Sustained Recovery of Na⁺-K⁺-ATPase Activity in Sciatic Nerve of Diabetic Mice by Administration of H7 or Calphostin C, Inhibitors of PKC

We have previously shown that intraperitoneal injection of H-7, an inhibitor of PKC, restores completely the activity of Na⁺-K⁺-ATPase in sciatic nerve of diabetic mice; however, the effect was transient, with a half-life of ∼1 h under the conditions used. This work assessed whether calphostin C, a new more potent and specific inhibitor of PKC, is also able to restore the activity of Na⁺-K⁺-ATPase in sciatic nerve of ALX-induced diabetic mice and also assessed if continuous administration of H-7 or calphostin C can afford sustained recovery of the ATPase. Small amounts of calphostin C (i.e., 2 μg/kg) restore entirely the activity of the enzyme. Larger doses (e.g., 30 μg/kg) can be administered with equal results. The ED₅₀ was ∼0.5 μg/kg. This indicates that calphostin C is ∼20,000 times more potent than H-7 in restoring the ATPase activity in diabetic mice. A single intraperitoneal injection of 1 or 10 μg/kg of calphostin C maintains the enzyme for 4 and 8 h, respectively. Administration of H-7 by continuous delivery from micro-osmotic pumps implanted in the back of the mice maintains the Na⁺-K⁺-ATPase for 24 h, although the activity decreases thereafter. This is the result of instability of H-7 in solution. Continuous administration of calphostin C maintains the activity of the ATPase at nearly normal values for at least 2 wk. The results support the hypothesis that, in sciatic nerve tissue of diabetic animals, the activity of PKC is increased, leading to higher phosphorylation of Na⁺-K⁺-ATPase, which results in the decreased activity observed. Also, the results show that inhibitors of PKC such as calphostin C can be of therapeutic interest.