The 4-kilodalton (39 to 43 amino acids) amyloid beta protein (beta AP), which is deposited as amyloid in the brains of patients with Alzheimer's diseases, is derived from a large protein, the amyloid beta protein precursor (beta APP). Human mononuclear leukemic (K562) cells expressing a beta AP-bearing, carboxyl-terminal beta APP derivative released significant amounts of a soluble 4-kilodalton beta APP derivative essentially identical to the beta AP deposited in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing full-length beta APP and M17 cells expressing only endogenous beta APP also released soluble 4-kilodalton beta AP, and a similar, if not identical, fragment was readily detected in cerebrospinal fluid from individuals with Alzheimer's disease and normal individuals. Thus cells normally produce and release soluble 4-kilodalton beta AP that is essentially identical to the 4-kilodalton beta AP deposited as insoluble amyloid fibrils in Alzheimer's disease.

Previous studies from this and other laboratories have shown that angiotensin II (AII) induces [Ca2"j transients in proximal tubular epithelium independent of phospholipase C. AII also stimulates formation of 5,6-epoxyeicosatrienoic acid (5,6-EET) from arachidonic acid by a cytochrome P450 epoxygenase and decreases Na' transport in the same concentration range. Because 5,6-EET mimics AII with regard to Na' transport, its effects on calcium mobilization were evaluated. ICa2i, was measured by video microscopy with the fluorescent indicator fura-2 employing cultured rabbit proximal tubule. AII-induced ICa2+i transients were enhanced by arachidonic acid and attenuated by ketoconazole, an inhibitor of cytochrome P450 epoxygenases. Arachidonic acid also elicited a ICa2i1, transient that was attenuated by ketoconazole. 5,6-EET augmented (Ca2'], similar to that seen with AII, but was unaffected by ketoconazole. By contrast, the other regioisomers (8,9-, 11,12-, and 14,15-EET) were much less potent. ICa2+i, transients resulted from influx through verapamil-and nifedipine-sensitive channels. These results suggest a novel mechanism for AII-induced Ca mobilization in proximal tubule involving cytochrome P450-dependent arachidonic asid metabolism and Ca influx through voltage-sensitive channels. (J. Clin. Invest. 1991.