Effect of high K+ exposure on phosphoinositide metabolism in frog skeletal muscle

Effects of high potassium and caffeine exposure on activities of Ca2+-dependent and Ca2+-independent protein kinase C in frog skeletal muscle

With the use of histone III-S as a protein kinase C (PKC) substrate, the activities of total and Ca2+-independent PKC in frog skeletal muscle were measured, and their difference is designated Ca2+-dependent PKC. In resting muscle, the total PKC was almost equally associated with the cytosol and membrane, and the ratio of membrane to cytosol PKC was about 1. However, the distribution of PKC was subtype dependent. About 60% of Ca2+-dependent PKC was located in the cytosol, whereas Ca2+-independent PKC was mainly associated with the membrane. High potassium exposure not only caused a significant translocation of Ca2+-dependent PKC from the cytosol to the membrane, but also changed the distribution of Ca2+-independent PKC, although to a lesser extent. However, in the preparations exposed to caffeine, the translocation of PKC occurred only in a Ca2+-dependent subtype. In addition, the biphasic change in membrane-associated PKC seen in high K+ exposed muscles was absent with caffeine treatment. The possible mechanisms of these differences are discussed.

3,4-Diaminopyridine induced hydrolysis of phosphoinositide in cultured neurons from embryo chick forebrain

The effect of 3,4-diaminopyridine (DAP) on phosphoinositide hydrolysis in cultured neurons from embryo chick forebrain has been studied. DAP produced a dose- and time-dependent accumulation of inositol phosphates. At 1 mM DAP a maximal effect was obtained. In Ca2+ free medium, DAP-activated turnover of phosphoinositide was reduced, but was still significant. Blocking Ca2+ entry with 200 microM Cd2+ also did not abolish the DAP-induced accumulation of inositol phosphates. As a comparison the effect of high K+ exposure was investigated. High K+ enhanced phosphoinositide hydrolysis, and this effect was also reduced by excluding Ca2+ influx. Moreover, DAP had no additional effect on the high K(+)-induced hydrolysis of phosphoinositide. Using oxonol-V, a depolarization of the membrane potential was seen in the neurons bathed in DAP containing medium. It is suggested that the depolarization may play a role in DAP-activated phosphoinositide turnover in cultured neurons of the embryo chick forebrain, but that Ca2+ entry is not necessary for this effect.