A diacylglycerol analogue reduces neuronal calcium currents independently of protein kinase C activation

DIACYLGLYCEROL analogues (for example 1,2-oleoylacetyl-glycerol, OAG) and phorbol esters are activators of protein kinase C, and have been widely used to study the function of this enzyme in both intact cells and cell-free preparations^1,2. Electrophysiological studies have shown that these activators can either depress^3-6 or increase Ca²⁺ currents^7-9, or decrease K⁺ currents^10,11 when applied outside the cell. It has been assumed that these effects are mediated by protein kinase C activation. Here we report that micromolar levels of OAG and phorbol esters depress Ca²⁺ currents in chick sensory neurons independently of their effect as activators of protein kinase C. The depression of the Ca²⁺ current is rapid and is unaffected by intracellular application of the protein kinase C inhibitors staurosporin, sphingosine and H-7¹². Furthermore, the activators were ineffective when applied intracellularly, indicating that their site of action is on the outside of the membrane.