Ornithine Decarboxylase Exhibits Negative Thermal Modulation in the Sea Star Asterias vulgaris: Potential Regulatory Role During Temperature-Dependent Testicular Growth

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Abstract
The common northern sea star Asterias vulgaris is exposed to seasonal variation in temperature from 02 to 17.degree. C. A. vulgaris exhibits an annual reproductive cycle, i.e., the testes increase slowly in size during fall and winter, and reach maximal size in early spring. Slow testicular growth in the winter has been attributed to low field temperatures. Previous studies indicate that the specific activity of ornithine decarboxylase and the levels of the polyamines putrescine, spermidine, and spermine decrease in mid-winter and increase in the spring, coincident with changes in field temperatures. Kinetic studies show that ornithine decarboxylase assayed from individuals collected in March exhibits negative thermal modulation (KM of ornithine is 0.22 mM and 0.65 mM at 15 and 0.degree. C, respectively). Q10 values are highest at low substrate concentrations and at low temperatures. We hypothesize that during the cold winter months a decrease in the amount of ODC and an increase in the apparent KM causes polyamine synthesis to decline, leading to decreased growth and development of the testis. We suggest that thermal modulation of ODC (and polyamine synthesis) is a mechanism by which seasonal temperature fluctuations influence seasonal spermatogenesis in A. vulgaris. We further suggest that growth of various tissues in many other ectothermal invertebrates may be similarly controlled.

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