Atrial Natriuretic Peptide Attenuates Elevations in Ca2+ and Protects Hepatocytes by Stimulating Net Plasma Membrane Ca2+ Efflux

Elevations in intracellular Ca2+ concentration and calpain activity are common early events in cellular injury, including that of hepatocytes. Atrial natriuretic peptide is a circulating hormone that has been shown to be hepatoprotective. The aim of this study was to examine the effects of atrial natriuretic peptide on potentially harmful elevations in cytosolic free Ca2+ and calpain activity induced by extracellular ATP in rat hepatocytes. We show that atrial natriuretic peptide, through protein kinase G, attenuated both the amplitude and duration of ATP-induced cytosolic Ca2+ rises in single hepatocytes. Atrial natriuretic peptide also prevented stimulation of calpain activity by ATP, taurolithocholate, or Ca2+ mobilization by thapsigargin and ionomycin. We therefore investigated the cellular Ca2+ handling mechanisms through which ANP attenuates this sustained elevation in cytosolic Ca2+. We show that atrial natriuretic peptide does not modulate the release from or re-uptake of Ca2+ into intracellular stores but, through protein kinase G, both stimulates plasma membrane Ca2+ efflux from and inhibits ATP-stimulated Ca2+ influx into hepatocytes. These findings suggest that stimulation of net plasma membrane Ca2+ efflux (to which both Ca2+ efflux stimulation and Ca2+ influx inhibition contribute) is the key process through which atrial natriuretic peptide attenuates elevations in cytosolic Ca2+ and calpain activity. Moreover we propose that plasma membrane Ca2+ efflux is a valuable, previously undiscovered, mechanism through which atrial natriuretic peptide protects rat hepatocytes, and perhaps other cell types, against Ca2+-dependent injury.