2ND-MESSENGER AND CAMP-DEPENDENT PROTEIN-KINASE RESPONSES TO DEHYDRATION AND ANOXIA STRESSES IN FROGS

The effects of whole body dehydration (up to 40% of total body water l ost) or anoxia exposure (up to 2 days under N-2 gas) at 5 OC on tissue levels of adenosine 3'-5' cyclic monophosphate (cAMP) and the percent age of cAMP-dependent protein kinase present as the free catalytic sub unit (PKAc), as well as the levels of the protein kinase C (PKC) secon d messenger, inositol 1,4,5-trisphosphate (IP3), were assessed in two anurans, the freeze-tolerant wood frog, Rana sylvatica, and the freeze -intolerant leopard frog, Rana pipiens. Dehydration of wood frogs resu lted in a rapid elevation of liver cAMP and PKAc; cAMP was 3.4-fold gr eater than control values in animals that had lost 5% of total body wa ter, whereas PKAc was elevated threefold in 20% dehydrated frogs. Thes e results indicate protein kinase A mediation of the liver glycogenoly sis and hyperglycemia that is induced by dehydration in this species. Skeletal muscle PKAc content also rose with dehydration but neither cA MP nor PKAc was affected by dehydration in leopard frog tissues. Anoxi a exposure had different effects on signal transduction systems. PKAc was elevated after I h anoxia in R. sylvatica brain and was sustained over time but the enzyme was unaffected in other organs; by contrast, R. pipiens showed variable responses by PKAc to anoxia in three organs . Both species showed rapid (within 30 min) and large (3 to 7.8-fold) increases in IP3 in liver of anoxic frogs that decreased slowly with c ontinued anoxia. IP3 also increased quickly in heart of anoxia-exposed wood frogs. This suggests that PKC may mediate various metabolic adju stments that promote hypoxia/anoxia resistance such as coordinating me tabolic rate depression. A progressive rise in liver IP3 during dehydr ation in wood frogs (reaching fourfold higher than controls in 40% deh ydrated animals) may also mediate similar hypoxia resistance adaptatio ns under this stress since anurans experience progressive hypoxia due to increased blood viscosity when water loss reaches high values. The patterns of second messenger and PKAc changes in wood frog liver durin g dehydration closely parallel the changes seen in these same paramete rs during natural freezing suggesting that the freeze tolerance of sel ected terrestrially hibernating anurans may have evolved out of variou s anuran mechanisms of dehydration resistance.