Relationship between calcium loading and impaired energy metabolism during Na+, K+ pump inhibition and metabolic inhibition in cultured neonatal rat cardiac myocytes.

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Abstract
This study tested the hypothesis that the initiating mechanism is a major determinant of the response to calcium (Ca) accumulation in myocardium. Cultured neonatal rat ventriculocytes were exposed to Na+, K+ pump inhibition with 1 mM ouabain and metabolic inhibition with 20 mM 2-deoxy-D-glucose and 1 mM cyanide (DOG-CN) for up to 2 h. Microspectrofluorometry of myocytes loaded with fura-2 showed that ouabain resulted in a relatively rapid increase in [Ca2+]i up to 2-3 microM (two to threefold above peak systolic level) and that DOG-CN produced an initial decrease and then a relatively slow increase in [Ca2+]i up to peak systolic level. Electron probe x-ray microanalysis (EPMA) showed prominent increases in Na and Ca and decreases in K and Mg in cytoplasm and mitochondria with both interventions, although the increases in Ca were greater with ouabain than DOG-CN. ATP was reduced by 58% after 1 and 2 h of ouabain and by 70 and 90% after 1 and 2 h of DOG-CN, respectively. Thus, ouabain produced greater calcium accumulation and less ATP reduction than DOG-CN. Upon return to normal medium for 30 min, myocytes showed recovery of most electrolyte alterations and resumption of normal Ca2+ transients after 1 h exposure to either ouabain or DOG-CN; however, recovery was less after 2 h of either treatment, with elevated [Ca2+]i maintained in many myocytes. We conclude that the severity of myocyte injury is influenced by the magnitude and duration of both ATP reduction and calcium accumulation.

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