Calcium in Renal Cells. Modulation of Calcium-dependent Activation of Phospholipase $A_2$.

Abstract

Calcium has been implicated as a regulatory factor in many physiological and pathophysiological processes in the renal cell. Under physiological conditions, the cytosolic free calcium concentration is maintained at approximately 100 nM. Most of the releasable cell $Ca^{2+}$ resides in the nonmitochondrial compartments. In addition to the plasma membrane $Ca^{2+}$ transport processes, there is a high-affinity, low-capacity buffering capability of nonmitochondrial organelles and a lower-affinity high-capacity mitochondrial $Ca^{2+}$ buffering capability. A critical enzymatic effector of $Ca^{2+}$ action in the cell is phospholipase $A_2$. By using digitonin-permeabilized renal mesangial cells, the [$Ca^{2+}$] dependency of phospholipase $A_2$ was characterized. The [$Ca^{2+}$] sensitivity was insufficient to explain the phospholipase $A_2$ activation observed with vasopressin. In both intact cells, as well as permeabilized cells, it was found that protein kinase C activation markedly enhanced the $Ca^{2+}$ calmodulin-dependent activation of phospholipase $A_2$. In response to platelet-derived growth factor, it was found that arachidonic acid release preceded phospholipase C activation. This suggests that other effectors besides $Ca^{2+}$ and protein kinase C may also be important for phospholipase $A_2$ activation. In an experimental model designed to mimic postischemic reperfusion damage to renal mitochondria, it was demonstrated that reactive oxygen species act synergistically with $Ca^{2+}$ to activate mitochondrial phospholipase $A_2$, which mediates damage to site I of the electron transport chain, the $F_1F_0$ ATPase, and the adenine nucleotide translocase. In conclusion, an adequate understanding of the physiological and pathophysiological roles of intracellular $Ca^{2+}$ relies, not only on the measurement of $Ca^{2+}$ concentration and the characterization of "$Ca^{2+}$-dependent" processes, but an appreciation of the complex synergistic interactions between $Ca^{2+}$ and other mediators of cellular activation and toxicity