Mechanisms of hemolysate-induced [Ca2+]i elevation in cerebral smooth muscle cells

Abstract

The effects of hemolysate on free cytosolic [Ca2+] ([Ca2+]i) homeostasis were studied in freshly isolated rat basilar artery smooth muscle cells using fura 2 and dual excitation wavelength microfluorimetry. Hemolysate reversibly produced a transient [Ca2+]i peak followed by a slowly decaying plateau which was absent in Ca(2+)-free solution. This effect of hemolysate was attenuated by 1) the sarcoplasmic reticulum Ca2+ pump inhibitors thapsigargin and cyclopiazonic acid, 2) the Ca2+ release-blocking agents ryanodine and dantrolene, 3) the cytochrome P-450 inhibitor econazole, and 4) the inorganic Ca2+ channel blocker lanthanum but was not significantly attenuated by 1) the receptor-regulated Ca2+ channel blocker SKF-96365 or 2) the voltage-dependent Ca2+ channel blocker nimodipine. Fractionation of hemolysate using membranes with specific pore sizes (0.5, 1, and 12-14 kDa) indicated that a component(s) > 0.5 but < 1 kDa could produce a similar [Ca2+]i peak and plateau while fractions > 1 and > 12-14 kDa produced a small and slow [Ca2+]i rise without a significant peak. ATP, which was found in hemolysate, produced a [Ca2+]i response similar to that of hemolysate. P2-purinoceptor antagonists significantly attenuated the effect of ATP, hemolysate, and the fractions < 1 and < 12-14 kDa. We conclude that hemolysate elevates [Ca2+]i by both releasing Ca2+ from internal stores and triggering Ca2+ entry, possibly from a voltage-independent Ca2+ influx pathway, an effect apparently identical to that of ATP.