It has recently been shown that HSL is present in skeletal muscle and can be activated by both adrenaline and muscle contractions (Langfort et al. 1999, 2000). The effects of the two stimuli are partially additive (Langfort et al. 2001). Adrenaline acts via protein kinase A. However, the mechanism involved in the contraction-mediated activation of HSL is totally unknown. This study was performed to determine whether protein kinase C (PKC) is involved in the contraction-mediated activation of HSL. The activated form of the enzyme can be measured with a triacylglycerol substrate (HSL (TG) activity). Incubated soleus muscles from humanely killed 70 g male rats were electrically stimulated either in the presence or absence of calphostin C (2.5 µM), which inhibits the diacylglycerol binding site on PKC. Calphostin C inhibited the contraction-mediated increase in HSL (TG) activity (control: 0.64 ± 0.04 (S.E.M.) (basal) vs. 0.99 ± 0.07 (contractions) mU (mg protein)^-1 (P < 0.05, ANOVA); Calphostin C: 0.71 ± 0.05 (basal) vs. 0.73 ± 0.04 (contractions) mU (mg protein)^-1 (P > 0.05), n = 8-9 in all groups). In contrast, calphostin C did not influence the HSL (TG) activity during adrenaline stimulation (P > 0.05). The basal enzyme activity was not influenced by calphostin C (P > 0.05). Also another PKC inhibitor, bisindolylmaleimide I (1 µM), which blocks the ATP binding site on PKC inhibited the contraction-mediated increase in HSL (TG) activity (control: 0.54 ± 0.03 (basal) vs. 1.01 ± 0.12 (contractions) mU (mg protein)^-1 (P < 0.05); bisindolylmaleimide I: 0.65 ± 0.02 (basal) vs. 0.73 ± 0.08 (contractions) mU (mg protein)^-1 (P > 0.05), n = 6 in all groups). Again, no effect was observed either during adrenaline stimulation or basal conditions (P > 0.05).

In conclusion, the study indicates that PKC is involved in the contraction-mediated activation of HSL.