardiac cells

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

Thyrotoxic myopathy in mice: accentuation by a creatine transport inhibitor

To demonstrate the importance of creatine and phosphocreatine in skeletal muscle during periods of metabolic stress, thyrotoxicosis was induced in mice fed the creatine transport inhibitor, beta-guanidinopropionic acid (beta-GPA). Adding 2% of beta-GPA to the diet of normal mice inhibited weight gain and caused a 75% reduction of creatine and phosphocreatine concentrations in skeletal muscle. Addition of 0.25% or 2% of thyroid powder to the diet of normal mice was associated with hyperactivity, cardiomegaly, and a high mortality rate. Superimposing thyrotoxicosis on mice already depleted of creatine and phosphocreatine resulted in degeneration of muscle fibers. These results indicate that high concentrations of creatine and phosphocreatine are essential for the maintenance of muscle integrity during periods of metabolic stress.

Hormone regulation of cardiac energy metabolism. I. Creatine transport across cell membranes of euthyroid and hyperthyroid rat heart

Hyperthyroid rat heart was studied with the purpose of identifying the mechanism for the significant decrease in total creatine (free creatine plus phosphocreatine) observed in this pathology and its consequences on heart function. Administration of L-thyroxine in doses of 50-100 micrograms/100 g of body weight during a week resulted in a reversible decrease of the total creatine by 40-50%. Simultaneously, remarkable changes in the creatine transport system across the cardiac cell membranes were observed: both the maximal rate of its active uptake and its passive movement along its concentration gradient were enhanced. In euthyroid hearts, the parameters of creatine uptake (Km approximately or equal to 0.05 mM, Vmax = 20 nmole/min/g dry weight) were similar to those for skeletal muscle and the passive movement of creatine was negligible. In hyperthyroid hearts the latter rate was enhanced to 0.4 mumole min/g dry weight, this showing reversible damages in the cell membrane structure induced by L-thyroxine. This conclusion is consistent with observed penetration of colloidal lanthanum into the cells of hyperthyroid hearts. Perfusion of hyperthyroid rat hearts with 50 mM creatine significantly restored creatine content in the cells, Hyperthyroid hearts with decreased creatine content were found to develop ischemic contracture more rapidly and in higher extent than the euthyroid hearts. Increased sensitivity to ischemic damage may be related to decreased efficiency of energy channeling via phosphocreatine pathway.

Mechanism of triiodothyronine-induced creatinuria in the rat

Mechanism of triiodothyronine (T3)-induced creatinuria was studied in rats after a single injection of T3 (100 microgram/100 g per s). Urinary creatine excretion increased markedly during the 1st day after T3 injection. Peak of creatinuria was observed during the 2nd day after T3 injection. Excretions of nonprotein nitrogen and potassium were not different between control and T3-injected animals throughout the experimental period. Creatine reabsorption in the kidney was not influenced by T3 treatment. Radioactive creatines in both plasma and urine after [14C]creatine injection were higher in T3-injected animals than in controls, whereas the uptake of radioactive creatine by muscle was lower in the former. Creatine content in muscles decreased significantly 48 h after T3 injection. Moreover, T3 increased a fraction derived from muscle in urinary creatine, but it did not influence that derived from creatine synthesized de novo. These results suggest that both decreased creatine uptake by muscle and increased release of creatine from muscle play important roles in T3-induced creatinuria.