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Acute exposure of 3T3-L1 undifferentiated fibroblasts to insulin or 4 beta-phorbol-12,13-dibutyrate (PDB) produced a moderate but significant stimulation of hexose transport (100% stimulation). In differentiated 3T3-L1 adipocytes, stimulation by insulin increased significantly (to 340%), while that by PDB remained at 130%. Total protein kinase C activity was 3-fold higher in 3T3-L1 fibroblast than adipocyte homogenates. PDB, but not insulin, induced migration of protein kinase C from the cytosol to the membrane, in both fibroblasts and adipocytes. Moreover, the hormone increased by 15% the protein kinase C activity of the cytosol. In 3T3-L1 fibroblasts, both insulin and PDB elicited a rapid (2 min lag) cytoplasmic alkalinization, measured with the fluorescent pH indicator bis-carboxyethyl carboxyfluorescein trapped in the cytoplasm. In 3T3-L1 adipocytes, PDB but not insulin elicited the cytoplasmic alkalinization. The alkalinization was prevented by amiloride or by replacing Na+ with either N-methylglucamine+ or K+. Stimulation of hexose transport by insulin or PDB was not affected by amiloride or Na+ substitution. It is concluded that: 1) Insulin and PDB have different effects on protein kinase C activity and subcellular distribution; 2) the responses of Na+/H+ exchange and hexose transport to insulin and PDB develop independently during differentiation of 3T3-L1 cells; 3) stimulation of Na+/H+ exchange and of hexose transport occur in parallel rather than in series in 3T3-L1 cells.
Klip et al. (Fri,) studied this question.