High levels of cytosolic free calcium inhibit dephosphorylation of insulin receptor and glycogen synthase.

Treatment of adipocytes with depolarizing concentrations of K+ (40 mM) for 60 min increased Ca2+]i from 158 +/- 28 nM to 328 +/- 38 nM. This significantly reduced (up to 80% inhibition) dephosphorylation of insulin receptor (IR), EGF receptor (EGF-R) and glycogen synthase (GS). The calcium channel blocker, nitrendipine (30 microM), or Ca2+ free medium completely prevented K(+)-induced inhibition of phosphoprotein phosphatase (PPTase). This effect of high Ca2+]i was completely reversible when the cells were returned into the non-depolarizing medium. Trypsin treatment (4 micrograms/ml) of the membrane fraction containing inhibited PPTase activity, restored dephosphorylation activity to normal suggesting that elevated Ca2+]i may inhibit PPTase by promoting its association with the inhibitors. These observations indicate that dephosphorylation of IR and GS can be regulated by Ca2+]i.