Effects of calcium loading and impaired energy production on metabolic and ultrastructural features of cell injury in cultured neonatal rat cardiac myocytes

We evaluated the contributions of calcium loading and impaired energy production to metabolic and ultrastructural manifestations of cell injury in a cultured neonatal rat ventriculocyte model. Direct calcium loading was produced by incubation in K(+)-free medium to inhibit the Na+,K(+)-ATPase and promote Na(+)-Ca2+ exchange, and inhibition of energy metabolism was produced by incubation with 30 microM iodoacetic acid (IAA). Measurements were made of total cell calcium, [3H] arachidonic acid (AA) release (an index of membrane phospholipid degradation), ATP, and ultrastructural features of cell damage. Inhibition of the Na(+),K(+) pump resulted in the rapid onset of cellular calcium loading, increased [3H]AA release, and moderate ATP reduction. After return to control medium for 24 hours, myocytes previously exposed to K(+)-free medium for 1 hour showed recovery of ATP level and little additional [3H]AA release. However, after 2 to 3 hours of calcium loading, the ATP level remained moderately depressed, residual [3H]AA release was greater, and a mixed population of relatively normal and severely damaged myocytes was observed by electron microscopy. IAA treatment for 1 hour resulted in moderate ATP reduction without calcium accumulation or [3H]AA release, whereas IAA treatment for 3 hours resulted in marked ATP reduction associated with calcium accumulation and [3H]AA release. Reversal experiments showed substantial recovery of ATP level after 1 hour of IAA exposure, and marked ATP depression and [3H]AA release associated with widespread irreversible injury after 3 hours. Thus, the data indicate that increased calcium accumulation itself can initiate accelerated membrane phospholipid degradation, but that progression to irreversible injury is influenced by other factors, including the magnitude of ATP depression associated with calcium loading.