The involvement of the Ca(2+)-dependent and Ca(2+)-independent, insulin-sensitive pathway in the alpha 1-receptor-control of glycogenolysis was investigated in hepatocytes from young adult, mature adult, and senescent rats. Upon chelation of extracellular Ca2+, phenylephrine caused a similar increase in glucose output that was potently inhibited by insulin, indicating the presence of both pathways in each age group. From the age-related decreasing sensitivity of the Ca(2+)-dependent pathway toward verapamil and nifedipine, and toward insulin, we suggest that the contribution of Ca(2+)-fluxes in eliciting glycogenolysis through the Ca(2+)-dependent pathway decreases upon aging. Both pathways were inhibited by the protein kinase C (PKC) activator, 4 beta-phorbol 12-myristate 13-acetate (PMA); the inhibitory effect was decreased in hepatocytes from mature adult and senescent rats. In conclusion, our results favor the idea that a Ca(2+)-dependent and a Ca(2+)-independent, insulin-sensitive pathway remain involved throughout the life span. We provided the evidence for an impaired regulatory role of protein kinase C and calcium in hepatocytes from the older age groups.
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