Figure 1.
Simplified representation of the pathway leading to the breakdown of glycogen into glucose in vertebrate liver. Protein kinase A (PKA) and its catalytic subunit (PKAc), glycogen phosphorylase kinase (PhK), glycogen phosphorylase (GPb and GPa) are the main enzymes involved in this process. The pathway involves a phosphorylation cascade, leading to glycogen breakdown. Inactive or dephosphorylated enzymes are italicized. Enzymes assayed in this study are inside grey boxes.
Figure 2.
Hepatic glycemic response during freezing and thawing.
Liver glycogen content (A) and glucose output (B) during freezing and thawing in Alaskan and Ohioan R. sylvatica as compared to that in unfrozen frogs (U) (mean ± SEM; N = 4–8). Glucose output (B) was calculated by subtracting the mean liver glycogen content (µmol liver −1) remaining at each sample from the mean liver glycogen content of unfrozen frogs. Asterisk indicates that the value differs from the mean for the corresponding sample of unfrozen frogs (Dunnett’s, P<0.05). Some overlapping values were slightly offset along the abscissa for clarity.
Figure 3.
Activity of liver PKAc during freezing and thawing.
Activity of hepatic PKAc (mean ± SEM; N = 4–8) in frozen and thawed Alaskan and Ohioan R. sylvatica as compared to that in unfrozen frogs (U). Asterisk indicates that the value differs from the mean for the corresponding sample of unfrozen frogs (Dunnett’s, P<0.05); dagger indicates that the value differs between populations (Bonferroni, P<0.05). Some overlapping values were slightly offset along the abscissa for clarity.
Table 1.
Percentage of enzyme present as PKAc and total PKA activity during freezing and thawing in R. sylvatica.
Figure 4.
Liver PKAc protein levels during freezing and thawing.
(A) Representative immunoblots of hepatic PKAc protein in Alaskan and Ohioan unfrozen (U), 48 h-frozen (F), and 120 h-thawed (T) frogs (2 samples per treatment). (B) Hepatic PKAc protein amounts (mean ± SEM; N = 4–8) during freezing (48 h) and thawing (120 h) in Alaskan and Ohioan R. sylvatica as compared to that in unfrozen frogs (U). Asterisk indicates that the value differs from the mean for the corresponding sample of unfrozen frogs (Dunnett’s, P<0.05); dagger indicates that the value differs between populations (Bonferroni, P<0.05).
Figure 5.
Activity of liver GPa during freezing and thawing.
Activity of hepatic GPa (mean ± SEM; N = 4–8) during freezing and thawing in Alaskan and Ohioan R. sylvatica as compared to that in unfrozen frogs (U). Asterisk indicates that the value differs from the mean for the corresponding sample of unfrozen frogs (Dunnett’s, P<0.05); dagger indicates that the value differs between populations (Bonferroni, P<0.05).
Table 2.
Percentage of enzyme present as GPa and total GP activity during freezing and thawing in R. sylvatica.
Figure 6.
Liver GP protein levels during freezing and thawing.
(A) Representative immunoblots of hepatic GP protein in Alaskan and Ohioan unfrozen (U), 48 h-frozen (F) and 120 h-thawed (T) frogs (2 samples per treatment). (B) Hepatic GP protein amounts (mean ± SEM; N = 4–8) during freezing (48 h) and thawing (120 h) in Alaskan and Ohioan R. sylvatica as compared to that in unfrozen frogs (U). Dagger indicates that the value differs between populations (Bonferroni, P<0.05).