PKC isoform expression, basal contractile function, and cTnI phosphorylation in young compared with older F344BN rat hearts. (A) Quantitative analysis showed no significant change in basal expression of PKCα, δ, and ε protein levels in myocytes from young adult (6 mo) versus older (26 +34 mo data = ≥26 mo) rats. PKC isoform expression is normalized to a portion of the silver-stained (Ag stain) gel. Quantitative results in each panel are expressed as mean ± SEM (n = number of rat samples in A and C). Statistical comparisons in A and C used an unpaired Student’s t test (*, P < 0.05). (B) Analysis of composite myocyte contractile function in 6-, 18-, 26-, and 34-mo-old rat myocytes paced at 0.2 Hz (n = number of myocytes from four or more different rats). Resting sarcomere length, peak shortening amplitude, the rates of shortening and relengthening, and the TTR_50% were analyzed with a one-way ANOVA and post hoc Dunnett’s tests (*, P < 0.05 versus 6 mo). (C) Quantitative analysis of cTnI phosphorylation at cTnI p-S23/24, p-S45, and total cTnI expression in 6- versus ≥26-mo-old rat hearts. Total cTnI levels are normalized to a portion of the silver-stained gel. The p-S45 levels are significantly elevated in rat hearts from ≥26-mo-olds compared with 6-mo-olds (middle panel), while there were no differences in cTnI p-S23/24 (left panel) or total cTnI (right panel) expression. Note that representative Western blots for A are shown in Fig. 5, A, B, and E, and the representative Western results for basal cTnI p-S23/24, p-S45, and cTnI are shown in Fig. 5 F.