The protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methyl-piperzine (H-7) prevents and reverses Ca(2+)-mediated injury in isolated rat hepatocyte couplets

Atrial natriuretic peptide attenuates elevations in Ca2+ and protects hepatocytes by stimulating net plasma membrane Ca2+ efflux

Elevations in intracellular Ca(2+) 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 Ca(2+) 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 Ca(2+) rises in single hepatocytes. Atrial natriuretic peptide also prevented stimulation of calpain activity by ATP, taurolithocholate, or Ca(2+) mobilization by thapsigargin and ionomycin. We therefore investigated the cellular Ca(2+) handling mechanisms through which ANP attenuates this sustained elevation in cytosolic Ca(2+). We show that atrial natriuretic peptide does not modulate the release from or re-uptake of Ca(2+) into intracellular stores but, through protein kinase G, both stimulates plasma membrane Ca(2+) efflux from and inhibits ATP-stimulated Ca(2+) influx into hepatocytes. These findings suggest that stimulation of net plasma membrane Ca(2+) efflux (to which both Ca(2+) efflux stimulation and Ca(2+) influx inhibition contribute) is the key process through which atrial natriuretic peptide attenuates elevations in cytosolic Ca(2+) and calpain activity. Moreover we propose that plasma membrane Ca(2+) efflux is a valuable, previously undiscovered, mechanism through which atrial natriuretic peptide protects rat hepatocytes, and perhaps other cell types, against Ca(2+)-dependent injury.

The inhibitory effect of prostaglandin E1 on oxidative stress-induced hepatocyte injury evaluated by calpain-mu activation

BACKGROUND

Prostaglandin E1 (PGE1) is known to inhibit ischemia-reperfusion injury of the liver. The calcium-dependent neutral proteinase, calpain-mu, is involved in oxidative stress-induced hepatocyte injury. We investigated the mechanisms of cytoprotection by PGE1, focusing on the elevation of intracellular calcium ([Ca2+]i), activation of calpain-mu, and calpain-mu-mediated activation of protein kinase C-alpha (PKC-alpha).

METHODS

Cultured hepatocytes were treated with various amounts of PGE1 (0, 0.1, 1.0, 10, and 100 ng/ml) for 30 min and subsequently with 0.5 mM tert-butyl hydroperoxide (TBHP). Cell injury was evaluated by the release of lactate dehydrogenase. Plasma membrane bleb formation was examined by phase contrast microscopy. Activation of calpain-mu and limited degradation of PKC-alpha was evaluated by Western blotting using antibodies that specifically recognize the amino-terminal regions of calpain-mu and PKC-alpha. [Ca2+]i was measured by confocal microscopy using Fluo-3AM.

RESULTS

LDH release from cells treated with 10 ng/ml PGE1 was significantly lower than from untreated cells (135 +/- 12 vs. 258 +/- 18 IU/L, respectively; P < 0.05). Morphologically, many blebs were observed in untreated cells, but very few were seen in those treated with 10 ng/ml PGE1. Western blotting revealed that the amount of activated calpain-mu and [Ca2+]i increased up to 1,300 nM at 35 min after the addition of TBHP (0.5 mmol/L) in control experiments (without PGE1). PGE1 (10 ng/ml) delayed the rise in [Ca2+]i for about 30 min, but did not suppress it completely. PKC-alpha decreased in experiments using PGE1 (10 ng/ml).

CONCLUSION

PGE1 exerts its cytoprotective effect in TBHP-induced hepatocyte injury partly by inhibiting Ca2+-calpain-mu-mediated mechanisms.