The membrane attack complex of complement (MAC) can induce reversible changes in cell membrane permeability resulting in significant but transient intracellular ionic changes in the absence of cell lysis. Because ion fluxes and cytosolic ionic changes are integral steps in the signaling cascade initiated when growth factors bind to their receptors, we hypothesized that the MAC-induced reversible changes in membrane permeability could stimulate cell proliferation. Using purified terminal complement components we have documented a mitogenic effect of the MAC for quiescent murine 3T3 cells. The MAC enhances the mitogenic effects of serum and PDGF, and also stimulates cell proliferation in the absence of other exogenous growth factors. MAC-induced mitogenesis represents a novel effect of the terminal complement complex that could contribute to focal tissue repair or pathological cell proliferation locally at sites of complement activation. (J. Clin. Invest. 1993.

The complement system has been implicated in the pathogenesis of cardiovascular disorders including ischemia and atherosclerosis. Selective deposition of C5b-9, the membrane attack complex of complement, has been histochemically documented in human myocardium early after reperfusion of ischemic areas and in infarcted zones. However, functional sequelae of the C5b-9 complex binding to myocardial cells have not been identified. Insertion of C5b-9 complexes into the membrane of other cell types can generate transient changes in membrane permeability in the absence of cell lysis. We demonstrate in beating isolated adult guinea pig and rat cardiac myocytes that human derived C5b-9 can transiently augment in a dose-dependent manner both basal cytosolic calcium concentration and calcium transients, resulting in a temporary increase in contractility. If similar changes occur in human heart cells in vivo, they could significantly affect myocardial performance and contribute to functional abnormalities seen in ischemia and other pathological conditions associated with complement activation.

We have previously shown that sublytic heterologous complement induces large but transient increases in erythrocyte membrane permeability. We now report that when erythrocytes are bystanders in zymosan-activated autologous serum, they increase their Na+ permeability 10-fold, indicating that autologous complement can also induce transient membrane lesions. When we isolated the effect of the C5b-9 membrane attack complex of complement by using human C5b-9 assembled from purified components, we found there was minimal lysis but efficient Na+ uptake. Suspension of the sublytically damaged erythrocytes in K+ medium caused the cells to lyse, which is consistent with the cells recruiting a compensatory K+ efflux similar to that observed when human erythrocytes were exposed to heterologous complement. Sublytic C5b-9 exposure also became lytic when extracellular Ca2+ was limited and when the cells were exposed to charybdotoxin, an inhibitor of the Ca(2+)- activated K+ channel. This indicates that Ca2+ is required for the functional termination of the C5b-9 lesion. We also show that the membrane hyperpolarization resulting from activation of the Ca(2+)- dependent K+ efflux does not influence the termination of the C5b-9 lesion. Thus, the influx of Ca2+ through the complement lesion initiates at least two apparently independent adaptive responses: (1) a process that terminates the leak; and (2) a K+ efflux that has a volume regulatory function. Our data support the potential of the sublytic C5b- 9 lesion to act as a physiologic mediator for autologous erythrocytes.

A low molecular weight compound (about 600 daltons) extracted from human cerebrospinal fluid, and sensitive to proteolytic enzymes, has been shown to mimic the specific inhibitory effects of cardiac glycosides on the Na+/K+ pump of erythrocytes. The compound, which was labelled CSF-inhibitor (CSF-I) and reconstituted in artificial rabbit CSF, was used to study its effects on the rate of CSF formation in rabbits. Three groups of adult New Zealand white rabbits of either sex, anesthetized with ketamine and xylazine, and artificially respired were subjected to ventriculocisternal perfusions. Baseline rates of CSF formation were obtained during the first 2 hours of perfusion when plain rabbit CSF was used as the perfusate. Thereafter, the animals were perfused for an additional 2 to 3 hours with either plain rabbits CSF (controls), rabbit CSF containing 10(-6) M ouabain (group 2) or CSF-I (group 3). The rate of CSF formation in control animals was observed to gradually decline with time (about 7.5% in 4 hours). The addition of ouabain to the perfusate caused an abrupt and marked 43% decline in the rate of formation while the addition of CSF-I resulted in a 57% decline. These results suggest that the peptide CSF-I which is present in human CSF may act as an endogenous regulator of CSF production.

We have previously shown that sublytic heterologous complement induces large but transient increases in erythrocyte membrane permeability. We now report that when erythrocytes are bystanders in zymosan-activated autologous serum, they increase their Na+ permeability 10-fold, indicating that autologous complement can also induce transient membrane lesions. When we isolated the effect of the C5b-9 membrane attack complex of complement by using human C5b-9 assembled from purified components, we found there was minimal lysis but efficient Na+ uptake. Suspension of the sublytically damaged erythrocytes in K+ medium caused the cells to lyse, which is consistent with the cells recruiting a compensatory K+ efflux similar to that observed when human erythrocytes were exposed to heterologous complement. Sublytic C5b-9 exposure also became lytic when extracellular Ca2+ was limited and when the cells were exposed to charybdotoxin, an inhibitor of the Ca(2+)- activated K+ channel. This indicates that Ca2+ is required for the functional termination of the C5b-9 lesion. We also show that the membrane hyperpolarization resulting from activation of the Ca(2+)- dependent K+ efflux does not influence the termination of the C5b-9 lesion. Thus, the influx of Ca2+ through the complement lesion initiates at least two apparently independent adaptive responses: (1) a process that terminates the leak; and (2) a K+ efflux that has a volume regulatory function. Our data support the potential of the sublytic C5b- 9 lesion to act as a physiologic mediator for autologous erythrocytes.